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tech / sci.electronics.design / Active differential power rail probe

SubjectAuthor
* Active differential power rail probeDominic Chan
+* Re: Active differential power rail probeolaf
|`* Re: Active differential power rail probeDominic Chan
| `- Re: Active differential power rail probeolaf
+* Re: Active differential power rail probewhit3rd
|`* Re: Active differential power rail probeDominic Chan
| `* Re: Active differential power rail probeMike Monett
|  `* Re: Active differential power rail probePhil Hobbs
|   +* Re: Active differential power rail probepiglet
|   |`- Re: Active differential power rail probeJeroen Belleman
|   +- Re: Active differential power rail probejlarkin
|   `* Re: Active differential power rail probeDominic Chan
|    +* Re: Active differential power rail probePhil Hobbs
|    |`* Re: Active differential power rail probeJohn Larkin
|    | +* Re: Active differential power rail probewhit3rd
|    | |+- Re: Active differential power rail probePhil Hobbs
|    | |`- Re: Active differential power rail probeJohn Larkin
|    | `* Re: Active differential power rail probePhil Hobbs
|    |  `- Re: Active differential power rail probeJohn Doe
|    `- Re: Active differential power rail probeJohn Doe
+* Re: Active differential power rail probejlarkin
|`* Re: Active differential power rail probeDominic Chan
| `- Re: Active differential power rail probejlarkin
+- Re: Active differential power rail probelegg
`- Re: Active differential power rail probeChris Jones

1
Active differential power rail probe

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Subject: Active differential power rail probe
From: domi...@arista.com (Dominic Chan)
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 by: Dominic Chan - Wed, 4 May 2022 03:48 UTC

I've been doing some power rail noise/ripple measurements lately. It seems like the generally agreed best practice is to solder a coax lead as close to the load as possible, taking care not to probe directly across backside decoupling caps to avoid underestimating HF noise. This coax lead is then connected to a power rail probe / 50R scope input.

Since the boards I'm working with have 0V tied to Earth, there is a ground loop formed when making these measurements. I've found that if I power the scope and board off the same power strip the environmental noise coupling into this loop is sufficiently small for good measurements.

This has got me wondering though, why isn't an active differential probe optimized for power rail measurements a thing? Perhaps it does exist? - If so please let me know!

Cheers, Dominic

Re: Active differential power rail probe

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From: ola...@criseis.ruhr.de (olaf)
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Subject: Re: Active differential power rail probe
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 by: olaf - Wed, 4 May 2022 04:13 UTC

Dominic Chan <dominic@arista.com> wrote:

>This has got me wondering though, why isn't an active differential
>probe optimized for power rail measurements a thing? Perhaps it does
>exist? - If so please let me know!

Tek offers the TPR4000. :-D And I think there is one from Agilent
in the same price range, too.

https://www.tek.com/en/datasheet/active-power-rail-probes

These probes are AC-coupled, so offering a direct connection for
High frequency and add the DC-Level to the output again.

Olaf

Re: Active differential power rail probe

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Subject: Re: Active differential power rail probe
From: domi...@arista.com (Dominic Chan)
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 by: Dominic Chan - Wed, 4 May 2022 04:46 UTC

On Wednesday, May 4, 2022 at 2:15:09 PM UTC+10, olaf wrote:
> Dominic Chan <dom...@arista.com> wrote:
>
>
> >This has got me wondering though, why isn't an active differential
> >probe optimized for power rail measurements a thing? Perhaps it does
> >exist? - If so please let me know!
> Tek offers the TPR4000. :-D And I think there is one from Agilent
> in the same price range, too.
>
> https://www.tek.com/en/datasheet/active-power-rail-probes
>
> These probes are AC-coupled, so offering a direct connection for
> High frequency and add the DC-Level to the output again.
>
> Olaf

Olaf those are the classic power rail probes - the DC level circuitry is 'active' but the AC path is purely passive, and of course it is a single ended probe.

Re: Active differential power rail probe

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Subject: Re: Active differential power rail probe
From: whit...@gmail.com (whit3rd)
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 by: whit3rd - Wed, 4 May 2022 04:49 UTC

On Tuesday, May 3, 2022 at 8:48:08 PM UTC-7, Dominic Chan wrote:
> I've been doing some power rail noise/ripple measurements lately.
....
> This has got me wondering though, why isn't an active differential probe optimized for power rail measurements a thing? Perhaps it does exist? - If so please let me know!

For a quick check, I just use two channels, with x10 passive probes, and difference 'em.
It's not terrific for accuracy, but power isn't required to be accurate.
Biggest drawback: you need both hands to hold the probes.

Re: Active differential power rail probe

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Subject: Re: Active differential power rail probe
From: domi...@arista.com (Dominic Chan)
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 by: Dominic Chan - Wed, 4 May 2022 05:58 UTC

On Wednesday, May 4, 2022 at 2:49:41 PM UTC+10, whit3rd wrote:
> On Tuesday, May 3, 2022 at 8:48:08 PM UTC-7, Dominic Chan wrote:
> > I've been doing some power rail noise/ripple measurements lately.
> ...
> > This has got me wondering though, why isn't an active differential probe optimized for power rail measurements a thing? Perhaps it does exist? - If so please let me know!
> For a quick check, I just use two channels, with x10 passive probes, and difference 'em.
> It's not terrific for accuracy, but power isn't required to be accurate.
> Biggest drawback: you need both hands to hold the probes.

Yep that works for a quick check, I've done the same for jank current measurements through a resistor.
The issue with x10 probes for power rail measurement is the measurement noise. E.g. Xilinx transceivers guidelines allow for 10mVpp from 10kHz to 80MHz. Using active differential probes on rails with even more stringent requirements also runs into the same measurement noise issue.

Since input cap / loading of a probe is pretty much irrelevant for power rail measurements, it seems to me that an active diff probe that is designed intentionally for power rail measurements could obtain much better noise performance.

E.g. the RT-ZD40 active diff probe has 3mVrms input referred noise, while 50 ohm oscilloscope frontends easily achieve sub 1mVrms.

Re: Active differential power rail probe

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 by: Mike Monett - Wed, 4 May 2022 08:30 UTC

Dominic Chan <dominic@arista.com> wrote:

> On Wednesday, May 4, 2022 at 2:49:41 PM UTC+10, whit3rd wrote:
>> On Tuesday, May 3, 2022 at 8:48:08 PM UTC-7, Dominic Chan wrote:
>> > I've been doing some power rail noise/ripple measurements lately. ...
>> > This has got me wondering though, why isn't an active differential
>> > prob
> e optimized for power rail measurements a thing? Perhaps it does exist?
> - I f so please let me know!
>> For a quick check, I just use two channels, with x10 passive probes,
>> and difference 'em. It's not terrific for accuracy, but power isn't
>> required to be accurate.
>
>> Biggest drawback: you need both hands to hold the probes.
>
> Yep that works for a quick check, I've done the same for jank current
> measurements through a resistor. The issue with x10 probes for power
> rail measurement is the measurement noise. E.g. Xilinx transceivers
> guidelines allow for 10mVpp from 10kHz to 80MHz. Using active
> differential probes on rails with even more stringent requirements also
> runs into the same measurement noise issue.
>
> Since input cap / loading of a probe is pretty much irrelevant for power
> rail measurements, it seems to me that an active diff probe that is
> designed intentionally for power rail measurements could obtain much
> better noise performance.
>
> E.g. the RT-ZD40 active diff probe has 3mVrms input referred noise,
> while 50 ohm oscilloscope frontends easily achieve sub 1mVrms.

For interest, Marco Reps shows how to get 0.001pf isolation on line power:

eXtReMe iSoLaTiOn ( Low Leakage Power in Precision Electronics )
https://www.youtube.com/watch?v=9JinSfCKuNQ

For even more isolation, a number of battery powered oscilloscopes are
available for under $200. For example, the FNIRSI-1013D is a 100 MHz dual
trace touch tablet:

http://www.fnirsi.cn/productinfo/556152.html

It is available at

https://www.aliexpress.com/i/4000861098295.html

Dave Jones, EEVblog doesn't think much of it, but whadda gonna do for CAD
$168.74 ?

https://www.youtube.com/watch?v=5iwtDwJlbWk

The lowest range is 50mV, but you could easily add a differential opamp for
low level work, such as the opa846:

https://www.ti.com/lit/ds/symlink/opa846.pdf

It is under $10 at Octopart:

https://octopart.com/search?q=opa846&currency=USD&specs=0

--
MRM

Re: Active differential power rail probe

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 by: jlar...@highlandsniptechnology.com - Wed, 4 May 2022 13:44 UTC

On Tue, 3 May 2022 20:48:04 -0700 (PDT), Dominic Chan
<dominic@arista.com> wrote:

>I've been doing some power rail noise/ripple measurements lately. It seems like the generally agreed best practice is to solder a coax lead as close to the load as possible, taking care not to probe directly across backside decoupling caps to avoid underestimating HF noise. This coax lead is then connected to a power rail probe / 50R scope input.
>
>Since the boards I'm working with have 0V tied to Earth, there is a ground loop formed when making these measurements. I've found that if I power the scope and board off the same power strip the environmental noise coupling into this loop is sufficiently small for good measurements.
>
>This has got me wondering though, why isn't an active differential probe optimized for power rail measurements a thing? Perhaps it does exist? - If so please let me know!
>
>Cheers, Dominic

Tek has a number of fully isolated scopes and outboard isolated
probes. We have a 4-channel isolated scope, TPS2024 I recall, that's
wonderful for working on power things. Clip the probe ground things
anywhere.

https://www.tek.com/en/search?keywords=isolated

--

Anybody can count to one.

- Robert Widlar

Re: Active differential power rail probe

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 by: olaf - Wed, 4 May 2022 14:08 UTC

Dominic Chan <dominic@arista.com> wrote:

>Olaf those are the classic power rail probes - the DC level
>circuitry is 'active' but the AC path is purely passive, and of
>course it is a single ended probe.

But the DC level allow more shifting than the scope alone,
so for most power supply it is enought.

Olaf

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 by: legg - Wed, 4 May 2022 14:17 UTC

On Tue, 3 May 2022 20:48:04 -0700 (PDT), Dominic Chan
<dominic@arista.com> wrote:

>I've been doing some power rail noise/ripple measurements lately. It seems like the generally agreed best practice is to solder a coax lead as close to the load as possible, taking care not to probe directly across backside decoupling caps to avoid underestimating HF noise. This coax lead is then connected to a power rail probe / 50R scope input.
>
>Since the boards I'm working with have 0V tied to Earth, there is a ground loop formed when making these measurements. I've found that if I power the scope and board off the same power strip the environmental noise coupling into this loop is sufficiently small for good measurements.
>
>This has got me wondering though, why isn't an active differential probe optimized for power rail measurements a thing? Perhaps it does exist? - If so please let me know!
>
>Cheers, Dominic

Noise, by definition, is not a DC thing.

For PARD (Peak And Random Deviation), you would include DC.

Differential and common-mode (AC) noise can be measured/contrasted
using passive or active combination, over a specific bandwidth.

Wideband diff probes are available for lower frequencies, but
reduce with BW and accuracy as DC maximum withstand levels increase.

Is somebody asking for this data? Ask for the measurement standards
being referenced. This will usually include a test set-up figure
and equipment requirements.

I built a 2-pin, battery-operated Ppk measurement probe for quick
'relative' PSU measurements that gave a digital reading of % in the
ancient past. This used the commodity ICL7106 to do a ratiometric
comparison between the DC (ref) and an ECL-type biased AC peak
detector, though the peak could be obtained as a number, as well.

It's not common test gear because it has very low volume demand.
Indications are sufficient, because real world testing usually
requires a >>2:1 performance margin on the test limit, so
tolerances are not a great issue - just repeatability.

Aim is to avoid trouble, not to track it down after the fact;
though it can be used as prediction/symptom of failure elsewhere.
Physical noise : Same thing.

Hence strict test setup and usefulness of field experience, in
tracking down measurement or application errors / physical
configuration pitfalls.
RL

Re: Active differential power rail probe

<8ewcK.1261374$zX2.843716@fx12.ams4>

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 by: Chris Jones - Wed, 4 May 2022 14:39 UTC

On 04/05/2022 13:48, Dominic Chan wrote:
> I've been doing some power rail noise/ripple measurements lately. It seems like the generally agreed best practice is to solder a coax lead as close to the load as possible, taking care not to probe directly across backside decoupling caps to avoid underestimating HF noise. This coax lead is then connected to a power rail probe / 50R scope input.
>
> Since the boards I'm working with have 0V tied to Earth, there is a ground loop formed when making these measurements. I've found that if I power the scope and board off the same power strip the environmental noise coupling into this loop is sufficiently small for good measurements.
>
> This has got me wondering though, why isn't an active differential probe optimized for power rail measurements a thing? Perhaps it does exist? - If so please let me know!
>
> Cheers, Dominic

I put some connectors on an AD830 because I have lots of them:

https://twitter.com/chrisgj198/status/330590675806605312

Re: Active differential power rail probe

<431866c0-a226-40dd-839f-c622869136af@electrooptical.net>

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From: pcdhSpam...@electrooptical.net (Phil Hobbs)
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 by: Phil Hobbs - Wed, 4 May 2022 16:42 UTC

Mike Monett wrote:
> Dominic Chan <dominic@arista.com> wrote:
>
>> On Wednesday, May 4, 2022 at 2:49:41 PM UTC+10, whit3rd wrote:
>>> On Tuesday, May 3, 2022 at 8:48:08 PM UTC-7, Dominic Chan wrote:
>>>> I've been doing some power rail noise/ripple measurements lately. ...
>>>> This has got me wondering though, why isn't an active differential
>>>> prob
>> e optimized for power rail measurements a thing? Perhaps it does exist?
>> - I f so please let me know!
>>> For a quick check, I just use two channels, with x10 passive probes,
>>> and difference 'em. It's not terrific for accuracy, but power isn't
>>> required to be accurate.
>>
>>> Biggest drawback: you need both hands to hold the probes.
>>
>> Yep that works for a quick check, I've done the same for jank current
>> measurements through a resistor. The issue with x10 probes for power
>> rail measurement is the measurement noise. E.g. Xilinx transceivers
>> guidelines allow for 10mVpp from 10kHz to 80MHz. Using active
>> differential probes on rails with even more stringent requirements also
>> runs into the same measurement noise issue.
>>
>> Since input cap / loading of a probe is pretty much irrelevant for power
>> rail measurements, it seems to me that an active diff probe that is
>> designed intentionally for power rail measurements could obtain much
>> better noise performance.
>>
>> E.g. the RT-ZD40 active diff probe has 3mVrms input referred noise,
>> while 50 ohm oscilloscope frontends easily achieve sub 1mVrms.
>
> For interest, Marco Reps shows how to get 0.001pf isolation on line power:
>
> eXtReMe iSoLaTiOn ( Low Leakage Power in Precision Electronics )
> https://www.youtube.com/watch?v=9JinSfCKuNQ
>
> For even more isolation, a number of battery powered oscilloscopes are
> available for under $200. For example, the FNIRSI-1013D is a 100 MHz dual
> trace touch tablet:
>
> http://www.fnirsi.cn/productinfo/556152.html
>
> It is available at
>
> https://www.aliexpress.com/i/4000861098295.html
>
> Dave Jones, EEVblog doesn't think much of it, but whadda gonna do for CAD
> $168.74 ?
>
> https://www.youtube.com/watch?v=5iwtDwJlbWk
>
> The lowest range is 50mV, but you could easily add a differential opamp for
> low level work, such as the opa846:
>
> https://www.ti.com/lit/ds/symlink/opa846.pdf
>
> It is under $10 at Octopart:
>
> https://octopart.com/search?q=opa846&currency=USD&specs=0
>
>
>
>
Another approach is to put a Mini-Circuits transformer across the
current sense resistor, and use a normal scope on the secondary. Needs
a ~1 uF coupling cap, of course, but that'll get you down to the low
kilohertz. Coupling is very high--their gizmos typically measure over
0.999, often about 0.9997.

Try the T1-6.

<https://www.minicircuits.com/pdfs/T1-6-X65+.pdf>

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

Re: Active differential power rail probe

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From: erichpwa...@hotmail.com (piglet)
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Subject: Re: Active differential power rail probe
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 by: piglet - Thu, 5 May 2022 05:35 UTC

On 04/05/2022 17:42, Phil Hobbs wrote:
> Another approach is to put a Mini-Circuits transformer across the
> current sense resistor, and use a normal scope on the secondary.  Needs
> a ~1 uF coupling cap, of course, but that'll get you down to the low
> kilohertz.  Coupling is very high--their gizmos typically measure over
> 0.999, often about 0.9997.
>
> Try the T1-6.
>
> <https://www.minicircuits.com/pdfs/T1-6-X65+.pdf>
>

What is the pri-sec capacitance of that transformer the datasheet is silent?

piglet

Re: Active differential power rail probe

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From: jer...@nospam.please (Jeroen Belleman)
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Subject: Re: Active differential power rail probe
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 by: Jeroen Belleman - Thu, 5 May 2022 07:54 UTC

piglet wrote:
> On 04/05/2022 17:42, Phil Hobbs wrote:
>> Another approach is to put a Mini-Circuits transformer across the
>> current sense resistor, and use a normal scope on the secondary.
>> Needs a ~1 uF coupling cap, of course, but that'll get you down to the
>> low kilohertz. Coupling is very high--their gizmos typically measure
>> over 0.999, often about 0.9997.
>>
>> Try the T1-6.
>>
>> <https://www.minicircuits.com/pdfs/T1-6-X65+.pdf>
>>
>
> What is the pri-sec capacitance of that transformer the datasheet is
> silent?
>
> piglet

About 13pF.

Jeroen Belleman

Re: Active differential power rail probe

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 by: jlar...@highlandsniptechnology.com - Thu, 5 May 2022 16:03 UTC

On Wed, 4 May 2022 12:42:39 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>Mike Monett wrote:
>> Dominic Chan <dominic@arista.com> wrote:
>>
>>> On Wednesday, May 4, 2022 at 2:49:41 PM UTC+10, whit3rd wrote:
>>>> On Tuesday, May 3, 2022 at 8:48:08 PM UTC-7, Dominic Chan wrote:
>>>>> I've been doing some power rail noise/ripple measurements lately. ...
>>>>> This has got me wondering though, why isn't an active differential
>>>>> prob
>>> e optimized for power rail measurements a thing? Perhaps it does exist?
>>> - I f so please let me know!
>>>> For a quick check, I just use two channels, with x10 passive probes,
>>>> and difference 'em. It's not terrific for accuracy, but power isn't
>>>> required to be accurate.
>>>
>>>> Biggest drawback: you need both hands to hold the probes.
>>>
>>> Yep that works for a quick check, I've done the same for jank current
>>> measurements through a resistor. The issue with x10 probes for power
>>> rail measurement is the measurement noise. E.g. Xilinx transceivers
>>> guidelines allow for 10mVpp from 10kHz to 80MHz. Using active
>>> differential probes on rails with even more stringent requirements also
>>> runs into the same measurement noise issue.
>>>
>>> Since input cap / loading of a probe is pretty much irrelevant for power
>>> rail measurements, it seems to me that an active diff probe that is
>>> designed intentionally for power rail measurements could obtain much
>>> better noise performance.
>>>
>>> E.g. the RT-ZD40 active diff probe has 3mVrms input referred noise,
>>> while 50 ohm oscilloscope frontends easily achieve sub 1mVrms.
>>
>> For interest, Marco Reps shows how to get 0.001pf isolation on line power:
>>
>> eXtReMe iSoLaTiOn ( Low Leakage Power in Precision Electronics )
>> https://www.youtube.com/watch?v=9JinSfCKuNQ
>>
>> For even more isolation, a number of battery powered oscilloscopes are
>> available for under $200. For example, the FNIRSI-1013D is a 100 MHz dual
>> trace touch tablet:
>>
>> http://www.fnirsi.cn/productinfo/556152.html
>>
>> It is available at
>>
>> https://www.aliexpress.com/i/4000861098295.html
>>
>> Dave Jones, EEVblog doesn't think much of it, but whadda gonna do for CAD
>> $168.74 ?
>>
>> https://www.youtube.com/watch?v=5iwtDwJlbWk
>>
>> The lowest range is 50mV, but you could easily add a differential opamp for
>> low level work, such as the opa846:
>>
>> https://www.ti.com/lit/ds/symlink/opa846.pdf
>>
>> It is under $10 at Octopart:
>>
>> https://octopart.com/search?q=opa846&currency=USD&specs=0
>>
>>
>>
>>
>Another approach is to put a Mini-Circuits transformer across the
>current sense resistor, and use a normal scope on the secondary. Needs
>a ~1 uF coupling cap, of course, but that'll get you down to the low
>kilohertz. Coupling is very high--their gizmos typically measure over
>0.999, often about 0.9997.
>
>Try the T1-6.
>
><https://www.minicircuits.com/pdfs/T1-6-X65+.pdf>
>
>Cheers
>
>Phil Hobbs

The DRQ series dual-winding inductors have crazy high coupling and are
good for low frequency cases. They come up to 1 mH.

--

Anybody can count to one.

- Robert Widlar

Re: Active differential power rail probe

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Subject: Re: Active differential power rail probe
From: domi...@arista.com (Dominic Chan)
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 by: Dominic Chan - Thu, 5 May 2022 22:13 UTC

On Wednesday, May 4, 2022 at 11:44:50 PM UTC+10, jla...@highlandsniptechnology.com wrote:
> On Tue, 3 May 2022 20:48:04 -0700 (PDT), Dominic Chan
> <dom...@arista.com> wrote:
>
> >I've been doing some power rail noise/ripple measurements lately. It seems like the generally agreed best practice is to solder a coax lead as close to the load as possible, taking care not to probe directly across backside decoupling caps to avoid underestimating HF noise. This coax lead is then connected to a power rail probe / 50R scope input.
> >
> >Since the boards I'm working with have 0V tied to Earth, there is a ground loop formed when making these measurements. I've found that if I power the scope and board off the same power strip the environmental noise coupling into this loop is sufficiently small for good measurements.
> >
> >This has got me wondering though, why isn't an active differential probe optimized for power rail measurements a thing? Perhaps it does exist? - If so please let me know!
> >
> >Cheers, Dominic
> Tek has a number of fully isolated scopes and outboard isolated
> probes. We have a 4-channel isolated scope, TPS2024 I recall, that's
> wonderful for working on power things. Clip the probe ground things
> anywhere.
>
> https://www.tek.com/en/search?keywords=isolated
>
>
>
> --
>
> Anybody can count to one.
>
> - Robert Widlar
Neat! Their isolated probes with 50 ohm probe input impedance is along the lines of what I was thinking of. Though I don't think true isolation is necessary, just some CMRR. Why isn't this a more common thing?

Re: Active differential power rail probe

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Subject: Re: Active differential power rail probe
From: domi...@arista.com (Dominic Chan)
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 by: Dominic Chan - Thu, 5 May 2022 22:19 UTC

On Thursday, May 5, 2022 at 2:42:53 AM UTC+10, Phil Hobbs wrote:
> Mike Monett wrote:
> > Dominic Chan <dom...@arista.com> wrote:
> >
> >> On Wednesday, May 4, 2022 at 2:49:41 PM UTC+10, whit3rd wrote:
> >>> On Tuesday, May 3, 2022 at 8:48:08 PM UTC-7, Dominic Chan wrote:
> >>>> I've been doing some power rail noise/ripple measurements lately. ...
> >>>> This has got me wondering though, why isn't an active differential
> >>>> prob
> >> e optimized for power rail measurements a thing? Perhaps it does exist?
> >> - I f so please let me know!
> >>> For a quick check, I just use two channels, with x10 passive probes,
> >>> and difference 'em. It's not terrific for accuracy, but power isn't
> >>> required to be accurate.
> >>
> >>> Biggest drawback: you need both hands to hold the probes.
> >>
> >> Yep that works for a quick check, I've done the same for jank current
> >> measurements through a resistor. The issue with x10 probes for power
> >> rail measurement is the measurement noise. E.g. Xilinx transceivers
> >> guidelines allow for 10mVpp from 10kHz to 80MHz. Using active
> >> differential probes on rails with even more stringent requirements also
> >> runs into the same measurement noise issue.
> >>
> >> Since input cap / loading of a probe is pretty much irrelevant for power
> >> rail measurements, it seems to me that an active diff probe that is
> >> designed intentionally for power rail measurements could obtain much
> >> better noise performance.
> >>
> >> E.g. the RT-ZD40 active diff probe has 3mVrms input referred noise,
> >> while 50 ohm oscilloscope frontends easily achieve sub 1mVrms.
> >
> > For interest, Marco Reps shows how to get 0.001pf isolation on line power:
> >
> > eXtReMe iSoLaTiOn ( Low Leakage Power in Precision Electronics )
> > https://www.youtube.com/watch?v=9JinSfCKuNQ
> >
> > For even more isolation, a number of battery powered oscilloscopes are
> > available for under $200. For example, the FNIRSI-1013D is a 100 MHz dual
> > trace touch tablet:
> >
> > http://www.fnirsi.cn/productinfo/556152.html
> >
> > It is available at
> >
> > https://www.aliexpress.com/i/4000861098295.html
> >
> > Dave Jones, EEVblog doesn't think much of it, but whadda gonna do for CAD
> > $168.74 ?
> >
> > https://www.youtube.com/watch?v=5iwtDwJlbWk
> >
> > The lowest range is 50mV, but you could easily add a differential opamp for
> > low level work, such as the opa846:
> >
> > https://www.ti.com/lit/ds/symlink/opa846.pdf
> >
> > It is under $10 at Octopart:
> >
> > https://octopart.com/search?q=opa846&currency=USD&specs=0
> >
> >
> >
> >
> Another approach is to put a Mini-Circuits transformer across the
> current sense resistor, and use a normal scope on the secondary. Needs
> a ~1 uF coupling cap, of course, but that'll get you down to the low
> kilohertz. Coupling is very high--their gizmos typically measure over
> 0.999, often about 0.9997.
>
> Try the T1-6.
>
> <https://www.minicircuits.com/pdfs/T1-6-X65+.pdf>
>
> Cheers
>
> Phil Hobbs
>
> --
> Dr Philip C D Hobbs
> Principal Consultant
> ElectroOptical Innovations LLC / Hobbs ElectroOptics
> Optics, Electro-optics, Photonics, Analog Electronics
> Briarcliff Manor NY 10510
>
> http://electrooptical.net
> http://hobbs-eo.com

I'm curious if you've ever tried using a transformer in front of a TIA to boost the photocurrent from a PD for effectively lower TIA input noise? (understandably the impedance looking into the transformer + TIA would be greater than the TIA alone).

Re: Active differential power rail probe

<e5ade819-ca91-86f3-3dbe-c4e722e5ab32@electrooptical.net>

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From: pcdhSpam...@electrooptical.net (Phil Hobbs)
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Subject: Re: Active differential power rail probe
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 by: Phil Hobbs - Fri, 6 May 2022 01:14 UTC

Dominic Chan wrote:
> On Thursday, May 5, 2022 at 2:42:53 AM UTC+10, Phil Hobbs wrote:
>> Mike Monett wrote:
>>> Dominic Chan <dom...@arista.com> wrote:
>>>
>>>> On Wednesday, May 4, 2022 at 2:49:41 PM UTC+10, whit3rd wrote:
>>>>> On Tuesday, May 3, 2022 at 8:48:08 PM UTC-7, Dominic Chan
>>>>> wrote:
>>>>>> I've been doing some power rail noise/ripple measurements
>>>>>> lately. ... This has got me wondering though, why isn't an
>>>>>> active differential prob
>>>> e optimized for power rail measurements a thing? Perhaps it
>>>> does exist? - I f so please let me know!
>>>>> For a quick check, I just use two channels, with x10 passive
>>>>> probes, and difference 'em. It's not terrific for accuracy,
>>>>> but power isn't required to be accurate.
>>>>
>>>>> Biggest drawback: you need both hands to hold the probes.
>>>>
>>>> Yep that works for a quick check, I've done the same for jank
>>>> current measurements through a resistor. The issue with x10
>>>> probes for power rail measurement is the measurement noise.
>>>> E.g. Xilinx transceivers guidelines allow for 10mVpp from 10kHz
>>>> to 80MHz. Using active differential probes on rails with even
>>>> more stringent requirements also runs into the same measurement
>>>> noise issue.
>>>>
>>>> Since input cap / loading of a probe is pretty much irrelevant
>>>> for power rail measurements, it seems to me that an active diff
>>>> probe that is designed intentionally for power rail
>>>> measurements could obtain much better noise performance.
>>>>
>>>> E.g. the RT-ZD40 active diff probe has 3mVrms input referred
>>>> noise, while 50 ohm oscilloscope frontends easily achieve sub
>>>> 1mVrms.
>>>
>>> For interest, Marco Reps shows how to get 0.001pf isolation on
>>> line power:
>>>
>>> eXtReMe iSoLaTiOn ( Low Leakage Power in Precision Electronics )
>>> https://www.youtube.com/watch?v=9JinSfCKuNQ
>>>
>>> For even more isolation, a number of battery powered
>>> oscilloscopes are available for under $200. For example, the
>>> FNIRSI-1013D is a 100 MHz dual trace touch tablet:
>>>
>>> http://www.fnirsi.cn/productinfo/556152.html
>>>
>>> It is available at
>>>
>>> https://www.aliexpress.com/i/4000861098295.html
>>>
>>> Dave Jones, EEVblog doesn't think much of it, but whadda gonna do
>>> for CAD $168.74 ?
>>>
>>> https://www.youtube.com/watch?v=5iwtDwJlbWk
>>>
>>> The lowest range is 50mV, but you could easily add a differential
>>> opamp for low level work, such as the opa846:
>>>
>>> https://www.ti.com/lit/ds/symlink/opa846.pdf
>>>
>>> It is under $10 at Octopart:
>>>
>>> https://octopart.com/search?q=opa846&currency=USD&specs=0
>>>
>>>
>>>
>>>
>> Another approach is to put a Mini-Circuits transformer across the
>> current sense resistor, and use a normal scope on the secondary.
>> Needs a ~1 uF coupling cap, of course, but that'll get you down to
>> the low kilohertz. Coupling is very high--their gizmos typically
>> measure over 0.999, often about 0.9997.
>>
>> Try the T1-6.
>>
>> <https://www.minicircuits.com/pdfs/T1-6-X65+.pdf>
>>

>
> I'm curious if you've ever tried using a transformer in front of a
> TIA to boost the photocurrent from a PD for effectively lower TIA
> input noise? (understandably the impedance looking into the
> transformer + TIA would be greater than the TIA alone).

I've occasionally used reactive matching networks for narrowband UHFish
things. At ordinary frequencies transformers don't help, because in dim
light the limiting noise source is the internal series resistance of the
photodiode.

Getting to that point requires a decent TIA design, of course--barefoot
op amps generally won't cut it.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

Re: Active differential power rail probe

<t51tpb$6ie$1@dont-email.me>

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From: always.l...@message.header (John Doe)
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Subject: Re: Active differential power rail probe
Date: Fri, 6 May 2022 01:31:23 -0000 (UTC)
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 by: John Doe - Fri, 6 May 2022 01:31 UTC

TIA = Transimpedance Amplifier

Re: Active differential power rail probe

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 by: jlar...@highlandsniptechnology.com - Fri, 6 May 2022 14:26 UTC

On Thu, 5 May 2022 15:13:39 -0700 (PDT), Dominic Chan
<dominic@arista.com> wrote:

>On Wednesday, May 4, 2022 at 11:44:50 PM UTC+10, jla...@highlandsniptechnology.com wrote:
>> On Tue, 3 May 2022 20:48:04 -0700 (PDT), Dominic Chan
>> <dom...@arista.com> wrote:
>>
>> >I've been doing some power rail noise/ripple measurements lately. It seems like the generally agreed best practice is to solder a coax lead as close to the load as possible, taking care not to probe directly across backside decoupling caps to avoid underestimating HF noise. This coax lead is then connected to a power rail probe / 50R scope input.
>> >
>> >Since the boards I'm working with have 0V tied to Earth, there is a ground loop formed when making these measurements. I've found that if I power the scope and board off the same power strip the environmental noise coupling into this loop is sufficiently small for good measurements.
>> >
>> >This has got me wondering though, why isn't an active differential probe optimized for power rail measurements a thing? Perhaps it does exist? - If so please let me know!
>> >
>> >Cheers, Dominic
>> Tek has a number of fully isolated scopes and outboard isolated
>> probes. We have a 4-channel isolated scope, TPS2024 I recall, that's
>> wonderful for working on power things. Clip the probe ground things
>> anywhere.
>>
>> https://www.tek.com/en/search?keywords=isolated
>>
>>
>>
>> --
>>
>> Anybody can count to one.
>>
>> - Robert Widlar
>Neat! Their isolated probes with 50 ohm probe input impedance is along the lines of what I was thinking of. Though I don't think true isolation is necessary, just some CMRR. Why isn't this a more common thing?

Some of the cheap ebay/amazon battery-powered scopes are actually not
bad. That does single-channel-isolated measurement.

https://www.amazon.com/gp/product/B081Q2KDJT/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

--

Anybody can count to one.

- Robert Widlar

Re: Active differential power rail probe

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From: jlar...@highland_atwork_technology.com (John Larkin)
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Date: Fri, 06 May 2022 11:50:14 -0700
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 by: John Larkin - Fri, 6 May 2022 18:50 UTC

On Thu, 5 May 2022 21:14:45 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>Dominic Chan wrote:
>> On Thursday, May 5, 2022 at 2:42:53 AM UTC+10, Phil Hobbs wrote:
>>> Mike Monett wrote:
>>>> Dominic Chan <dom...@arista.com> wrote:
>>>>
>>>>> On Wednesday, May 4, 2022 at 2:49:41 PM UTC+10, whit3rd wrote:
>>>>>> On Tuesday, May 3, 2022 at 8:48:08 PM UTC-7, Dominic Chan
>>>>>> wrote:
>>>>>>> I've been doing some power rail noise/ripple measurements
>>>>>>> lately. ... This has got me wondering though, why isn't an
>>>>>>> active differential prob
>>>>> e optimized for power rail measurements a thing? Perhaps it
>>>>> does exist? - I f so please let me know!
>>>>>> For a quick check, I just use two channels, with x10 passive
>>>>>> probes, and difference 'em. It's not terrific for accuracy,
>>>>>> but power isn't required to be accurate.
>>>>>
>>>>>> Biggest drawback: you need both hands to hold the probes.
>>>>>
>>>>> Yep that works for a quick check, I've done the same for jank
>>>>> current measurements through a resistor. The issue with x10
>>>>> probes for power rail measurement is the measurement noise.
>>>>> E.g. Xilinx transceivers guidelines allow for 10mVpp from 10kHz
>>>>> to 80MHz. Using active differential probes on rails with even
>>>>> more stringent requirements also runs into the same measurement
>>>>> noise issue.
>>>>>
>>>>> Since input cap / loading of a probe is pretty much irrelevant
>>>>> for power rail measurements, it seems to me that an active diff
>>>>> probe that is designed intentionally for power rail
>>>>> measurements could obtain much better noise performance.
>>>>>
>>>>> E.g. the RT-ZD40 active diff probe has 3mVrms input referred
>>>>> noise, while 50 ohm oscilloscope frontends easily achieve sub
>>>>> 1mVrms.
>>>>
>>>> For interest, Marco Reps shows how to get 0.001pf isolation on
>>>> line power:
>>>>
>>>> eXtReMe iSoLaTiOn ( Low Leakage Power in Precision Electronics )
>>>> https://www.youtube.com/watch?v=9JinSfCKuNQ
>>>>
>>>> For even more isolation, a number of battery powered
>>>> oscilloscopes are available for under $200. For example, the
>>>> FNIRSI-1013D is a 100 MHz dual trace touch tablet:
>>>>
>>>> http://www.fnirsi.cn/productinfo/556152.html
>>>>
>>>> It is available at
>>>>
>>>> https://www.aliexpress.com/i/4000861098295.html
>>>>
>>>> Dave Jones, EEVblog doesn't think much of it, but whadda gonna do
>>>> for CAD $168.74 ?
>>>>
>>>> https://www.youtube.com/watch?v=5iwtDwJlbWk
>>>>
>>>> The lowest range is 50mV, but you could easily add a differential
>>>> opamp for low level work, such as the opa846:
>>>>
>>>> https://www.ti.com/lit/ds/symlink/opa846.pdf
>>>>
>>>> It is under $10 at Octopart:
>>>>
>>>> https://octopart.com/search?q=opa846&currency=USD&specs=0
>>>>
>>>>
>>>>
>>>>
>>> Another approach is to put a Mini-Circuits transformer across the
>>> current sense resistor, and use a normal scope on the secondary.
>>> Needs a ~1 uF coupling cap, of course, but that'll get you down to
>>> the low kilohertz. Coupling is very high--their gizmos typically
>>> measure over 0.999, often about 0.9997.
>>>
>>> Try the T1-6.
>>>
>>> <https://www.minicircuits.com/pdfs/T1-6-X65+.pdf>
>>>
>
>>
>> I'm curious if you've ever tried using a transformer in front of a
>> TIA to boost the photocurrent from a PD for effectively lower TIA
>> input noise? (understandably the impedance looking into the
>> transformer + TIA would be greater than the TIA alone).
>
>I've occasionally used reactive matching networks for narrowband UHFish
>things. At ordinary frequencies transformers don't help, because in dim
>light the limiting noise source is the internal series resistance of the
>photodiode.
>
>Getting to that point requires a decent TIA design, of course--barefoot
>op amps generally won't cut it.
>
>Cheers
>
>Phil Hobbs

Somebody, SRS I think, makes a lab amplifier with a transformer in the
front end. The noise level is way sub 1 nv/rthz. A good (well
shielded!) transformer is noiseless voltage gain.

I recall that ribbon microphones like to have a transformer. They are
very low voltage, very low impedance sources.

Given a low impedance source, a DC-coupled transformer-enhanced amp
wouldn't be difficult.

--

If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties.
Francis Bacon

Re: Active differential power rail probe

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Subject: Re: Active differential power rail probe
From: whit...@gmail.com (whit3rd)
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 by: whit3rd - Fri, 6 May 2022 20:03 UTC

On Friday, May 6, 2022 at 11:50:27 AM UTC-7, John Larkin wrote:

> Given a low impedance source, a DC-coupled transformer-enhanced amp
> wouldn't be difficult.

Er... if it's a power supply being probed, how do you transformer-enhance
without putting a short on the power supply?

For low impedance at high ground-relative voltages, the current-sense
instrument amps like INA283 more-or-less fill that niche. Input current
is in the dozens-of-microamps range.

Re: Active differential power rail probe

<e1d8c152-1d7c-9749-37c4-c33399c95274@electrooptical.net>

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From: pcdhSpam...@electrooptical.net (Phil Hobbs)
Newsgroups: sci.electronics.design
Subject: Re: Active differential power rail probe
Date: Fri, 6 May 2022 16:09:30 -0400
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 by: Phil Hobbs - Fri, 6 May 2022 20:09 UTC

John Larkin wrote:
> On Thu, 5 May 2022 21:14:45 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>> Dominic Chan wrote:
>>> On Thursday, May 5, 2022 at 2:42:53 AM UTC+10, Phil Hobbs wrote:
>>>> Mike Monett wrote:
>>>>> Dominic Chan <dom...@arista.com> wrote:
>>>>>
>>>>>> On Wednesday, May 4, 2022 at 2:49:41 PM UTC+10, whit3rd wrote:
>>>>>>> On Tuesday, May 3, 2022 at 8:48:08 PM UTC-7, Dominic Chan
>>>>>>> wrote:
>>>>>>>> I've been doing some power rail noise/ripple measurements
>>>>>>>> lately. ... This has got me wondering though, why isn't an
>>>>>>>> active differential prob
>>>>>> e optimized for power rail measurements a thing? Perhaps it
>>>>>> does exist? - I f so please let me know!
>>>>>>> For a quick check, I just use two channels, with x10 passive
>>>>>>> probes, and difference 'em. It's not terrific for accuracy,
>>>>>>> but power isn't required to be accurate.
>>>>>>
>>>>>>> Biggest drawback: you need both hands to hold the probes.
>>>>>>
>>>>>> Yep that works for a quick check, I've done the same for jank
>>>>>> current measurements through a resistor. The issue with x10
>>>>>> probes for power rail measurement is the measurement noise.
>>>>>> E.g. Xilinx transceivers guidelines allow for 10mVpp from 10kHz
>>>>>> to 80MHz. Using active differential probes on rails with even
>>>>>> more stringent requirements also runs into the same measurement
>>>>>> noise issue.
>>>>>>
>>>>>> Since input cap / loading of a probe is pretty much irrelevant
>>>>>> for power rail measurements, it seems to me that an active diff
>>>>>> probe that is designed intentionally for power rail
>>>>>> measurements could obtain much better noise performance.
>>>>>>
>>>>>> E.g. the RT-ZD40 active diff probe has 3mVrms input referred
>>>>>> noise, while 50 ohm oscilloscope frontends easily achieve sub
>>>>>> 1mVrms.
>>>>>
>>>>> For interest, Marco Reps shows how to get 0.001pf isolation on
>>>>> line power:
>>>>>
>>>>> eXtReMe iSoLaTiOn ( Low Leakage Power in Precision Electronics )
>>>>> https://www.youtube.com/watch?v=9JinSfCKuNQ
>>>>>
>>>>> For even more isolation, a number of battery powered
>>>>> oscilloscopes are available for under $200. For example, the
>>>>> FNIRSI-1013D is a 100 MHz dual trace touch tablet:
>>>>>
>>>>> http://www.fnirsi.cn/productinfo/556152.html
>>>>>
>>>>> It is available at
>>>>>
>>>>> https://www.aliexpress.com/i/4000861098295.html
>>>>>
>>>>> Dave Jones, EEVblog doesn't think much of it, but whadda gonna do
>>>>> for CAD $168.74 ?
>>>>>
>>>>> https://www.youtube.com/watch?v=5iwtDwJlbWk
>>>>>
>>>>> The lowest range is 50mV, but you could easily add a differential
>>>>> opamp for low level work, such as the opa846:
>>>>>
>>>>> https://www.ti.com/lit/ds/symlink/opa846.pdf
>>>>>
>>>>> It is under $10 at Octopart:
>>>>>
>>>>> https://octopart.com/search?q=opa846&currency=USD&specs=0
>>>>>
>>>>>
>>>>>
>>>>>
>>>> Another approach is to put a Mini-Circuits transformer across the
>>>> current sense resistor, and use a normal scope on the secondary.
>>>> Needs a ~1 uF coupling cap, of course, but that'll get you down to
>>>> the low kilohertz. Coupling is very high--their gizmos typically
>>>> measure over 0.999, often about 0.9997.
>>>>
>>>> Try the T1-6.
>>>>
>>>> <https://www.minicircuits.com/pdfs/T1-6-X65+.pdf>
>>>>
>>
>>>
>>> I'm curious if you've ever tried using a transformer in front of a
>>> TIA to boost the photocurrent from a PD for effectively lower TIA
>>> input noise? (understandably the impedance looking into the
>>> transformer + TIA would be greater than the TIA alone).
>>
>> I've occasionally used reactive matching networks for narrowband UHFish
>> things. At ordinary frequencies transformers don't help, because in dim
>> light the limiting noise source is the internal series resistance of the
>> photodiode.
>>
>> Getting to that point requires a decent TIA design, of course--barefoot
>> op amps generally won't cut it.

>
> Somebody, SRS I think, makes a lab amplifier with a transformer in the
> front end. The noise level is way sub 1 nv/rthz. A good (well
> shielded!) transformer is noiseless voltage gain.
>
> I recall that ribbon microphones like to have a transformer. They are
> very low voltage, very low impedance sources.
>
> Given a low impedance source, a DC-coupled transformer-enhanced amp
> wouldn't be difficult.
>

Sure, they're used a fair amount for low-Z resistive stuff. I have a
bunch of old mu-metal shielded transformers for ribbon mic preamps that
I've never actually used for anything. ;)

Win and Paul made a pretty cool low-noise front end for a ribbon mic in
AOE3.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

Re: Active differential power rail probe

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From: pcdhSpam...@electrooptical.net (Phil Hobbs)
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 by: Phil Hobbs - Fri, 6 May 2022 20:45 UTC

whit3rd wrote:
> On Friday, May 6, 2022 at 11:50:27 AM UTC-7, John Larkin wrote:
>
>> Given a low impedance source, a DC-coupled transformer-enhanced amp
>> wouldn't be difficult.
>
> Er... if it's a power supply being probed, how do you transformer-enhance
> without putting a short on the power supply?

With a "feedbeside" network, i.e. you DC couple the DC and AC couple the AC.

>
> For low impedance at high ground-relative voltages, the current-sense
> instrument amps like INA283 more-or-less fill that niche. Input current
> is in the dozens-of-microamps range.
>

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

Re: Active differential power rail probe

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From: jlar...@highland_atwork_technology.com (John Larkin)
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Subject: Re: Active differential power rail probe
Date: Fri, 06 May 2022 14:48:34 -0700
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 by: John Larkin - Fri, 6 May 2022 21:48 UTC

On Fri, 6 May 2022 13:03:22 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

>On Friday, May 6, 2022 at 11:50:27 AM UTC-7, John Larkin wrote:
>
>> Given a low impedance source, a DC-coupled transformer-enhanced amp
>> wouldn't be difficult.
>
>Er... if it's a power supply being probed, how do you transformer-enhance
>without putting a short on the power supply?

Er... use a capacitor.

--

If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties.
Francis Bacon

Re: Active differential power rail probe

<t56vqj$c32$2@dont-email.me>

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 by: John Doe - Sat, 7 May 2022 23:36 UTC

AOE3 = The Art of Electronics 3rd Edition

1
server_pubkey.txt

rocksolid light 0.9.7
clearnet tor