I have (or will have) a 1000 bar pressure sensor, it's a full bridge
with the resistance given as 3500 ohms +/- 20%. I've no way of
pressurising it accurately.

But it's supplied with calibration data comprising sensitivity and a
list of offsets against temperature, and I can measure the temperature.
So far, so correctable.

However, the data supplied is in the form of mV offset against
temperature along with a single mV/bar figure *when the bridge is driven
from a constant 1mA*. I don't want to drive it that way, I have a
voltage reference and can use the differential ADC ratiometrically. The
sensor would end up being excited by 2.500V which would be about 700uA
which is within the allowable range.

I feel there must be some way of using the supplied calibration offsets,
but I'm not sure exactly how. (Also, doesn't everybody do bridges
ratiometrically?)

[Constant current is tricky with only a 3.3V supply, though the data
sheet says I could go as low as 0.5mA, but I don't really want the extra
complexity.]

Ideas as to how I'd do the math(s)?

--
Cheers
Clive

On Friday, August 6, 2021 at 1:56:14 AM UTC+10, Clive Arthur wrote:
> I have (or will have) a 1000 bar pressure sensor, it's a full bridge
> with the resistance given as 3500 ohms +/- 20%. I've no way of
> pressurising it accurately.
>
> But it's supplied with calibration data comprising sensitivity and a
> list of offsets against temperature, and I can measure the temperature.
> So far, so correctable.
>
> However, the data supplied is in the form of mV offset against
> temperature along with a single mV/bar figure *when the bridge is driven
> from a constant 1mA*. I don't want to drive it that way, I have a
> voltage reference and can use the differential ADC ratiometrically. The
> sensor would end up being excited by 2.500V which would be about 700uA
> which is within the allowable range.
>
> I feel there must be some way of using the supplied calibration offsets,
> but I'm not sure exactly how. (Also, doesn't everybody do bridges
> ratiometrically?)
>
> [Constant current is tricky with only a 3.3V supply, though the data
> sheet says I could go as low as 0.5mA, but I don't really want the extra
> complexity.]
>
> Ideas as to how I'd do the math(s)?

Work out - measure - what the current actually is and scale the correction by "actual current" divided by 1mA. It means multiply by roughly 0.7 in your application, which doesn't sound too bad.

People do bridges ratiometrically, but the calibration shop will do whatever is easiest for them, which usually means sticking with what they've always done..

--
Bill Sloman, Sydney

On 05/08/2021 17:33, Anthony William Sloman wrote:
> On Friday, August 6, 2021 at 1:56:14 AM UTC+10, Clive Arthur wrote:

<snip>

>> Ideas as to how I'd do the math(s)?
>
> Work out - measure - what the current actually is and scale the correction by "actual current" divided by 1mA. It means multiply by roughly 0.7 in your application, which doesn't sound too bad.

Measuring the current at 2.5V (or measuring the bridge resistance) is
something I'd like to avoid if possible. It seems to me I *should* have
all the information I need, but I can't quite work it out.

> People do bridges ratiometrically, but the calibration shop will do whatever is easiest for them, which usually means sticking with what they've always done..
>

Yes, that seems likely.

--
Cheers
Clive

Clive Arthur wrote:
> I have (or will have) a 1000 bar pressure sensor, it's a full bridge
> with the resistance given as 3500 ohms +/- 20%.  I've no way of
> pressurising it accurately.
>
> But it's supplied with calibration data comprising sensitivity and a
> list of offsets against temperature, and I can measure the temperature.
> So far, so correctable.
>
> However, the data supplied is in the form of mV offset against
> temperature along with a single mV/bar figure *when the bridge is driven
> from a constant 1mA*.  I don't want to drive it that way, I have a
> voltage reference and can use the differential ADC ratiometrically.  The
> sensor would end up being excited by 2.500V which would be about 700uA
> which is within the allowable range.
>
> I feel there must be some way of using the supplied calibration offsets,
> but I'm not sure exactly how.  (Also, doesn't everybody do bridges
> ratiometrically?)
>
> [Constant current is tricky with only a 3.3V supply, though the data
> sheet says I could go as low as 0.5mA, but I don't really want the extra
> complexity.]
>
> Ideas as to how I'd do the math(s)?
>

It's a resistance sensor, so just turn the offsets into resistance
changes using V=IR, then plug that into your bridge calculation.

Cheers

Phil Hobbs
(Who is this very minute doing something similar with a PbSe
photoconductor, which is the same idea but more complicated.)

--
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

On 05/08/2021 18:06, Phil Hobbs wrote:
> Clive Arthur wrote:
>> I have (or will have) a 1000 bar pressure sensor, it's a full bridge
>> with the resistance given as 3500 ohms +/- 20%.  I've no way of
>> pressurising it accurately.
>>
>> But it's supplied with calibration data comprising sensitivity and a
>> list of offsets against temperature, and I can measure the
>> temperature. So far, so correctable.
>>
>> However, the data supplied is in the form of mV offset against
>> temperature along with a single mV/bar figure *when the bridge is
>> driven from a constant 1mA*.  I don't want to drive it that way, I
>> have a voltage reference and can use the differential ADC
>> ratiometrically.  The sensor would end up being excited by 2.500V
>> which would be about 700uA which is within the allowable range.
>>
>> I feel there must be some way of using the supplied calibration
>> offsets, but I'm not sure exactly how.  (Also, doesn't everybody do
>> bridges ratiometrically?)
>>
>> [Constant current is tricky with only a 3.3V supply, though the data
>> sheet says I could go as low as 0.5mA, but I don't really want the
>> extra complexity.]
>>
>> Ideas as to how I'd do the math(s)?
>>
>
> It's a resistance sensor, so just turn the offsets into resistance
> changes using V=IR, then plug that into your bridge calculation.

Yes, of course! Thanks.

--
Cheers
Clive

On 05/08/2021 18:06, Phil Hobbs wrote:
> Clive Arthur wrote:
>> I have (or will have) a 1000 bar pressure sensor, it's a full bridge
>> with the resistance given as 3500 ohms +/- 20%.  I've no way of
>> pressurising it accurately.
>>
>> But it's supplied with calibration data comprising sensitivity and a
>> list of offsets against temperature, and I can measure the
>> temperature. So far, so correctable.
>>
>> However, the data supplied is in the form of mV offset against
>> temperature along with a single mV/bar figure *when the bridge is
>> driven from a constant 1mA*.  I don't want to drive it that way, I
>> have a voltage reference and can use the differential ADC
>> ratiometrically.  The sensor would end up being excited by 2.500V
>> which would be about 700uA which is within the allowable range.
>>
>> I feel there must be some way of using the supplied calibration
>> offsets, but I'm not sure exactly how.  (Also, doesn't everybody do
>> bridges ratiometrically?)
>>
>> [Constant current is tricky with only a 3.3V supply, though the data
>> sheet says I could go as low as 0.5mA, but I don't really want the
>> extra complexity.]
>>
>> Ideas as to how I'd do the math(s)?
>>
>
> It's a resistance sensor, so just turn the offsets into resistance
> changes using V=IR, then plug that into your bridge calculation.

Unfortunately, I can't make it work without knowing the bridge resistance.

Let's say a full bridge with two pairs of resistors

____|____
| |
R1 R2
|--(V)--|
R2 R1
|_______|
_|_

For Vex excitation, V = Vex * (R2-R1)/(R1+R2)

At 1mA constant current excitation Vex = 1E-3 (R1+R2)/2

So V = 1E-3 (R2-R1)/2

I know (or I'm told) the V at 1mA excitation under some defined
conditions, and I want to know the V at, say, Vex = 2.5V. But I can't.
Bugger.

--
Cheers
Clive

On Fri, 6 Aug 2021 17:02:51 +0100, Clive Arthur
<clive@nowaytoday.co.uk> wrote:

>On 05/08/2021 18:06, Phil Hobbs wrote:
>> Clive Arthur wrote:
>>> I have (or will have) a 1000 bar pressure sensor, it's a full bridge
>>> with the resistance given as 3500 ohms +/- 20%.  I've no way of
>>> pressurising it accurately.
>>>
>>> But it's supplied with calibration data comprising sensitivity and a
>>> list of offsets against temperature, and I can measure the
>>> temperature. So far, so correctable.
>>>
>>> However, the data supplied is in the form of mV offset against
>>> temperature along with a single mV/bar figure *when the bridge is
>>> driven from a constant 1mA*.  I don't want to drive it that way, I
>>> have a voltage reference and can use the differential ADC
>>> ratiometrically.  The sensor would end up being excited by 2.500V
>>> which would be about 700uA which is within the allowable range.
>>>
>>> I feel there must be some way of using the supplied calibration
>>> offsets, but I'm not sure exactly how.  (Also, doesn't everybody do
>>> bridges ratiometrically?)
>>>
>>> [Constant current is tricky with only a 3.3V supply, though the data
>>> sheet says I could go as low as 0.5mA, but I don't really want the
>>> extra complexity.]
>>>
>>> Ideas as to how I'd do the math(s)?
>>>
>>
>> It's a resistance sensor, so just turn the offsets into resistance
>> changes using V=IR, then plug that into your bridge calculation.
>
>Unfortunately, I can't make it work without knowing the bridge resistance.
>
>Let's say a full bridge with two pairs of resistors
>
>
>____|____
>| |
>R1 R2
>|--(V)--|
>R2 R1
>|_______|
> _|_
>
>For Vex excitation, V = Vex * (R2-R1)/(R1+R2)
>
>At 1mA constant current excitation Vex = 1E-3 (R1+R2)/2
>
>So V = 1E-3 (R2-R1)/2
>
>I know (or I'm told) the V at 1mA excitation under some defined
>conditions, and I want to know the V at, say, Vex = 2.5V. But I can't.
> Bugger.

There could be a modest accuracy penalty for using a constant-voltage
excitation, as opposed to cc. It depends on the tempco of the
resistors. If they are silicon, the difference could be significant.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

On a sunny day (Fri, 6 Aug 2021 17:02:51 +0100) it happened Clive Arthur
<clive@nowaytoday.co.uk> wrote in <sejmfd$oiq$1@dont-email.me>:

>On 05/08/2021 18:06, Phil Hobbs wrote:
>> Clive Arthur wrote:
>>> I have (or will have) a 1000 bar pressure sensor, it's a full bridge
>>> with the resistance given as 3500 ohms +/- 20%.  I've no way of
>>> pressurising it accurately.
>>>
>>> But it's supplied with calibration data comprising sensitivity and a
>>> list of offsets against temperature, and I can measure the
>>> temperature. So far, so correctable.
>>>
>>> However, the data supplied is in the form of mV offset against
>>> temperature along with a single mV/bar figure *when the bridge is
>>> driven from a constant 1mA*.  I don't want to drive it that way, I
>>> have a voltage reference and can use the differential ADC
>>> ratiometrically.  The sensor would end up being excited by 2.500V
>>> which would be about 700uA which is within the allowable range.
>>>
>>> I feel there must be some way of using the supplied calibration
>>> offsets, but I'm not sure exactly how.  (Also, doesn't everybody do
>>> bridges ratiometrically?)
>>>
>>> [Constant current is tricky with only a 3.3V supply, though the data
>>> sheet says I could go as low as 0.5mA, but I don't really want the
>>> extra complexity.]
>>>
>>> Ideas as to how I'd do the math(s)?
>>>
>>
>> It's a resistance sensor, so just turn the offsets into resistance
>> changes using V=IR, then plug that into your bridge calculation.
>
>Unfortunately, I can't make it work without knowing the bridge resistance.
>
>Let's say a full bridge with two pairs of resistors
>
>
>____|____
>| |
>R1 R2
>|--(V)--|
>R2 R1
>|_______|
> _|_
>
>For Vex excitation, V = Vex * (R2-R1)/(R1+R2)
>
>At 1mA constant current excitation Vex = 1E-3 (R1+R2)/2
>
>So V = 1E-3 (R2-R1)/2
>
>I know (or I'm told) the V at 1mA excitation under some defined
>conditions, and I want to know the V at, say, Vex = 2.5V. But I can't.
> Bugger.

Cannot be that hard to make a curent mirror at 3.3V,
2 transistors and a resistor?

On Fri, 06 Aug 2021 17:06:23 GMT, Jan Panteltje
<pNaOnStPeAlMtje@yahoo.com> wrote:

>On a sunny day (Fri, 6 Aug 2021 17:02:51 +0100) it happened Clive Arthur
><clive@nowaytoday.co.uk> wrote in <sejmfd$oiq$1@dont-email.me>:
>
>>On 05/08/2021 18:06, Phil Hobbs wrote:
>>> Clive Arthur wrote:
>>>> I have (or will have) a 1000 bar pressure sensor, it's a full bridge
>>>> with the resistance given as 3500 ohms +/- 20%.  I've no way of
>>>> pressurising it accurately.
>>>>
>>>> But it's supplied with calibration data comprising sensitivity and a
>>>> list of offsets against temperature, and I can measure the
>>>> temperature. So far, so correctable.
>>>>
>>>> However, the data supplied is in the form of mV offset against
>>>> temperature along with a single mV/bar figure *when the bridge is
>>>> driven from a constant 1mA*.  I don't want to drive it that way, I
>>>> have a voltage reference and can use the differential ADC
>>>> ratiometrically.  The sensor would end up being excited by 2.500V
>>>> which would be about 700uA which is within the allowable range.
>>>>
>>>> I feel there must be some way of using the supplied calibration
>>>> offsets, but I'm not sure exactly how.  (Also, doesn't everybody do
>>>> bridges ratiometrically?)
>>>>
>>>> [Constant current is tricky with only a 3.3V supply, though the data
>>>> sheet says I could go as low as 0.5mA, but I don't really want the
>>>> extra complexity.]
>>>>
>>>> Ideas as to how I'd do the math(s)?
>>>>
>>>
>>> It's a resistance sensor, so just turn the offsets into resistance
>>> changes using V=IR, then plug that into your bridge calculation.
>>
>>Unfortunately, I can't make it work without knowing the bridge resistance.
>>
>>Let's say a full bridge with two pairs of resistors
>>
>>
>>____|____
>>| |
>>R1 R2
>>|--(V)--|
>>R2 R1
>>|_______|
>> _|_
>>
>>For Vex excitation, V = Vex * (R2-R1)/(R1+R2)
>>
>>At 1mA constant current excitation Vex = 1E-3 (R1+R2)/2
>>
>>So V = 1E-3 (R2-R1)/2
>>
>>I know (or I'm told) the V at 1mA excitation under some defined
>>conditions, and I want to know the V at, say, Vex = 2.5V. But I can't.
>> Bugger.
>
>Cannot be that hard to make a curent mirror at 3.3V,
>2 transistors and a resistor?
>

Discrete transistor based mirrors are horrible. Integrated ones are
fair.

Clive Arthur wrote:
> On 05/08/2021 18:06, Phil Hobbs wrote:
>> Clive Arthur wrote:
>>> I have (or will have) a 1000 bar pressure sensor, it's a full bridge
>>> with the resistance given as 3500 ohms +/- 20%.  I've no way of
>>> pressurising it accurately.
>>>
>>> But it's supplied with calibration data comprising sensitivity and a
>>> list of offsets against temperature, and I can measure the
>>> temperature. So far, so correctable.
>>>
>>> However, the data supplied is in the form of mV offset against
>>> temperature along with a single mV/bar figure *when the bridge is
>>> driven from a constant 1mA*.  I don't want to drive it that way, I
>>> have a voltage reference and can use the differential ADC
>>> ratiometrically.  The sensor would end up being excited by 2.500V
>>> which would be about 700uA which is within the allowable range.
>>>
>>> I feel there must be some way of using the supplied calibration
>>> offsets, but I'm not sure exactly how.  (Also, doesn't everybody do
>>> bridges ratiometrically?)
>>>
>>> [Constant current is tricky with only a 3.3V supply, though the data
>>> sheet says I could go as low as 0.5mA, but I don't really want the
>>> extra complexity.]
>>>
>>> Ideas as to how I'd do the math(s)?
>>>
>>
>> It's a resistance sensor, so just turn the offsets into resistance
>> changes using V=IR, then plug that into your bridge calculation.
>
> Unfortunately, I can't make it work without knowing the bridge resistance.
>
> Let's say a full bridge with two pairs of resistors
>
>
> ____|____
> |       |
> R1     R2
> |--(V)--|
> R2     R1
> |_______|
>    _|_
>
> For Vex excitation, V = Vex * (R2-R1)/(R1+R2)
>
> At 1mA constant current excitation Vex = 1E-3 (R1+R2)/2
>
> So V = 1E-3 (R2-R1)/2
>
> I know (or I'm told) the V at 1mA excitation under some defined
> conditions, and I want to know the V at, say, Vex = 2.5V.  But I can't.
>  Bugger.

Post the datasheet. I suspect you're making this way too hard.

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

On 06/08/2021 19:50, Phil Hobbs wrote:
> Clive Arthur wrote:
>> On 05/08/2021 18:06, Phil Hobbs wrote:
>>> Clive Arthur wrote:
>>>> I have (or will have) a 1000 bar pressure sensor, it's a full bridge
>>>> with the resistance given as 3500 ohms +/- 20%.  I've no way of
>>>> pressurising it accurately.
>>>>
>>>> But it's supplied with calibration data comprising sensitivity and a
>>>> list of offsets against temperature, and I can measure the
>>>> temperature. So far, so correctable.
>>>>
>>>> However, the data supplied is in the form of mV offset against
>>>> temperature along with a single mV/bar figure *when the bridge is
>>>> driven from a constant 1mA*.  I don't want to drive it that way, I
>>>> have a voltage reference and can use the differential ADC
>>>> ratiometrically.  The sensor would end up being excited by 2.500V
>>>> which would be about 700uA which is within the allowable range.
>>>>
>>>> I feel there must be some way of using the supplied calibration
>>>> offsets, but I'm not sure exactly how.  (Also, doesn't everybody do
>>>> bridges ratiometrically?)
>>>>
>>>> [Constant current is tricky with only a 3.3V supply, though the data
>>>> sheet says I could go as low as 0.5mA, but I don't really want the
>>>> extra complexity.]
>>>>
>>>> Ideas as to how I'd do the math(s)?
>>>>
>>>
>>> It's a resistance sensor, so just turn the offsets into resistance
>>> changes using V=IR, then plug that into your bridge calculation.
>>
>> Unfortunately, I can't make it work without knowing the bridge
>> resistance.
>>
>> Let's say a full bridge with two pairs of resistors
>>
>>
>> ____|____
>> |       |
>> R1     R2
>> |--(V)--|
>> R2     R1
>> |_______|
>>     _|_
>>
>> For Vex excitation, V = Vex * (R2-R1)/(R1+R2)
>>
>> At 1mA constant current excitation Vex = 1E-3 (R1+R2)/2
>>
>> So V = 1E-3 (R2-R1)/2
>>
>> I know (or I'm told) the V at 1mA excitation under some defined
>> conditions, and I want to know the V at, say, Vex = 2.5V.  But I
>> can't.   Bugger.
>
> Post the datasheet.  I suspect you're making this way too hard.
>
> Cheers
>
> Phil Hobbs
>>
>
>
Keller series 7 LI is the part, or close to it. They'll give you
calibration stuff with 1mA constant current excitation. Ain't no way to
work this into a constant voltage ratiometric thing without
device-specific measurements

--
Cheers
Clive

On Fri, 6 Aug 2021 22:33:36 +0100, Clive Arthur
<clive@nowaytoday.co.uk> wrote:

>On 06/08/2021 19:50, Phil Hobbs wrote:
>> Clive Arthur wrote:
>>> On 05/08/2021 18:06, Phil Hobbs wrote:
>>>> Clive Arthur wrote:
>>>>> I have (or will have) a 1000 bar pressure sensor, it's a full bridge
>>>>> with the resistance given as 3500 ohms +/- 20%.  I've no way of
>>>>> pressurising it accurately.
>>>>>
>>>>> But it's supplied with calibration data comprising sensitivity and a
>>>>> list of offsets against temperature, and I can measure the
>>>>> temperature. So far, so correctable.
>>>>>
>>>>> However, the data supplied is in the form of mV offset against
>>>>> temperature along with a single mV/bar figure *when the bridge is
>>>>> driven from a constant 1mA*.  I don't want to drive it that way, I
>>>>> have a voltage reference and can use the differential ADC
>>>>> ratiometrically.  The sensor would end up being excited by 2.500V
>>>>> which would be about 700uA which is within the allowable range.
>>>>>
>>>>> I feel there must be some way of using the supplied calibration
>>>>> offsets, but I'm not sure exactly how.  (Also, doesn't everybody do
>>>>> bridges ratiometrically?)
>>>>>
>>>>> [Constant current is tricky with only a 3.3V supply, though the data
>>>>> sheet says I could go as low as 0.5mA, but I don't really want the
>>>>> extra complexity.]
>>>>>
>>>>> Ideas as to how I'd do the math(s)?
>>>>>
>>>>
>>>> It's a resistance sensor, so just turn the offsets into resistance
>>>> changes using V=IR, then plug that into your bridge calculation.
>>>
>>> Unfortunately, I can't make it work without knowing the bridge
>>> resistance.
>>>
>>> Let's say a full bridge with two pairs of resistors
>>>
>>>
>>> ____|____
>>> |       |
>>> R1     R2
>>> |--(V)--|
>>> R2     R1
>>> |_______|
>>>     _|_
>>>
>>> For Vex excitation, V = Vex * (R2-R1)/(R1+R2)
>>>
>>> At 1mA constant current excitation Vex = 1E-3 (R1+R2)/2
>>>
>>> So V = 1E-3 (R2-R1)/2
>>>
>>> I know (or I'm told) the V at 1mA excitation under some defined
>>> conditions, and I want to know the V at, say, Vex = 2.5V.  But I
>>> can't.   Bugger.
>>
>> Post the datasheet.  I suspect you're making this way too hard.
>>
>> Cheers
>>
>> Phil Hobbs
>>>
>>
>>
>Keller series 7 LI is the part, or close to it. They'll give you
>calibration stuff with 1mA constant current excitation. Ain't no way to
>work this into a constant voltage ratiometric thing without
>device-specific measurements

Cheat a little!

Vref - good resistor - bridge - ground.

Have the resistor make close to 1 mA.

Have the ADC measure the diff drop across the resistor, then the diff
bridge output, divide.

We measure 4-wire RTDs that way.

What's the bridge resistance?

On Saturday, August 7, 2021 at 3:21:39 AM UTC+10, John Larkin wrote:
> On Fri, 06 Aug 2021 17:06:23 GMT, Jan Panteltje <pNaOnSt...@yahoo.com> wrote:
> >On a sunny day (Fri, 6 Aug 2021 17:02:51 +0100) it happened Clive Arthur
> ><cl...@nowaytoday.co.uk> wrote in <sejmfd$oiq$1...@dont-email.me>:
> >>On 05/08/2021 18:06, Phil Hobbs wrote:
> >>> Clive Arthur wrote:

<snip>

> >Cannot be that hard to make a current mirror at 3.3V,
> >2 transistors and a resistor?
> >
> Discrete transistor based mirrors are horrible. Integrated ones are fair.

You can buy dual transistors which are both on the same substrate. Analog Devices make a few.

https://www.analog.com/en/parametricsearch/10988#/

The gains can match to 0.5%. Stick a pair of 0.1% resistors in series with each emitter, and they do a bit better. Throw in a trimpot, and they can do better still.

People who do (or once did) electronic design know about this kind of stuff. John Larkin doesn't seem to.

--
Bill Sloman, Sydney

On 07/08/2021 03:27, Anthony William Sloman wrote:
> On Saturday, August 7, 2021 at 3:21:39 AM UTC+10, John Larkin wrote:
>> On Fri, 06 Aug 2021 17:06:23 GMT, Jan Panteltje <pNaOnSt...@yahoo.com> wrote:
>>> On a sunny day (Fri, 6 Aug 2021 17:02:51 +0100) it happened Clive Arthur
>>> <cl...@nowaytoday.co.uk> wrote in <sejmfd$oiq$1...@dont-email.me>:
>>>> On 05/08/2021 18:06, Phil Hobbs wrote:
>>>>> Clive Arthur wrote:
>
> <snip>
>
>>> Cannot be that hard to make a current mirror at 3.3V,
>>> 2 transistors and a resistor?
>>>
>> Discrete transistor based mirrors are horrible. Integrated ones are fair.
>
> You can buy dual transistors which are both on the same substrate. Analog Devices make a few.
>
> https://www.analog.com/en/parametricsearch/10988#/
>
> The gains can match to 0.5%. Stick a pair of 0.1% resistors in series with each emitter, and they do a bit better. Throw in a trimpot, and they can do better still.
>
> People who do (or once did) electronic design know about this kind of stuff. John Larkin doesn't seem to.
>
0.5% is about right for the sensor, but it would have to be accurate up
to 180'C. Something with feedback would seem better on the face of it,
but it's well worth a look.

--
Cheers
Clive

On 06/08/2021 23:08, John Larkin wrote:
> On Fri, 6 Aug 2021 22:33:36 +0100, Clive Arthur
> <clive@nowaytoday.co.uk> wrote:
>
>> On 06/08/2021 19:50, Phil Hobbs wrote:
>>> Clive Arthur wrote:
>>>> On 05/08/2021 18:06, Phil Hobbs wrote:
>>>>> Clive Arthur wrote:
>>>>>> I have (or will have) a 1000 bar pressure sensor, it's a full bridge
>>>>>> with the resistance given as 3500 ohms +/- 20%.  I've no way of
>>>>>> pressurising it accurately.
>>>>>>
>>>>>> But it's supplied with calibration data comprising sensitivity and a
>>>>>> list of offsets against temperature, and I can measure the
>>>>>> temperature. So far, so correctable.
>>>>>>
>>>>>> However, the data supplied is in the form of mV offset against
>>>>>> temperature along with a single mV/bar figure *when the bridge is
>>>>>> driven from a constant 1mA*.  I don't want to drive it that way, I
>>>>>> have a voltage reference and can use the differential ADC
>>>>>> ratiometrically.  The sensor would end up being excited by 2.500V
>>>>>> which would be about 700uA which is within the allowable range.
>>>>>>
>>>>>> I feel there must be some way of using the supplied calibration
>>>>>> offsets, but I'm not sure exactly how.  (Also, doesn't everybody do
>>>>>> bridges ratiometrically?)
>>>>>>
>>>>>> [Constant current is tricky with only a 3.3V supply, though the data
>>>>>> sheet says I could go as low as 0.5mA, but I don't really want the
>>>>>> extra complexity.]
>>>>>>
>>>>>> Ideas as to how I'd do the math(s)?
>>>>>>
>>>>>
>>>>> It's a resistance sensor, so just turn the offsets into resistance
>>>>> changes using V=IR, then plug that into your bridge calculation.
>>>>
>>>> Unfortunately, I can't make it work without knowing the bridge
>>>> resistance.
>>>>
>>>> Let's say a full bridge with two pairs of resistors
>>>>
>>>>
>>>> ____|____
>>>> |       |
>>>> R1     R2
>>>> |--(V)--|
>>>> R2     R1
>>>> |_______|
>>>>     _|_
>>>>
>>>> For Vex excitation, V = Vex * (R2-R1)/(R1+R2)
>>>>
>>>> At 1mA constant current excitation Vex = 1E-3 (R1+R2)/2
>>>>
>>>> So V = 1E-3 (R2-R1)/2
>>>>
>>>> I know (or I'm told) the V at 1mA excitation under some defined
>>>> conditions, and I want to know the V at, say, Vex = 2.5V.  But I
>>>> can't.   Bugger.
>>>
>>> Post the datasheet.  I suspect you're making this way too hard.
>>>
>>> Cheers
>>>
>>> Phil Hobbs
>>>>
>>>
>>>
>> Keller series 7 LI is the part, or close to it. They'll give you
>> calibration stuff with 1mA constant current excitation. Ain't no way to
>> work this into a constant voltage ratiometric thing without
>> device-specific measurements
>
> Cheat a little!
>
> Vref - good resistor - bridge - ground.
>
> Have the resistor make close to 1 mA.
>
> Have the ADC measure the diff drop across the resistor, then the diff
> bridge output, divide.
>
> We measure 4-wire RTDs that way.
>
> What's the bridge resistance?
>

The bridge is 3500R +/-20%. The given calibration figures are for 1mA,
but they do say that you can run from between 500uA and 1.5mA and scale
accordingly.

So, I could run it at 700uA which is nominally 2.45V, then use 850R
(okay 820R) from 3.3V supply, or maybe a ref. As it happens, the ADC
I'm using has a spare differential channel and its input can go as high
as the supply. It also has programmable gain, so that looks like a
result thanks.

Not quite as simple as ratiometric, but hey.

--
Cheers
Clive

On Sat, 7 Aug 2021 10:39:13 +0100, Clive Arthur
<clive@nowaytoday.co.uk> wrote:

>On 07/08/2021 03:27, Anthony William Sloman wrote:
>> On Saturday, August 7, 2021 at 3:21:39 AM UTC+10, John Larkin wrote:
>>> On Fri, 06 Aug 2021 17:06:23 GMT, Jan Panteltje <pNaOnSt...@yahoo.com> wrote:
>>>> On a sunny day (Fri, 6 Aug 2021 17:02:51 +0100) it happened Clive Arthur
>>>> <cl...@nowaytoday.co.uk> wrote in <sejmfd$oiq$1...@dont-email.me>:
>>>>> On 05/08/2021 18:06, Phil Hobbs wrote:
>>>>>> Clive Arthur wrote:
>>
>> <snip>
>>
>>>> Cannot be that hard to make a current mirror at 3.3V,
>>>> 2 transistors and a resistor?
>>>>
>>> Discrete transistor based mirrors are horrible. Integrated ones are fair.
>>
>> You can buy dual transistors which are both on the same substrate. Analog Devices make a few.
>>
>> https://www.analog.com/en/parametricsearch/10988#/
>>
>> The gains can match to 0.5%. Stick a pair of 0.1% resistors in series with each emitter, and they do a bit better. Throw in a trimpot, and they can do better still.
>>
>> People who do (or once did) electronic design know about this kind of stuff. John Larkin doesn't seem to.
>>
>0.5% is about right for the sensor, but it would have to be accurate up
>to 180'C. Something with feedback would seem better on the face of it,
>but it's well worth a look.

Well, throw in several trimpots and temperature cycle the rig and
tweak the pots until the tempco and supply sensitivity are low enough.
That should only take a couple of days per unit. That seems to be the
professional way to do it.

That sounds better than my single resistor ratiometric suggestion.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

On Sat, 7 Aug 2021 11:25:25 +0100, Clive Arthur
<clive@nowaytoday.co.uk> wrote:

>On 06/08/2021 23:08, John Larkin wrote:
>> On Fri, 6 Aug 2021 22:33:36 +0100, Clive Arthur
>> <clive@nowaytoday.co.uk> wrote:
>>
>>> On 06/08/2021 19:50, Phil Hobbs wrote:
>>>> Clive Arthur wrote:
>>>>> On 05/08/2021 18:06, Phil Hobbs wrote:
>>>>>> Clive Arthur wrote:
>>>>>>> I have (or will have) a 1000 bar pressure sensor, it's a full bridge
>>>>>>> with the resistance given as 3500 ohms +/- 20%.  I've no way of
>>>>>>> pressurising it accurately.
>>>>>>>
>>>>>>> But it's supplied with calibration data comprising sensitivity and a
>>>>>>> list of offsets against temperature, and I can measure the
>>>>>>> temperature. So far, so correctable.
>>>>>>>
>>>>>>> However, the data supplied is in the form of mV offset against
>>>>>>> temperature along with a single mV/bar figure *when the bridge is
>>>>>>> driven from a constant 1mA*.  I don't want to drive it that way, I
>>>>>>> have a voltage reference and can use the differential ADC
>>>>>>> ratiometrically.  The sensor would end up being excited by 2.500V
>>>>>>> which would be about 700uA which is within the allowable range.
>>>>>>>
>>>>>>> I feel there must be some way of using the supplied calibration
>>>>>>> offsets, but I'm not sure exactly how.  (Also, doesn't everybody do
>>>>>>> bridges ratiometrically?)
>>>>>>>
>>>>>>> [Constant current is tricky with only a 3.3V supply, though the data
>>>>>>> sheet says I could go as low as 0.5mA, but I don't really want the
>>>>>>> extra complexity.]
>>>>>>>
>>>>>>> Ideas as to how I'd do the math(s)?
>>>>>>>
>>>>>>
>>>>>> It's a resistance sensor, so just turn the offsets into resistance
>>>>>> changes using V=IR, then plug that into your bridge calculation.
>>>>>
>>>>> Unfortunately, I can't make it work without knowing the bridge
>>>>> resistance.
>>>>>
>>>>> Let's say a full bridge with two pairs of resistors
>>>>>
>>>>>
>>>>> ____|____
>>>>> |       |
>>>>> R1     R2
>>>>> |--(V)--|
>>>>> R2     R1
>>>>> |_______|
>>>>>     _|_
>>>>>
>>>>> For Vex excitation, V = Vex * (R2-R1)/(R1+R2)
>>>>>
>>>>> At 1mA constant current excitation Vex = 1E-3 (R1+R2)/2
>>>>>
>>>>> So V = 1E-3 (R2-R1)/2
>>>>>
>>>>> I know (or I'm told) the V at 1mA excitation under some defined
>>>>> conditions, and I want to know the V at, say, Vex = 2.5V.  But I
>>>>> can't.   Bugger.
>>>>
>>>> Post the datasheet.  I suspect you're making this way too hard.
>>>>
>>>> Cheers
>>>>
>>>> Phil Hobbs
>>>>>
>>>>
>>>>
>>> Keller series 7 LI is the part, or close to it. They'll give you
>>> calibration stuff with 1mA constant current excitation. Ain't no way to
>>> work this into a constant voltage ratiometric thing without
>>> device-specific measurements
>>
>> Cheat a little!
>>
>> Vref - good resistor - bridge - ground.
>>
>> Have the resistor make close to 1 mA.
>>
>> Have the ADC measure the diff drop across the resistor, then the diff
>> bridge output, divide.
>>
>> We measure 4-wire RTDs that way.
>>
>> What's the bridge resistance?
>>
>
>The bridge is 3500R +/-20%. The given calibration figures are for 1mA,
>but they do say that you can run from between 500uA and 1.5mA and scale
>accordingly.
>
>So, I could run it at 700uA which is nominally 2.45V, then use 850R
>(okay 820R) from 3.3V supply, or maybe a ref. As it happens, the ADC
>I'm using has a spare differential channel and its input can go as high
>as the supply. It also has programmable gain, so that looks like a
>result thanks.
>
>Not quite as simple as ratiometric, but hey.

It is ratiometric, because you need to divide! A stable ref is good
because the ADC won't measure the current and the bridge output
simultaneously.

You can use the drop across the curreent sense resistor *as* the ADC
ref, but that can have issues.

The diff-input 24-bit delta-sigma ADCs are perfect for ratiometric
measurements.

TI's ADS1247 is nice. Some of the Analog Devices parts are weird and
flakey on the digital side. TI knows digital!

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

On Sunday, August 8, 2021 at 12:20:12 AM UTC+10, jla...@highlandsniptechnology.com wrote:
> On Sat, 7 Aug 2021 10:39:13 +0100, Clive Arthur
> <cl...@nowaytoday.co.uk> wrote:
>
> >On 07/08/2021 03:27, Anthony William Sloman wrote:
> >> On Saturday, August 7, 2021 at 3:21:39 AM UTC+10, John Larkin wrote:
> >>> On Fri, 06 Aug 2021 17:06:23 GMT, Jan Panteltje <pNaOnSt...@yahoo.com> wrote:
> >>>> On a sunny day (Fri, 6 Aug 2021 17:02:51 +0100) it happened Clive Arthur
> >>>> <cl...@nowaytoday.co.uk> wrote in <sejmfd$oiq$1...@dont-email.me>:
> >>>>> On 05/08/2021 18:06, Phil Hobbs wrote:
> >>>>>> Clive Arthur wrote:
> >>
> >> <snip>
> >>
> >>>> Cannot be that hard to make a current mirror at 3.3V,
> >>>> 2 transistors and a resistor?
> >>>>
> >>> Discrete transistor based mirrors are horrible. Integrated ones are fair.
> >>
> >> You can buy dual transistors which are both on the same substrate. Analog Devices make a few.
> >>
> >> https://www.analog.com/en/parametricsearch/10988#/
> >>
> >> The gains can match to 0.5%. Stick a pair of 0.1% resistors in series with each emitter, and they do a bit better. Throw in a trimpot, and they can do better still.
> >>
> >> People who do (or once did) electronic design know about this kind of stuff. John Larkin doesn't seem to.
> >>
> >0.5% is about right for the sensor, but it would have to be accurate up
> >to 180'C. Something with feedback would seem better on the face of it,
> >but it's well worth a look.
>
> Well, throw in several trimpots and temperature cycle the rig and
> tweak the pots until the tempco and supply sensitivity are low enough.
> That should only take a couple of days per unit. That seems to be the
> professional way to do it.

To John Larkin. The offsets are temperature dependent - the manufacturer has already done the temperature cycling - so the temperature cycling set-up shouldn't be necessary, but John Larkin's kind of "professional" probably wants to have something expensive he can bill the customer for.
> That sounds better than my single resistor ratiometric suggestion.

It is the obvious way to do it but Clive Arthur has a problem translating the manufacturer's off-set numbers into numbers appropriate to that set-up.

Once he has a good idea if the actual resistances in any particular bridge, he should be able to work or the actual current going through them under any partiular operating condition well enough to translate the off-set numbers to the value appropriate to that particular situation, but some people do find math intimidating.

Presumably the offsets are small, so he won't have to work out the actual current all that accurately.

--
Bill Sloman, Sydney

On Sunday, August 8, 2021 at 12:30:27 AM UTC+10, jla...@highlandsniptechnology.com wrote:
> On Sat, 7 Aug 2021 11:25:25 +0100, Clive Arthur <cl...@nowaytoday.co.uk> wrote:
> >On 06/08/2021 23:08, John Larkin wrote:
> >> On Fri, 6 Aug 2021 22:33:36 +0100, Clive Arthur
> >> <cl...@nowaytoday.co.uk> wrote:
> >>
> >>> On 06/08/2021 19:50, Phil Hobbs wrote:
> >>>> Clive Arthur wrote:
> >>>>> On 05/08/2021 18:06, Phil Hobbs wrote:
> >>>>>> Clive Arthur wrote:

<snip>

> >Not quite as simple as ratiometric, but hey.
> It is ratiometric, because you need to divide! A stable ref is good
> because the ADC won't measure the current and the bridge output
> simultaneously.

Curious. The last one used did. The output chopped down the input from the reference until it matched the voltage being digitised.
> You can use the drop across the current sense resistor *as* the ADC
> ref, but that can have issues.
>
> The diff-input 24-bit delta-sigma ADCs are perfect for ratiometric
> measurements.

They may put out 24 bits, but most them are only 20-bit accurate. The 24-bits come out as three 8-bit words. which makes marketing happy, but the last four bits aren't all that informative.
> TI's ADS1247 is nice. Some of the Analog Devices parts are weird and flakey on the digital side. TI knows digital!

Or claims to. Their analog circuits tended to suck, and some of their data sheets did lie by omission.

--
Bill Sloman, Sydney

On Thu, 5 Aug 2021 16:56:07 +0100, Clive Arthur
<clive@nowaytoday.co.uk> wrote:

>I have (or will have) a 1000 bar pressure sensor, it's a full bridge
>with the resistance given as 3500 ohms +/- 20%. I've no way of
>pressurising it accurately.
>
>But it's supplied with calibration data comprising sensitivity and a
>list of offsets against temperature, and I can measure the temperature.
>So far, so correctable.
>
>However, the data supplied is in the form of mV offset against
>temperature along with a single mV/bar figure *when the bridge is driven
>from a constant 1mA*. I don't want to drive it that way, I have a
>voltage reference and can use the differential ADC ratiometrically. The
>sensor would end up being excited by 2.500V which would be about 700uA
>which is within the allowable range.
>
>I feel there must be some way of using the supplied calibration offsets,
>but I'm not sure exactly how. (Also, doesn't everybody do bridges
>ratiometrically?)
>
>[Constant current is tricky with only a 3.3V supply, though the data
>sheet says I could go as low as 0.5mA, but I don't really want the extra
>complexity.]
>
>Ideas as to how I'd do the math(s)?

If I understand you correctly (?) the resistances in the bridge change
with temperature so you're going to be getting a different calibration
without that information. Can I assume you've already consulted the
manufacturer about what magnitude of error to expect?

Seems like constant current excitation is the easiest solution that
preserves full usability of that calibration data.

It wouldn't be too hard to make a constant voltage buffer that
provided a current signal you could measure, but that seems more
complex than just making a constant current source to begin with.

--
Best regards,
Spehro Pefhany

On Sat, 7 Aug 2021 08:00:55 -0700 (PDT), Anthony William Sloman
<bill.sloman@ieee.org> wrote:

>On Sunday, August 8, 2021 at 12:30:27 AM UTC+10, jla...@highlandsniptechnology.com wrote:
>> On Sat, 7 Aug 2021 11:25:25 +0100, Clive Arthur <cl...@nowaytoday.co.uk> wrote:
>> >On 06/08/2021 23:08, John Larkin wrote:
>> >> On Fri, 6 Aug 2021 22:33:36 +0100, Clive Arthur
>> >> <cl...@nowaytoday.co.uk> wrote:
>> >>
>> >>> On 06/08/2021 19:50, Phil Hobbs wrote:
>> >>>> Clive Arthur wrote:
>> >>>>> On 05/08/2021 18:06, Phil Hobbs wrote:
>> >>>>>> Clive Arthur wrote:
>
><snip>
>
>> >Not quite as simple as ratiometric, but hey.
>> It is ratiometric, because you need to divide! A stable ref is good
>> because the ADC won't measure the current and the bridge output
>> simultaneously.
>
>Curious. The last one used did. The output chopped down the input from the reference until it matched the voltage being digitised.
>
>> You can use the drop across the current sense resistor *as* the ADC
>> ref, but that can have issues.
>>
>> The diff-input 24-bit delta-sigma ADCs are perfect for ratiometric
>> measurements.
>
>They may put out 24 bits, but most them are only 20-bit accurate. The 24-bits come out as three 8-bit words. which makes marketing happy, but the last four bits aren't all that informative.
>
>> TI's ADS1247 is nice. Some of the Analog Devices parts are weird and flakey on the digital side. TI knows digital!
>
>Or claims to. Their analog circuits tended to suck, and some of their data sheets did lie by omission.

Before the Burr-Brown acquistion they were not as good as they are
now.
--
Best regards,
Spehro Pefhany

On Sat, 07 Aug 2021 16:09:49 -0400, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

>On Sat, 7 Aug 2021 08:00:55 -0700 (PDT), Anthony William Sloman
><bill.sloman@ieee.org> wrote:
>
>>On Sunday, August 8, 2021 at 12:30:27 AM UTC+10, jla...@highlandsniptechnology.com wrote:
>>> On Sat, 7 Aug 2021 11:25:25 +0100, Clive Arthur <cl...@nowaytoday.co.uk> wrote:
>>> >On 06/08/2021 23:08, John Larkin wrote:
>>> >> On Fri, 6 Aug 2021 22:33:36 +0100, Clive Arthur
>>> >> <cl...@nowaytoday.co.uk> wrote:
>>> >>
>>> >>> On 06/08/2021 19:50, Phil Hobbs wrote:
>>> >>>> Clive Arthur wrote:
>>> >>>>> On 05/08/2021 18:06, Phil Hobbs wrote:
>>> >>>>>> Clive Arthur wrote:
>>
>><snip>
>>
>>> >Not quite as simple as ratiometric, but hey.
>>> It is ratiometric, because you need to divide! A stable ref is good
>>> because the ADC won't measure the current and the bridge output
>>> simultaneously.
>>
>>Curious. The last one used did. The output chopped down the input from the reference until it matched the voltage being digitised.
>>
>>> You can use the drop across the current sense resistor *as* the ADC
>>> ref, but that can have issues.
>>>
>>> The diff-input 24-bit delta-sigma ADCs are perfect for ratiometric
>>> measurements.
>>
>>They may put out 24 bits, but most them are only 20-bit accurate. The 24-bits come out as three 8-bit words. which makes marketing happy, but the last four bits aren't all that informative.
>>
>>> TI's ADS1247 is nice. Some of the Analog Devices parts are weird and flakey on the digital side. TI knows digital!
>>
>>Or claims to. Their analog circuits tended to suck, and some of their data sheets did lie by omission.
>
>Before the Burr-Brown acquistion they were not as good as they are
>now.

The THS series are about the most radical opamps around. High voltage
and insanely fast.

As everyone else creeps down towards 5 volts, TI has some nice opamps
in the 30-60 volt range.

OPA547 is slick. 60 volts, DPAK, and programmable current limit.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

On Sunday, August 8, 2021 at 7:05:00 AM UTC+10, jla...@highlandsniptechnology.com wrote:
> On Sat, 07 Aug 2021 16:09:49 -0400, Spehro Pefhany
> <spef...@interlogDOTyou.knowwhat> wrote:
>
> >On Sat, 7 Aug 2021 08:00:55 -0700 (PDT), Anthony William Sloman
> ><bill....@ieee.org> wrote:
> >
> >>On Sunday, August 8, 2021 at 12:30:27 AM UTC+10, jla...@highlandsniptechnology.com wrote:
> >>> On Sat, 7 Aug 2021 11:25:25 +0100, Clive Arthur <cl...@nowaytoday.co.uk> wrote:
> >>> >On 06/08/2021 23:08, John Larkin wrote:
> >>> >> On Fri, 6 Aug 2021 22:33:36 +0100, Clive Arthur
> >>> >> <cl...@nowaytoday.co.uk> wrote:
> >>> >>
> >>> >>> On 06/08/2021 19:50, Phil Hobbs wrote:
> >>> >>>> Clive Arthur wrote:
> >>> >>>>> On 05/08/2021 18:06, Phil Hobbs wrote:
> >>> >>>>>> Clive Arthur wrote:
> >>
> >><snip>
> >>
> >>> >Not quite as simple as ratiometric, but hey.
> >>> It is ratiometric, because you need to divide! A stable ref is good
> >>> because the ADC won't measure the current and the bridge output
> >>> simultaneously.
> >>
> >>Curious. The last one used did. The output chopped down the input from the reference until it matched the voltage being digitised.
> >>
> >>> You can use the drop across the current sense resistor *as* the ADC
> >>> ref, but that can have issues.
> >>>
> >>> The diff-input 24-bit delta-sigma ADCs are perfect for ratiometric
> >>> measurements.
> >>
> >>They may put out 24 bits, but most them are only 20-bit accurate. The 24-bits come out as three 8-bit words. which makes marketing happy, but the last four bits aren't all that informative.
> >>
> >>> TI's ADS1247 is nice. Some of the Analog Devices parts are weird and flakey on the digital side. TI knows digital!
> >>
> >>Or claims to. Their analog circuits tended to suck, and some of their data sheets did lie by omission.
> >
> >Before the Burr-Brown acquistion they were not as good as they are
> >now.'

Burr-Brown were good, but under-capitalised. The TI take-over may have given them access to better fabs.

> The THS series are about the most radical opamps around. High voltage
> and insanely fast.
>
> As everyone else creeps down towards 5 volts, TI has some nice opamps
> in the 30-60 volt range.
>
> OPA547 is slick. 60 volts, DPAK, and programmable current limit.

But it is " A laser-trimmed monolithic integrated circuit " so it probably isn't made in a regular TI feb.

Everybody is creeping down towards 5V because smaller devices can't take higher voltages. You could use a fine line process to make bigger transistors, but Analog Devices have a very fancy process that throws in ion-implanted high-quality PNP transistors - Burr-Brown might be accessing something similar.

--
Bill Sloman, Sydney