Hi, all,

I'm in a bit of a rush, doing revs to a testing board for a prototype
lidar ASIC that turns out to be..., um..., well.... Let's just say it's
a _tiny_bit_delicate_ electrically. Yeah, just a little delicate,
that's it. (Not our design or our nickel, fortunately.)

It runs on a single supply, so the only sequencing issues are with the
signal pins. Thus the test board is going to bring up V_DD smoothly and
not too slowly: a monotonic edge of about 100 us. While that's going
on, all the inputs and outputs will see 100k ohms to ground. (All the
enables are positive-true, which makes this easier.)

The digital pins are all inputs, so they're just buffered with
SN74LVC1G125s (tri-state Schmitt noninverting), with the aforementioned
100k to ground on their outputs.

The analog outputs don't have much drive, but can go pretty fast if you
keep the capacitance down. There are a fair few of them, so I want to
mux them down before the op amp buffers (OPA2626es--surprisingly nice
chips).

Because we're not made of money, and the test boards aren't ITAR, we're
getting them made and stuffed at JLCPCB. Thus we care a lot about what
their tame distributor LCSC has in stock.

They're fairly light on analog muxes, it turns out. Going through what
they do have, I was reminded just how awful the capacitance of old-timey
mux parts is--way over 100 pF at the common pin of an 8-1, blech.

They do have these very nice-looking USB switches, type PI3USB14-A, for
pretty cheap ($0.45 @ 1ea) and in plentiful stock.

They look beautiful--6 ohms R_on, 4 pF C_off typical--but apart from one
crosstalk number, the datasheet gives only sketchy hints about their
actual analog performance--bandwidth and crosstalk, but nothing about
charge injection, switching times, make-before-break, resistance
linearity, or distortion.

Has any of you used them for actual analog stuff?

Thanks

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 Thu, 25 Apr 2024 12:28:42 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>Hi, all,
>
>I'm in a bit of a rush, doing revs to a testing board for a prototype
>lidar ASIC that turns out to be..., um..., well.... Let's just say it's
>a _tiny_bit_delicate_ electrically. Yeah, just a little delicate,
>that's it. (Not our design or our nickel, fortunately.)
>
>It runs on a single supply, so the only sequencing issues are with the
>signal pins. Thus the test board is going to bring up V_DD smoothly and
>not too slowly: a monotonic edge of about 100 us. While that's going
>on, all the inputs and outputs will see 100k ohms to ground. (All the
>enables are positive-true, which makes this easier.)
>
>The digital pins are all inputs, so they're just buffered with
>SN74LVC1G125s (tri-state Schmitt noninverting), with the aforementioned
>100k to ground on their outputs.
>
>The analog outputs don't have much drive, but can go pretty fast if you
>keep the capacitance down. There are a fair few of them, so I want to
>mux them down before the op amp buffers (OPA2626es--surprisingly nice
>chips).
>
>Because we're not made of money, and the test boards aren't ITAR, we're
>getting them made and stuffed at JLCPCB. Thus we care a lot about what
>their tame distributor LCSC has in stock.
>
>They're fairly light on analog muxes, it turns out. Going through what
>they do have, I was reminded just how awful the capacitance of old-timey
>mux parts is--way over 100 pF at the common pin of an 8-1, blech.
>
>They do have these very nice-looking USB switches, type PI3USB14-A, for
>pretty cheap ($0.45 @ 1ea) and in plentiful stock.
>
>They look beautiful--6 ohms R_on, 4 pF C_off typical--but apart from one
>crosstalk number, the datasheet gives only sketchy hints about their
>actual analog performance--bandwidth and crosstalk, but nothing about
>charge injection, switching times, make-before-break, resistance
>linearity, or distortion.
>
>Has any of you used them for actual analog stuff?
>
>Thanks
>
>Phil Hobbs

We use TS3USB30EDGSR to switch/mux fast signals. We mostly use them as
setup-time switches, things like polarity switching and low/50r source
impedances, things like that, so we haven't explored dynamics much.
They do propagate fast signals nicely.

How about using relays?

On 26/04/2024 2:28 am, Phil Hobbs wrote:
> Hi, all,
>
> I'm in a bit of a rush, doing revs to a testing board for a prototype
> lidar ASIC that turns out to be..., um..., well....  Let's just say it's
> a _tiny_bit_delicate_ electrically.  Yeah, just a little delicate,
> that's it. (Not our design or our nickel, fortunately.)
>
> It runs on a single supply, so the only sequencing issues are with the
> signal pins. Thus the test board is going to bring up V_DD smoothly and
> not too slowly: a monotonic edge of about 100 us.  While that's going
> on, all the inputs and outputs will see 100k ohms to ground. (All the
> enables are positive-true, which makes this easier.)
>
> The digital pins are all inputs, so they're just buffered with
> SN74LVC1G125s (tri-state Schmitt noninverting), with the aforementioned
> 100k to ground on their outputs.
>
> The analog outputs don't have much drive, but can go pretty fast if you
> keep the capacitance down.  There are a fair few of them, so I want to
> mux them down before the op amp buffers (OPA2626es--surprisingly nice
> chips).
>
> Because we're not made of money, and the test boards aren't ITAR, we're
> getting them made and stuffed at JLCPCB.  Thus we care a lot about what
> their tame distributor LCSC has in stock.
>
> They're fairly light on analog muxes, it turns out.  Going through what
> they do have, I was reminded just how awful the capacitance of old-timey
> mux parts is--way over 100 pF at the common pin of an 8-1, blech.
>
> They do have these very nice-looking USB switches, type PI3USB14-A, for
> pretty cheap ($0.45 @ 1ea) and in plentiful stock.
>
> They look beautiful--6 ohms R_on, 4 pF C_off typical--but apart from one
> crosstalk number, the datasheet gives only sketchy hints about their
> actual analog performance--bandwidth and crosstalk, but nothing about
> charge injection, switching times, make-before-break, resistance
> linearity, or distortion.
>
> Has any of you used them for actual analog stuff?
>
> Thanks
>
> Phil Hobbs
>

As long as you don't need the signals to swing near the positive rail, I
suggest you google "H-mode mixer" as a lot of amateur radio enthusiasts
have built mixers using bus switches, basically just nmos fets with a
gate driver. For example:
https://martein.home.xs4all.nl/pa3ake/hmode/switches.html

They may not be better than the PI3USB14-A but I have a better guess of
what is in them.

On 27/04/2024 1:01 am, Chris Jones wrote:
> On 26/04/2024 2:28 am, Phil Hobbs wrote:
>> Hi, all,
>>
>> I'm in a bit of a rush, doing revs to a testing board for a prototype
>> lidar ASIC that turns out to be..., um..., well....  Let's just say
>> it's a _tiny_bit_delicate_ electrically.  Yeah, just a little
>> delicate, that's it. (Not our design or our nickel, fortunately.)
>>
>> It runs on a single supply, so the only sequencing issues are with the
>> signal pins. Thus the test board is going to bring up V_DD smoothly
>> and not too slowly: a monotonic edge of about 100 us.  While that's
>> going on, all the inputs and outputs will see 100k ohms to ground.
>> (All the enables are positive-true, which makes this easier.)
>>
>> The digital pins are all inputs, so they're just buffered with
>> SN74LVC1G125s (tri-state Schmitt noninverting), with the
>> aforementioned 100k to ground on their outputs.
>>
>> The analog outputs don't have much drive, but can go pretty fast if
>> you keep the capacitance down.  There are a fair few of them, so I
>> want to mux them down before the op amp buffers
>> (OPA2626es--surprisingly nice chips).
>>
>> Because we're not made of money, and the test boards aren't ITAR,
>> we're getting them made and stuffed at JLCPCB.  Thus we care a lot
>> about what their tame distributor LCSC has in stock.
>>
>> They're fairly light on analog muxes, it turns out.  Going through
>> what they do have, I was reminded just how awful the capacitance of
>> old-timey mux parts is--way over 100 pF at the common pin of an 8-1,
>> blech.
>>
>> They do have these very nice-looking USB switches, type PI3USB14-A,
>> for pretty cheap ($0.45 @ 1ea) and in plentiful stock.
>>
>> They look beautiful--6 ohms R_on, 4 pF C_off typical--but apart from
>> one crosstalk number, the datasheet gives only sketchy hints about
>> their actual analog performance--bandwidth and crosstalk, but nothing
>> about charge injection, switching times, make-before-break, resistance
>> linearity, or distortion.
>>
>> Has any of you used them for actual analog stuff?
>>
>> Thanks
>>
>> Phil Hobbs
>>
>
> As long as you don't need the signals to swing near the positive rail, I
> suggest you google "H-mode mixer" as a lot of amateur radio enthusiasts
> have built mixers using bus switches, basically just nmos fets with a
> gate driver. For example:
> https://martein.home.xs4all.nl/pa3ake/hmode/switches.html
>
> They may not be better than the PI3USB14-A but I have a better guess of
> what is in them.

Ah, I see they also tried ones with both NMOS and PMOS devices in the
switches. The FSA3157 seems to have somewhat more complete
specifications than the one you mentioned.

If you don't happen to need it to swing near the positive rail, you
might get less capacitance with a NMOS-only type like the venerable FST3125.

I hope the ones that also have PMOS devices in the switches keep the
backgates of the PMOS at the rail (like a 4016), and don't do the
horrible backgate switching trickery to reduce Ron like in the old 4066.
I think some of those might have been so bad that they could momentarily
short your signal to the rail, and even if not, they certainly had
another big charge injection mechanism.

On Sat, 27 Apr 2024 01:12:13 +1000, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

>On 27/04/2024 1:01 am, Chris Jones wrote:
>> On 26/04/2024 2:28 am, Phil Hobbs wrote:
>>> Hi, all,
>>>
>>> I'm in a bit of a rush, doing revs to a testing board for a prototype
>>> lidar ASIC that turns out to be..., um..., well....  Let's just say
>>> it's a _tiny_bit_delicate_ electrically.  Yeah, just a little
>>> delicate, that's it. (Not our design or our nickel, fortunately.)
>>>
>>> It runs on a single supply, so the only sequencing issues are with the
>>> signal pins. Thus the test board is going to bring up V_DD smoothly
>>> and not too slowly: a monotonic edge of about 100 us.  While that's
>>> going on, all the inputs and outputs will see 100k ohms to ground.
>>> (All the enables are positive-true, which makes this easier.)
>>>
>>> The digital pins are all inputs, so they're just buffered with
>>> SN74LVC1G125s (tri-state Schmitt noninverting), with the
>>> aforementioned 100k to ground on their outputs.
>>>
>>> The analog outputs don't have much drive, but can go pretty fast if
>>> you keep the capacitance down.  There are a fair few of them, so I
>>> want to mux them down before the op amp buffers
>>> (OPA2626es--surprisingly nice chips).
>>>
>>> Because we're not made of money, and the test boards aren't ITAR,
>>> we're getting them made and stuffed at JLCPCB.  Thus we care a lot
>>> about what their tame distributor LCSC has in stock.
>>>
>>> They're fairly light on analog muxes, it turns out.  Going through
>>> what they do have, I was reminded just how awful the capacitance of
>>> old-timey mux parts is--way over 100 pF at the common pin of an 8-1,
>>> blech.
>>>
>>> They do have these very nice-looking USB switches, type PI3USB14-A,
>>> for pretty cheap ($0.45 @ 1ea) and in plentiful stock.
>>>
>>> They look beautiful--6 ohms R_on, 4 pF C_off typical--but apart from
>>> one crosstalk number, the datasheet gives only sketchy hints about
>>> their actual analog performance--bandwidth and crosstalk, but nothing
>>> about charge injection, switching times, make-before-break, resistance
>>> linearity, or distortion.
>>>
>>> Has any of you used them for actual analog stuff?
>>>
>>> Thanks
>>>
>>> Phil Hobbs
>>>
>>
>> As long as you don't need the signals to swing near the positive rail, I
>> suggest you google "H-mode mixer" as a lot of amateur radio enthusiasts
>> have built mixers using bus switches, basically just nmos fets with a
>> gate driver. For example:
>> https://martein.home.xs4all.nl/pa3ake/hmode/switches.html
>>
>> They may not be better than the PI3USB14-A but I have a better guess of
>> what is in them.
>
>Ah, I see they also tried ones with both NMOS and PMOS devices in the
>switches. The FSA3157 seems to have somewhat more complete
>specifications than the one you mentioned.

The 3157 is nice too, SPDT in a small package, with specified charge
injection. We pay 7 cents.

>
>If you don't happen to need it to swing near the positive rail, you
>might get less capacitance with a NMOS-only type like the venerable FST3125.

Optimistically, complementary P and N fets would have cancelling
charge injection. But that doesn't always work (another sad story.)

>
>I hope the ones that also have PMOS devices in the switches keep the
>backgates of the PMOS at the rail (like a 4016), and don't do the
>horrible backgate switching trickery to reduce Ron like in the old 4066.
>I think some of those might have been so bad that they could momentarily
>short your signal to the rail, and even if not, they certainly had
>another big charge injection mechanism.

On 2024-04-26 14:03, John Larkin wrote:
> On Sat, 27 Apr 2024 01:12:13 +1000, Chris Jones
> <lugnut808@spam.yahoo.com> wrote:
>
>> On 27/04/2024 1:01 am, Chris Jones wrote:
>>> On 26/04/2024 2:28 am, Phil Hobbs wrote:
>>>> Hi, all,
>>>>
>>>> I'm in a bit of a rush, doing revs to a testing board for a prototype
>>>> lidar ASIC that turns out to be..., um..., well....  Let's just say
>>>> it's a _tiny_bit_delicate_ electrically.  Yeah, just a little
>>>> delicate, that's it. (Not our design or our nickel, fortunately.)
>>>>
>>>> It runs on a single supply, so the only sequencing issues are with the
>>>> signal pins. Thus the test board is going to bring up V_DD smoothly
>>>> and not too slowly: a monotonic edge of about 100 us.  While that's
>>>> going on, all the inputs and outputs will see 100k ohms to ground.
>>>> (All the enables are positive-true, which makes this easier.)
>>>>
>>>> The digital pins are all inputs, so they're just buffered with
>>>> SN74LVC1G125s (tri-state Schmitt noninverting), with the
>>>> aforementioned 100k to ground on their outputs.
>>>>
>>>> The analog outputs don't have much drive, but can go pretty fast if
>>>> you keep the capacitance down.  There are a fair few of them, so I
>>>> want to mux them down before the op amp buffers
>>>> (OPA2626es--surprisingly nice chips).
>>>>
>>>> Because we're not made of money, and the test boards aren't ITAR,
>>>> we're getting them made and stuffed at JLCPCB.  Thus we care a lot
>>>> about what their tame distributor LCSC has in stock.
>>>>
>>>> They're fairly light on analog muxes, it turns out.  Going through
>>>> what they do have, I was reminded just how awful the capacitance of
>>>> old-timey mux parts is--way over 100 pF at the common pin of an 8-1,
>>>> blech.
>>>>
>>>> They do have these very nice-looking USB switches, type PI3USB14-A,
>>>> for pretty cheap ($0.45 @ 1ea) and in plentiful stock.
>>>>
>>>> They look beautiful--6 ohms R_on, 4 pF C_off typical--but apart from
>>>> one crosstalk number, the datasheet gives only sketchy hints about
>>>> their actual analog performance--bandwidth and crosstalk, but nothing
>>>> about charge injection, switching times, make-before-break, resistance
>>>> linearity, or distortion.
>>>>
>>>> Has any of you used them for actual analog stuff?
>>>>
>>>> Thanks
>>>>
>>>> Phil Hobbs
>>>>
>>>
>>> As long as you don't need the signals to swing near the positive rail, I
>>> suggest you google "H-mode mixer" as a lot of amateur radio enthusiasts
>>> have built mixers using bus switches, basically just nmos fets with a
>>> gate driver. For example:
>>> https://martein.home.xs4all.nl/pa3ake/hmode/switches.html
>>>
>>> They may not be better than the PI3USB14-A but I have a better guess of
>>> what is in them.
>>
>> Ah, I see they also tried ones with both NMOS and PMOS devices in the
>> switches. The FSA3157 seems to have somewhat more complete
>> specifications than the one you mentioned.
>
>
>
> The 3157 is nice too, SPDT in a small package, with specified charge
> injection. We pay 7 cents.

The onsemi FSA3157 is obsolete, unfortunately. There are TI and Diodes
Inc versions that you can still get,

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 2024-04-26 11:12, Chris Jones wrote:
> On 27/04/2024 1:01 am, Chris Jones wrote:
>> On 26/04/2024 2:28 am, Phil Hobbs wrote:
>>> Hi, all,
>>>
>>> I'm in a bit of a rush, doing revs to a testing board for a prototype
>>> lidar ASIC that turns out to be..., um..., well....  Let's just say
>>> it's a _tiny_bit_delicate_ electrically.  Yeah, just a little
>>> delicate, that's it. (Not our design or our nickel, fortunately.)
>>>
>>> It runs on a single supply, so the only sequencing issues are with
>>> the signal pins. Thus the test board is going to bring up V_DD
>>> smoothly and not too slowly: a monotonic edge of about 100 us.  While
>>> that's going on, all the inputs and outputs will see 100k ohms to
>>> ground. (All the enables are positive-true, which makes this easier.)
>>>
>>> The digital pins are all inputs, so they're just buffered with
>>> SN74LVC1G125s (tri-state Schmitt noninverting), with the
>>> aforementioned 100k to ground on their outputs.
>>>
>>> The analog outputs don't have much drive, but can go pretty fast if
>>> you keep the capacitance down.  There are a fair few of them, so I
>>> want to mux them down before the op amp buffers
>>> (OPA2626es--surprisingly nice chips).
>>>
>>> Because we're not made of money, and the test boards aren't ITAR,
>>> we're getting them made and stuffed at JLCPCB.  Thus we care a lot
>>> about what their tame distributor LCSC has in stock.
>>>
>>> They're fairly light on analog muxes, it turns out.  Going through
>>> what they do have, I was reminded just how awful the capacitance of
>>> old-timey mux parts is--way over 100 pF at the common pin of an 8-1,
>>> blech.
>>>
>>> They do have these very nice-looking USB switches, type PI3USB14-A,
>>> for pretty cheap ($0.45 @ 1ea) and in plentiful stock.
>>>
>>> They look beautiful--6 ohms R_on, 4 pF C_off typical--but apart from
>>> one crosstalk number, the datasheet gives only sketchy hints about
>>> their actual analog performance--bandwidth and crosstalk, but nothing
>>> about charge injection, switching times, make-before-break,
>>> resistance linearity, or distortion.
>>>
>>> Has any of you used them for actual analog stuff?
>>>
>>> Thanks
>>>
>>> Phil Hobbs
>>>
>>
>> As long as you don't need the signals to swing near the positive rail,
>> I suggest you google "H-mode mixer" as a lot of amateur radio
>> enthusiasts have built mixers using bus switches, basically just nmos
>> fets with a gate driver. For example:
>> https://martein.home.xs4all.nl/pa3ake/hmode/switches.html
>>
>> They may not be better than the PI3USB14-A but I have a better guess
>> of what is in them.

Thanks. The CMOS mux mixer was AFAICT first popularized by Ed Oxner of
Siliconix back in the early '80s. They make nice strong mixers, for sure.

>
> Ah, I see they also tried ones with both NMOS and PMOS devices in the
> switches. The FSA3157 seems to have somewhat more complete
> specifications than the one you mentioned.

Wouldn't be hard. ;)

> If you don't happen to need it to swing near the positive rail, you
> might get less capacitance with a NMOS-only type like the venerable
> FST3125.

Obsolete, unfortunately.

>
> I hope the ones that also have PMOS devices in the switches keep the
> backgates of the PMOS at the rail (like a 4016), and don't do the
> horrible backgate switching trickery to reduce Ron like in the old 4066.
> I think some of those might have been so bad that they could momentarily
> short your signal to the rail, and even if not, they certainly had
> another big charge injection mechanism.

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 Fri, 26 Apr 2024 19:30:58 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 2024-04-26 14:03, John Larkin wrote:
>> On Sat, 27 Apr 2024 01:12:13 +1000, Chris Jones
>> <lugnut808@spam.yahoo.com> wrote:
>>
>>> On 27/04/2024 1:01 am, Chris Jones wrote:
>>>> On 26/04/2024 2:28 am, Phil Hobbs wrote:
>>>>> Hi, all,
>>>>>
>>>>> I'm in a bit of a rush, doing revs to a testing board for a prototype
>>>>> lidar ASIC that turns out to be..., um..., well....  Let's just say
>>>>> it's a _tiny_bit_delicate_ electrically.  Yeah, just a little
>>>>> delicate, that's it. (Not our design or our nickel, fortunately.)
>>>>>
>>>>> It runs on a single supply, so the only sequencing issues are with the
>>>>> signal pins. Thus the test board is going to bring up V_DD smoothly
>>>>> and not too slowly: a monotonic edge of about 100 us.  While that's
>>>>> going on, all the inputs and outputs will see 100k ohms to ground.
>>>>> (All the enables are positive-true, which makes this easier.)
>>>>>
>>>>> The digital pins are all inputs, so they're just buffered with
>>>>> SN74LVC1G125s (tri-state Schmitt noninverting), with the
>>>>> aforementioned 100k to ground on their outputs.
>>>>>
>>>>> The analog outputs don't have much drive, but can go pretty fast if
>>>>> you keep the capacitance down.  There are a fair few of them, so I
>>>>> want to mux them down before the op amp buffers
>>>>> (OPA2626es--surprisingly nice chips).
>>>>>
>>>>> Because we're not made of money, and the test boards aren't ITAR,
>>>>> we're getting them made and stuffed at JLCPCB.  Thus we care a lot
>>>>> about what their tame distributor LCSC has in stock.
>>>>>
>>>>> They're fairly light on analog muxes, it turns out.  Going through
>>>>> what they do have, I was reminded just how awful the capacitance of
>>>>> old-timey mux parts is--way over 100 pF at the common pin of an 8-1,
>>>>> blech.
>>>>>
>>>>> They do have these very nice-looking USB switches, type PI3USB14-A,
>>>>> for pretty cheap ($0.45 @ 1ea) and in plentiful stock.
>>>>>
>>>>> They look beautiful--6 ohms R_on, 4 pF C_off typical--but apart from
>>>>> one crosstalk number, the datasheet gives only sketchy hints about
>>>>> their actual analog performance--bandwidth and crosstalk, but nothing
>>>>> about charge injection, switching times, make-before-break, resistance
>>>>> linearity, or distortion.
>>>>>
>>>>> Has any of you used them for actual analog stuff?
>>>>>
>>>>> Thanks
>>>>>
>>>>> Phil Hobbs
>>>>>
>>>>
>>>> As long as you don't need the signals to swing near the positive rail, I
>>>> suggest you google "H-mode mixer" as a lot of amateur radio enthusiasts
>>>> have built mixers using bus switches, basically just nmos fets with a
>>>> gate driver. For example:
>>>> https://martein.home.xs4all.nl/pa3ake/hmode/switches.html
>>>>
>>>> They may not be better than the PI3USB14-A but I have a better guess of
>>>> what is in them.
>>>
>>> Ah, I see they also tried ones with both NMOS and PMOS devices in the
>>> switches. The FSA3157 seems to have somewhat more complete
>>> specifications than the one you mentioned.
>>
>>
>>
>> The 3157 is nice too, SPDT in a small package, with specified charge
>> injection. We pay 7 cents.
>
>The onsemi FSA3157 is obsolete, unfortunately. There are TI and Diodes
>Inc versions that you can still get,
>
>Cheers
>
>Phil Hobbs

My MAX record shows OnSemi NLASB3157DFT2G as an acceptable sub. They
hires sadists to make up part numbers.

I like HH Smith, with part numbers like 101.

On 2024-04-26 20:07, John Larkin wrote:
> On Fri, 26 Apr 2024 19:30:58 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>> On 2024-04-26 14:03, John Larkin wrote:
>>> On Sat, 27 Apr 2024 01:12:13 +1000, Chris Jones
>>> <lugnut808@spam.yahoo.com> wrote:
>>>
>>>> On 27/04/2024 1:01 am, Chris Jones wrote:
>>>>> On 26/04/2024 2:28 am, Phil Hobbs wrote:
>>>>>> Hi, all,
>>>>>>
>>>>>> I'm in a bit of a rush, doing revs to a testing board for a prototype
>>>>>> lidar ASIC that turns out to be..., um..., well....  Let's just say
>>>>>> it's a _tiny_bit_delicate_ electrically.  Yeah, just a little
>>>>>> delicate, that's it. (Not our design or our nickel, fortunately.)
>>>>>>
>>>>>> It runs on a single supply, so the only sequencing issues are with the
>>>>>> signal pins. Thus the test board is going to bring up V_DD smoothly
>>>>>> and not too slowly: a monotonic edge of about 100 us.  While that's
>>>>>> going on, all the inputs and outputs will see 100k ohms to ground.
>>>>>> (All the enables are positive-true, which makes this easier.)
>>>>>>
>>>>>> The digital pins are all inputs, so they're just buffered with
>>>>>> SN74LVC1G125s (tri-state Schmitt noninverting), with the
>>>>>> aforementioned 100k to ground on their outputs.
>>>>>>
>>>>>> The analog outputs don't have much drive, but can go pretty fast if
>>>>>> you keep the capacitance down.  There are a fair few of them, so I
>>>>>> want to mux them down before the op amp buffers
>>>>>> (OPA2626es--surprisingly nice chips).
>>>>>>
>>>>>> Because we're not made of money, and the test boards aren't ITAR,
>>>>>> we're getting them made and stuffed at JLCPCB.  Thus we care a lot
>>>>>> about what their tame distributor LCSC has in stock.
>>>>>>
>>>>>> They're fairly light on analog muxes, it turns out.  Going through
>>>>>> what they do have, I was reminded just how awful the capacitance of
>>>>>> old-timey mux parts is--way over 100 pF at the common pin of an 8-1,
>>>>>> blech.
>>>>>>
>>>>>> They do have these very nice-looking USB switches, type PI3USB14-A,
>>>>>> for pretty cheap ($0.45 @ 1ea) and in plentiful stock.
>>>>>>
>>>>>> They look beautiful--6 ohms R_on, 4 pF C_off typical--but apart from
>>>>>> one crosstalk number, the datasheet gives only sketchy hints about
>>>>>> their actual analog performance--bandwidth and crosstalk, but nothing
>>>>>> about charge injection, switching times, make-before-break, resistance
>>>>>> linearity, or distortion.
>>>>>>
>>>>>> Has any of you used them for actual analog stuff?
>>>>>>
>>>>>> Thanks
>>>>>>
>>>>>> Phil Hobbs
>>>>>>
>>>>>
>>>>> As long as you don't need the signals to swing near the positive rail, I
>>>>> suggest you google "H-mode mixer" as a lot of amateur radio enthusiasts
>>>>> have built mixers using bus switches, basically just nmos fets with a
>>>>> gate driver. For example:
>>>>> https://martein.home.xs4all.nl/pa3ake/hmode/switches.html
>>>>>
>>>>> They may not be better than the PI3USB14-A but I have a better guess of
>>>>> what is in them.
>>>>
>>>> Ah, I see they also tried ones with both NMOS and PMOS devices in the
>>>> switches. The FSA3157 seems to have somewhat more complete
>>>> specifications than the one you mentioned.
>>>
>>>
>>>
>>> The 3157 is nice too, SPDT in a small package, with specified charge
>>> injection. We pay 7 cents.
>>
>> The onsemi FSA3157 is obsolete, unfortunately. There are TI and Diodes
>> Inc versions that you can still get,
>>
>> Cheers
>>
>> Phil Hobbs
>
> My MAX record shows OnSemi NLASB3157DFT2G as an acceptable sub. They
> hires sadists to make up part numbers.
>
> I like HH Smith, with part numbers like 101.
>
Since everybody gave up on having the part numbers fit on the package,
the sky's the limit.

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