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From: John Larkin <jjSNIPlarkin@highNONOlandtechnology.com>
Newsgroups: sci.electronics.design
Subject: Re: PI3USB14-A dual 4>1 USB switch for analog
Date: Fri, 26 Apr 2024 17:07:10 -0700
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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.