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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 11:03:08 -0700
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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.