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NNTP-Posting-Date: Fri, 26 Apr 2024 23:30:59 +0000
Subject: Re: PI3USB14-A dual 4>1 USB switch for analog
Newsgroups: sci.electronics.design
References: <df7333b6-e5ff-922a-7aea-effd4ea235c4@electrooptical.net>
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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Message-ID: <e5694455-f96a-a74e-5eb5-ef913d4f5434@electrooptical.net>
Date: Fri, 26 Apr 2024 19:30:58 -0400
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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