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NNTP-Posting-Date: Sat, 27 Apr 2024 23:46:14 +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: <0069906b-da42-fda4-46ce-4167d9584e8d@electrooptical.net>
Date: Sat, 27 Apr 2024 19:46:13 -0400
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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