Re: PI3USB14-A dual 4>1 USB switch for analog

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,
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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.)
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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.)
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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.
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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).
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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.
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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.
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They do have these very nice-looking USB switches, type PI3USB14-A, for pretty cheap ($0.45 @ 1ea) and in plentiful stock.
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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.
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Has any of you used them for actual analog stuff?
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Thanks
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Phil Hobbs
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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
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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