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

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