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

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