ssr's as analog switches

The '4066' thread above reminded me of this question: how good are
opto-mos solid-state relays as analog switches?

I recently experimented with a Claire CPC1008N, nice little part. But
on the board where it was used, it turned out to be the bad-boy for DC
offset, something we didn't expect. Seems like heating from the led
causes thermoelectrics in the switch side, on the order of 600 nV per
mA of led current. This is nasty enough that we plan to replace the
ssr's with real (latching) relays next rev. We reduced the led drive
as low as we dared, to about 1.5 mA (it trips around 0.5) and got the
offset below 1 uV, barely acceptable here.

Any experience with stuff like this? With leakage?

I'd do some more experiments, but one would potentially have to test a
lot of parts, and pcb layout (thermal symmetry issues) would seem to
matter, much hassle, so we'll just bail to the relay for now.

I guess that a silicon chip, gold or aluminum wire bonds, and copper
leads would make a bunch of thermocouples, tens of uV per deg C each,
and the heat from the led (say, 200 k/w gross thermal resistance?)
would have to be dumped *very* symmetrically to avoid some terrible
offsets.

Hey, any linear IC must have similar problems, especially something
like an LT1028, fairly high dissipation and low advertised offsets. In
cases like that, I suppose it just settles to some constant
thermoelectric offset after a few minutes (if not loaded much) so
nobody notices. I sometimes use 1028's with very low-value feedback
networks (to keep Johnson noise down) and add a cheap unity-gain opamp
inside the loop, after the 1028, just to drive the feedback resistors
without heating the main chip.

Relays are wonderful.

John

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Relevant Pages

  • Re: ssrs as analog switches
    ... >opto-mos solid-state relays as analog switches? ... >causes thermoelectrics in the switch side, on the order of 600 nV per ... >ssr's with real relays next rev. ... >offset below 1 uV, barely acceptable here. ...
    (sci.electronics.design)
  • Re: relays
    ... "Latching" relays will, others will not. ... How do analog switches that use semiconductors compare to relays? ... >is easy to navigate and search yet Newark is cheaper and has tons of stuff ...
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  • Re: Analog switch
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