Signal relays vs LC2MOS at low levels

[Spehro Pefhany]

At some point do you think about measureing a bunch? (sounds time consumin=
g)
So what's wrong with analog switches? too much leakage? from where? =20
can you bootstrap it somehow?=20
(I know about as much about analog swtiches as I do about relays..=20
not much, every time I've used either they've worked fine.)

George H. =20

Not sure how to get worst-case with testing, maybe run through a
solder process and then bake at 125°C+ for a while.


Best regards,
Spehro Pefhany

 

[Adrian Tuddenham]

Hi, all:-

I'm in the process of designing relatively high-precision signal
conditioning and data acquisition system front end.

It's desirable to have some kind of gain/range switching to maximize
the dynamic range over different operating modes.

Available options for switching are ADGxxx type analog multiplexers
(voltages are relatively high.. say +/-15V so HCxx are out) or "2A"
telecom style signal relays as are quite commonly used in bench-top
instruments.

All the relay data sheets that I've looked at have disturbingly high
recommended minimum voltages/currents. Say 10uA or 100uV-10mV (usually
the latter). I'd prefer to have just 10-20pA or so at < 1uV. With some
compromise I could arrange to have 10uA flowing through the contacts,
but that would result in a (possibly variable) ~1.5uV voltage drop. If
it varied with vibration that would be Bad. There's a reference to
some with special Silver-Palladium contacts for low-level loads, rated
at 10mA max, but I see no availability.

Any actual experience with these things? It might be possible to
"exercise" the contacts occasionally, or just before a measurement.

Also, on the ADGs, the typical leakage on the best ones is in the 10's
of pA (according to the data sheets, I have not yet made any
measurements). Any experience on the effects of long term radiation?
Usually things get worse, but how much worse?

Also a bit worried about the "sealed" relays in low pressure.

One point which may not be directly relevant in this particular case,
but is of more general help when designing with relays:

Make sure they are mounted with the contact gap in the vertical plane.

I know the manufacturers claim they can be mounted any way up, but they
give a lot less trouble and last a lot longer when the dirt is able to
drop out of the contact gap. This was common knowledge in the 1920s,
but seems to have disappeared from the textbooks and engineering courses
since then.