Steve Hardy wrote:
>
> > From: Scott Dattalo <sdattalo@UNIX.SRI.COM>
> >[cut]
> > Why do you use the relays? Do you need the low on resistance they
> > provide? Do you need the the galvanic contact-to-coil isolation?
> >
> > If it's an isolation issue, have you considered solid state
> > relays? These are available with on resistances <20 ohms (which
> > is extremely large compared to a relay), isolation barriers
> > as large as 3kv, and contact ratings as high as 400V (if not
> > higher now).
>
> Uh, I think the guy wanted low level analogue control, not
> phase control of his electric furnace.  (Steve waxing sarcastic
> here...)
>
> I think for the ultimate in low noise application he is correct
> in selecting relays (so long as he doesn't want to switch too
> rapidly).  The mercury-wetted contact has much lower noise than
> a FET (its 'RDSon' is heaps lower) and there is less capacitive
> coupling from the control - FET gate capacitance is at least
> a few pf.
>

Low noise, Low level, small signal, pick your favorite subjective
analog term. Solid state relays such as CP Clare's (formally
Theta-J) LAA-110 perform extremely well in a high-performance
analog circuits. These devices are essentially opto-isolators
with MOSFET outputs instead of the more typical Bipolar transistor
outputs.

Will they perform well in your application? I have no idea. But
I've used them in several designs. In one case, the solid state relays
were used to 1) Multiplex 24 differential analog signals, 2) Switch
Gain settings, and 3) Switch voltage references (part of the
compensation system). Three ranges were available +/- 1V, +/- 5V,
and +/- 15V. About 14.5 bits of dynamic range was obtained while
operating over a -20 to 70 degree C temperature range. (14.5 bits?
yes 14.5 bits; I was using a V/F converter for the A/D converter.
The usable counts were approximately 2^14.5 ~ 24,000.) Granted,
the bandwidth of the signals was extremely low, but it worked
for my application.

Some of disadvantages of these SS relays are:
1) The Capacitance of the contact is about 35pf when it is off.
Relays can be as low as 1pf.
2) The on resistance is between 1 and 20 ohms (depending on the
device you choose). Relays can be found with just milli-ohm on
resistances.
3) Can not phase-control Electric furnaces.

Some advantages
1) Cost. Last I checked they're about 2USD in qty.
2) Smaller foot print. 6pin through all dip and I've seen but
not used surface mount versions.
3) No inductive kick back when the relay is turned off.
4) No relay contact "bounce".
5) Speed; switching times vary from tens to hundreds of microseconds.
6) If you're lucky, you can phase-control an Electric furnace.

I'm sure there are more pros and cons.

Scott