I have an application where I need to range switch between 1G and 50G feedback resistors in a TIA for a capacitively coupled surface voltage probe. (I'm selling the customer a fancy high-Z bootstrap amp as well, but they want an improved TIA as a first step.)
I'm looking at using relays, of course, but there's an interesting wrinkle: I have to short out the 1G resistor when it's in 50G mode, because otherwise the Johnson noise of the 1G will couple through the relay contact capacitance and dominate the noise above a few kHz.
Sooo, I'm taking a good look at some small relays. The best ones so far are the Omron G6K ones, but even those are dominated by capacitance between coil and contacts--the main capacitive coupling goes
open contact -> coil -> (closed contact + pole)
(it's about 0.6 pF each -> 0.3 pF total).
It looks as though I'll have to figure out a way to bootstrap the relay coil, or else use a reed relay and a big external magnet.
Any better ideas?
Cheers
Phil Hobbs