This is an amplifier for measuring very low currents (~pA) by measuring the voltage across a big high-precision feedback resistor (RH2HVS, 1e+11 ohms). The resistor and this other silver component (KP F) are both inserted into sockets (not soldered) so they can be easily swapped out.

What is the silver thing, I am guessing a capacitor (making for a sensitive RC circuit) but the "KP F" lettering is not getting me anywhere. It looks like foil encased in glass. Any ideas what it is (and where I might be able to get more)?

 

Hello,

It looks like a styroflex capacitor.
Those are often used in circuits that need high temperature stability.
There might be more info on the other side of the component.

Bertus

 

Another word for that is, "polystyrene". Very low leakage, rather stable amount of capacitance across time and temperature.
The main problem is that the label is on the other side.

 

Could it be a compensation cap in a current to voltage opamp circuit?

 

Thanks!

Yes the board is normally located inside a housing that is evacuated to 1e-3 bars, and heated a few degrees above room temperature for stability.

The resistor Johnson noise would presumably be reduced by cooling, but then condensation happens...

 

Ah - on Vishay.com I found that "KP" = polypropylene.

An this:
"Polypropylene (KP) films are used in high frequency or high voltage applications due to their very low dissipation factor and high dielectric strength. These films are used in AC and pulse capacitors and interference suppression capacitors for mains applications."

Now to get the capacitance value....I may have to pull it off the board.

 

OK - get this - the cap is a 2 pF Styroflex.

I have googled and cannot find a supplier that carries these in 2 pF - anyone have any leads?

 

OK - get this - the cap is a 2 pF Styroflex.

I have googled and cannot find a supplier that carries these in 2 pF - anyone have any leads?

I seriously doubt that the capacitor is a 2 pF capacitor. There is 2 pf of stray capacitance in those leads. Get a grip on reality. If that value was measured then I would question the measurement method and or the instrument.

 

2pF cap coupled with 1e+12 ohm feedback resistor - RC time constant is 2 seconds. It is an amplifier for measuring small currents (on order of pA) by passing the current through the resistor and measuring the voltage across.