I am receiving a DC signal through a device called a Faraday cup. It is effectively a conductive piece of copper and when a gas is ionized nearby and the ions collide with the cup it is accepted as a current.
I want to do the following:
Convert this current into a voltage, the current is low so the resistance of the cup itself isn't quite enough to convert it all into voltage so I've included a resistor in the circuit as shown in the figure. The other end of the cup is tied to ground through a small bleed resistor as shown.
Take this low voltage input and amplify it, preferably 100 times.
Minimize noise and impedance issues whenever possible.
So to this end I was thinking of the following circuit.
I will have a low pass filter set up as the input to the first opamp, the buffer, so I'm able to allow the lower (DC) current inputs into the op amp since that is what I desire. I want the resistor to be high enough to convert the current to voltage as I mentioned before, but capacitance can be changed to make the cutoff frequency decently low (So I can get as close to DC as possible).
On the other hand I'll have a high pass filter going to ground because I want all those high frequency noise and other stuff to be removed from my circuit.
Then the output of the buffer is fed into a non inverting amplifier with a gain of 10, then another with a gain of 10. The reason for this is because of bandwidth limitations on the LM358. So when all is said and done I want to have a decently high output voltage in the 1-3V range with as minimal noise as I can get.
Does this seem like a decent set up or are there ways for me to improve it? I had considered using an instrumentation amplifier but currently am not because:
I've haven't been able to get the gain to work, ie I'd simulate 100 gain but hooking that up I'd only be getting 20-40.
Single chip InstrAmp are a bit pricey
I just need to compare the faraday to ground so the two buffered inputs wouldn't benefit me as I'd just need to ground the inverting input.