small signal at elevated voltage

[JDJ]

Hi folks, I've got a small signal (mV's generated by a nA current flowing though a ~10 MOhm resistor) that is floating on top of a ~30 V battery (the negative side at gnd), and I'm curious how I should configure (if it's even possible) my analog channels for measurement.  For your reference, I've got a PCIe-6259.  

Best regards,

JDJ 

[Henrik_Volkers]

Is an AC measurement possible?

How does your mV signal look like?

 First quick and dirty try : a 470nF C in series to your input and two antiparallel 1n4148 to protect your input from the 'turn on spike' 

Would need some external circuitry: Subtract the 30 V with an OP.

Message Edited by Henrik Volkers on 05-11-2010 08:19 AM

[JDJ]

Hi Henrik,

While there is interesting high-frequency content, there are also some slowly varying components that I care about in the signal, so an AC measurement is out.  You bring up an interesting point though: I do in fact take a time derivative of the signal later on, so DC offsets are irrelevant.  I could, as you say, do summing on a HV, high-speed opamp to get the signal in the card's range without worrying about the error in the offset.

Best regards,

JDJ