# Charge Amplifier with low impedance

Hello, I am looking for help designing a charge amplifier that has very low input impedance.  My basic circuit design looks like this:

Here, R1 is a many-turn coil with resistance from 3 ohm to 130 ohm.  I am looking to measure the charge that flows through the coil due to changing magnetic fields -- I expect around 0.5 to 3 nC.  However, the magnetic field takes about 1 s to change, so I have been using C1 = 30 nF and R2 = 30 Mohm.  That combination of R1 and R2 produces a lot of noise when there is no signal in the coil, and other charge amplifiers (like the AD8488) seem to be made only for pulses.  Is there anything that would work for my application?

• Hello,

I believe you are misinterpreting a basic parameter for your inductors. The coulomb is a measure of charge, and associated with capacitors or element with a similar kind electrostatic field. The dual of this (if there is one) for an inductor would be flux, or flux density.

I also believe you're looking for a way to measure energy stored in your inductor from some sort of transducer. I'm not an expert on such matters, but if this fits your goals you can get a lot of help from companies that make inductors, such as Coilcraft Inc, or from companies that make magnetic cores.

Good luck,

js

• Hi,

Thanks for the reply, but I am not mistaken in my application.  I have a large 8-inch diameter coil of about 200 -1000 turns, depending on the wire gauge.  The inductance of the coil is very, very low, as is its resistance.  I am creating small changes in magnetic field, which then create currents in the coil.  I am not so much interested in the current, but the total charge (the integration of the current) --  I would like to measure the total charge that goes through the coil.  I am pretty sure a charge amplifier, as the electronic analog of a ballistic galvanometer, is what I want to build.

The problem is that the charge amplifiers all want large impedances at the input, and I have a small impedance.