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I have an amplifier in mind, you could think of it as an ac coupled differential amplifier, where the coupling capacitance is extremely small (<1pF) and you can assume the input is a sinusoidal differential voltage function. The amplitude of the source is over 1 Million volts.

The conclusion that I've come to is that I want to measure the charge that moves back and forth from one capacitive input to the other over each cycle, which is something like

[tex]10nC \star Sin[\omega t] \hbox{ where } \omega \hbox{ is about 240Hz}[/tex]

The most important goals are long term stability, temperature stability, common mode rejection, and low noise. In my latest prototype the amplifier has two stages. A dual-supply op-amp integrator on each input to give a voltage signal proportional to the charge, and then the output goes into a fully differential amplifier and then to the ADC. The signal is already relatively large compared to the noise, but I am attempting to achieve better than .01% accuracy. There is also a badly distorted 60Hz component in the noise, which i am trying to combat by adding more CMRR and symmetrical inputs.

I would like to produce a newer version of this with a single supply, and a completely differential signal chain.

From reading a few other posts on the forums, I have learned a little bit more about the parameters of amplifiers, but let me ask a few questions to be sure I have it right.

At first I thought that a very high impedance instrumentation amplifier would be the best choice, since I am measuring a high impedance source. I was looking at the INA116, with an input bias current of and considering all sorts of guard ring and cable shielding possibilities. Alternatively, I have looked at the AD8231 zero-drift programmable instrumentation amplifier. I like the AD8231 better because its got better drift characteristics and CMRR.

Each input of the instrumentation amplifier would be biased through a 10-100Mohm resistor to a mid-supply temp stabilized voltage reference. The inputs would have a capacitor between them to produce a voltage of about +- 30mV. The amplifier would be configured for a gain of 32. After the instrumentation amplifier everything will be low impedance and relatively easy to manage. With respect to the first input stage, is there anything that will end up biting me later if I go with the AD8231 vs. any other arrangement?

While examining this problem I have noticed certain things about the different breeds of amplifier. Considering op-amps, fully differential amplifiers, and instrumentation amplifiers, can anyone elaborate on qualities that are favored for each type? For example, the input bias current on most fully differential amplifiers is enormous compared to many op-amps and in-amps, like 3 microamps compared to 20 picoamps.

The purpose of this project is to regulate the voltage of a electrostatic particle accelerator.

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# Component selection for special purpose amplifier

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