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DDC112 for precision electrometer

Fulvio Pompili
Prodigy 10 points
Other Parts Discussed in Thread: LMP7721, LMC6001, IVC102, DDC112

Hello,

I am starting the design of a precision electrometer whose purpose is to measure the charge stored in a 10nF or 22nF ceramic capacitor.  The capacitor is embedded in a package with a connector and it is charged elsewhere, far from the electrometer. After charging, the capacitor is connected to the electrometer and its charge is to be measured with a resolution of, at least, 1 pC.  The maximum charge to be measured is about 1100 pC.

I am considering various options:

1) Building the electrometer from basic blocks, i.e. using a LMP7721 or LMC6001 as integrator, followed by A/D conversion chain. In this case I think I should use shielded reed relays for resetting the integrator in order to reduce the injected charge. In any case it seems to be a demanding solution, especially in prototyping stage.

2) Using an IVC102 as switched integrator, followed by A/D conversion chain.  The charge injected by IVC102 internal switches is not negligible.

3) The most compact solution, the one I would prefer: using a DDC112 as switched integrator and A/D in a single chip.

Reading DDC112 datasheet, it is not clear to me if it is suitable for my application, mostly for the following reasons:

a) The high value of the capacitor connected to the input (10nF - 22nF)  could create problems of noise and stability.  However I have to say that I am not interested in the time evolution of the signal, but only in the value of the total charge stored in the input capacitor.

b) The "step nature" of the source: once the input switches are closed, the input current rises as a step because there is a charged capacitor at the input.  In principle, I could limit the maximum value of the current with a series resistor which should leave the total charge value unaffected.  I wonder if the fast rising edge of the current could be a problem for the operation of the whole circuit.

c) The DDC112 datasheet report a maximum full-scale range of 1000pC with a 250 pF external capacitor.  Since I need a full scale slightly higher, say 1100 -1200 pC, what would happen with a 330 pF external capacitor?

Any suggestion?

Thank you for support.

Fulvio Pompili
University of Rome - Italy