Peter L. Peres wrote...

>I am looking for an electrometer opamp. I have looked at LMC6001, AD549,
>OPA129 and a few other choices (including discrete JFET input and some
>input current compensation schemes with 'normal' opamps) , but I'd like =
to
>know opinions from people who have used them really. The device to be
>measured is an experimental capacitive sensor with ~5 pF of capacitance.=
 I
>want to measure charge on it. This amp is to be a buffer between the
>sensor and other measuring equipment.
>
>Any pointers are welcome. The amp should be compensated for unity gain.
>Single rail psu capability and including V- in CMR is an advantage.
>LMC6001 does this.

I use a lot of National Semiconductor's LMC6462, LMC6482, LMC6484 and
LMC6464 series of opamps.  These are all unity-gain stable, work well on
a 5V single supply, have rail-to-rail I/O, and have input currents down
in the femtoampere region.

One parameter you may need to consider in an opamp for this application
is input capacitance; most opamps will have at least a few pF of input
capacitance, and some will have much more.  With a capacitive sensor of
only ~5 pF, there will be some attenuation caused by the opamp's input
capacitance.

If the signal out of your sensor is AC and your application doesn't care
about DC errors through the buffer amp, a discrete JFET circuit designed
for low input capacitance might be best.  The NSC application notes have
some that might be suitable.

Dave

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