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'[PICLIST] Electrometer amplifier: ideas ?'
2001\12\23@152715 by Peter L. Peres

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Hi all,

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.

tia,

Peter

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2001\12\23@160142 by Dave Dilatush

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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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2001\12\23@173250 by Friedel Bruening

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Have you ever looked into INA116 ? This may safe you from quite a hazel.

http://www.burr-brown.com

Friedel

At 10:30 p.m. 23/12/01 +0200, you wrote:
{Quote hidden}

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2001\12\25@122137 by Peter L. Peres

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Dave D., thanks, I have considered the package pin capacitance and it is
more or less ok iff it is very stable. Maybe I will use a discrete JFET
front end which has a lower package capacitance. The main idea is to have
stable readings for up to 1 second (or at least predictable slope). This
includes thermostating the amplifier probably. I've done that before, it
works well. I have done some crude testing using a section of a TL074 and
I need it about 10 to 100 times better than that. The DIL TL074 drops
roughly 1V/sec on package capacitance alone + 5pF external at room
temperature. This is in line with the data sheet. If I'll add a JFET stage
before it I may even get there without a special opamp.

thanks,

Peter

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