Is this op-amp uV/C that high?

I

AD8551_8552_8...

What exactly is an "electrometer"?

Just be very careful about condensation.

John

--
Evacuated, back-filled with dry N2, and hermetically sealed oughta do
it.

JF

A Voltmeter with infinite input impedance.

Way to much work, just servo the temperature to stay at Room temp. George

AD8551_8552_8...

You got some good posts below. My question is can you contact the factory and pay for screened devices?

I never designed any precision op-amp, but it has been my experience that drift gets worse when you add hooks to the circuit to correct for offset. That is, a part with high offset (untrimmed) may have lower drift than the same part that has been trimmed.

datasheet:

Yep. Nothing has been said, AFAICR, about bandwidth. In similar cases I've made a zero-measure-zero-measure... system. Then you don't give a hoot about offset or offset drift.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
Obama snickers, claiming McCain is running for Bush\'s third term,
While he thinks running for Carter\'s second term is a good thing?

...

Maybe something like this? The thermal mass of the liquid-filled cylinder would help stabilize the temperature, acting as a sort of output bypass capacitor (provided the cylinder did not somehow lose its seal).

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Don't pay for screened devices. It's unlikely to be easy for the factory (unless they already do it-and charge extra for 'B' versions). It's also unlikely to survive shipping and soldering and aging. Your best bet is to trim it after all assembly is complete.

Heck, thermal gradients in the printed circuit board can create thermocouple voltages, you HAVE to trim it after assembly and warmup.

It's possible to trim a uA741 for minimum Voffset (the usual way), or to sense the input stage current at the adjust pins, and trim for matched emitter current. Zeroing the emitter-current imbalance in the input stage results in minimum offset voltage temperature coefficient. Many precision amplifiers (the technique was pioneered in the uA725) are designed so that the offset voltage and the thermal drift are simultaneously minimized. That's true, at least, for bipolar-input precision amplifiers...

Thanks for all the helpful advice! BTW is there a better site to post than from google groups? A few days ago I typed a good size post, clicked Send, and google said I made too many posts in a given time period.

Anyhow, I'll try to add more insulation around the DUT to dampen out temp changes, and will implement a mechanical switch that when pulled will flip the DUT polarity across the electrometer circuit, which should eliminate the concern for temp drift, hopefully. Sometimes designs doesn't work as good as on paper, but I'll see. :-)

The circuit was build yesterday with the exception of filtering noise. I placed a fixed resistor as the input for initial testing. Oddly enough when the resistor is flipped the electrometer output changes from 30mV to 500mV DC! For now all I can think of is that there's a bit more wiring in one position. So perhaps stray noise from radio station for instance being rectified by the op-amp? Today I'll try to move the circuit around to see if it's due to RF noise, and perhaps place the circuit inside the shield. I have extensive shielding waiting on the side, but it's a pain to work with a circuit inside the shield. ... Ah, could it be from lack of grounding? It's battery operated, so while the circuit is outside the shield there's not much ground material. Not sure, as this is my first real electrometer op- amp.

Thanks for all the help!! Paul

I'm wondering. The INA116P datasheet shows max Ib as 25fA. So that's taking such a chip and measuring the *total* bias current, correct? I mean, they're not subtracting thermoelectric effects are they?

Thanks, Paul

If you want to do this regularly, get a real newsreader program and a server subscription; Forte Agent can furnish both.

Even 30 mV means something goofy is going on.

But be careful about the relay: it can inject charge when it switches, and it can have many microvolts of thermal EMF with ugly time constants. The tiny telecom-type latching relays are great thermally, since you only have to pulse the coil for a millisecond to get them to flip.

John

or

y

When you say, "be careful about the relay: it can inject charge when it switches, and it can have many microvolts of thermal EMF with ugly time constants." Are you referring to a current driven coil type relay or a mechanical relay/switch? What I had in mind was a small mechanical switch setup such that the slight tug of a non-conductive string would flip the mechanical switch. The string would enter the shield through a pin hole and to the mechanical switch.

Are these the type of relays:

Part #: PB1117-ND

?name=3DPB1117-N= D

Part #: PB1091CT-ND

?name=3DPB1091CT=

-ND

Thanks, Paul

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Hi,

I'm using a dual non-magnetic SPDT mechanical micro-switch to essentially reverse the DUT across the electrometer input. I couldn't find a DPDT in this non-magnetic switch. The electrometer gain is 3 and the second stage op-amp (OP07C) gain is 213 for a total gain of

639. I know, the OP07C design is old. It's what I have on hand, relatively low Vos, nice and slow =3D good for DC. The OP07C chips are new. Also they don't have chopper noise. I'd be more than happy for an improved equivalent recommendation. The 2nd stage op-amp will eventually be used as a voltage-to-current converter since the feedback will be an LED, but for now the feedback is a 100K ohm resistor. Also there's an offset pot for the second stage op-amp. The input DUT is 850K ohms. Each of the electrometer op-amp input pins is tied to ground by a 29M ohm resistor. The datasheet shows 100M ohms resistors, but I don't have any.

An update: I placed the circuit inside the shield and as expected/ hoped the output went down to normal values regardless if the input was reversed or not. So I guess it was just RF signals (radio stations and such) causing the problem. Electrometer op-amps are probably more sensitive.

Regards, Paul

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BTW, as much bias current as possible was eliminated by bending the input pins up in the air and soldering the input pins to the DUT without touching the board. Also the entire electrometer op-amp is held in the air by the wires so that no part of the electrometer is touching the board. This not only produces the least amount of bias current leakage, but also allows me to easily clean the entire electrometer top, bottom, and sides of any residue or dust. I've read about that technique in several femto bias current op-amp datasheets. Air is considered nearly perfect in terms of leakage.

Paul