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Need Help Identifying IC

MBrantley

Jan 21, 2015
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Hello everyone,

I am attempting to identify the ICs shown below. This small circuit was taken off one of our instruments and the two long leads (CURRENT INPUT 1 & CURRENT INPUT 2) are connected to plates from which current is induced by gas phase ions. The outputs (VOLTAGE OUTPUT 1 and VOLTAGE OUTPUT 2) are digitized by an ADC. I am assuming PIN 1 and PIN 2 are Vcc and Ground, however I'm not sure how to determine which one since there are no discernible markings on the IC itself. This board is typically under ultra-high vacuum (roughly 5x10E-10 torr) if they helps in any manner. How would one go about identifying what these ICs are?

Thank you for your time,

Unknown_Preamp.png 20160127_203058_resized.jpg 20160127_203133_resized.jpg 20160127_203142_resized.jpg
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Well, "what they are" is easy. They're simply op-amps.

The part number... could be almost anything.

- pin 2 - inverting input
- pin 3 - non-inverting input
- pin 4 - Vcc
- pin 6 - output
- pin 7 - gnd

What value are the resistors?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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And what is the supply voltage?

Oh, and by the way, if you've touched the board other than with gloves, I would recommend you clean it before putting it back.
 

MBrantley

Jan 21, 2015
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Hi Steve, thanks for the response. I'm unsure of the supply voltage - the board that was originally driving this is defunct and the manufacturer has gone through two different buyouts. The resistors measured as 5 MOhm.

This board is identical to the one in the instrument, it's not actually removed from the instrument. However, yes in general we keep our UHV components very clean. :)
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Ok, given that the resistors are fairly low resistance (as compared to gigaohms) then you can possibly use a fet input op-amp. You would be looking for one which has low input current and has a valid input voltage range which includes the negative supply rail. Clearly it also needs to be able to drive the output to the negative supply rail as well.

A reasonable test would be a 9v battery in series with 20M connected to the input (w.r.t. Ground). Measure the output voltage of both your potential replacement and your spare. Differences in output will be due to either resister tolerance or input current.

similarly, shorting the input to ground via a 20M resistor should produce a similar output (0).
 
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