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Differential scope probe--with gain

(1/1)

Wimberleytech:
This weekend, under pressure to get some results, I needed to measure current in a feedback path--a relatively low current--microamps.  In this application, I could afford to put a series resistor (suffer the burden voltage that resulted) to get something to measure with my scope using two probes and taking the difference (CH1-CH2).  However, there was just not enough sensitivity for a good reading.

Some time back, I made an EKG amplifier using an AD624, so I thought, hey why not?  I got it out of the "project" bin, modified the gain to 100 and used it to make the measurement.  Fortunately, at a gain of 100, the BW was sufficient, and there were no common-mode issues to contend with when powered with +/-15V.

It worked.  I used a 1k resistor, which seems big, but for this application, not an issue.

Showing the schematic is a bit laughable because the AD624 requires almost no external components but here it is anyway.

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