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Accurate Low Amp Current Probe Advice

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MarkL:
Sorry for the delay in getting this test done, but here's my AM503B drift with an A6302.  I duplicated your test with a 30mA test current over a 30 minute measurement time and after a 40 minute warmup.  I pressed the "AUTOBLANACE" with the power supply disconnected just before the recording started.  The AM503B was set to 10mA/div.  By the time the test was done, the AUTOBALANCE button was blinking, indicating it was time for a re-balance. but I don't know exactly when the blinking started.

This particular AM503B came to me with a dead battery, which I have since replaced, but I have not gone through the re-calibration procedure.  So, the absolute current reading is expected to be off.

I think it does not exhibit as much short-term variation as shown in your test.

Edit: Additional details: Constant 30mA current supplied by a HP 3631A and measured by an HP 3478A DMM.  AM503B output measured by an HP 3456 DMM with a 50R pass-through terminator.

MarkL:
Here's another drift run using the exact same setup and procedure as outlined previously.  The only difference is that everything has been powered for two or three hours, and the monitoring time is a little longer.

The absolute current output from the AM503B is now closer to the actual (as read by the 3478A) and drift is slightly less.

KungFuJosh:

--- Quote from: MarkL on November 08, 2024, 05:08:48 am ---Here's another drift run

--- End quote ---

It looks like your PP is ~0.5mA? After switching the 4 larger caps, mine went from 4.5mA to ~0.2mA PP. I'll run the test again later today with my DMMs instead of using the scope.

Thanks,
Josah

MarkL:

--- Quote from: KungFuJosh on November 08, 2024, 12:30:41 pm ---
--- Quote from: MarkL on November 08, 2024, 05:08:48 am ---Here's another drift run

--- End quote ---
It looks like your PP is ~0.5mA? After switching the 4 larger caps, mine went from 4.5mA to ~0.2mA PP. I'll run the test again later today with my DMMs instead of using the scope.
...

--- End quote ---
Do you mean Pk-Pk drift?  Yes, it's in that range for both plots.  If you mean Pk-Pk noise, with a scope I'm measuring 5mA, and the AC RMS noise as 0.5mA, both with the 20MHz BW limiter on and no current going through the probe.

I'm surprised that replacing the filter caps improved your drift, if that's what you mean.  It's completely within reason that it improved the noise, especially if there was a 120Hz component.  An FFT would show that clearly.

Given the random variation in your plot, perhaps the scope was measuring the DC level AND the noise.  A DMM is a better approach for measuring slowly changing DC levels, since it's going to reject all the other garbage.  I'm interested to see your results.

MarkL:
I know we're talking about the AM503B in this thread, but since I have it set up I thought I would share what the drift looks like on the older AM503.  Same test scenario, and using the same A6302 probe.

The AM503 is an all analog implementation, with front panel buttons and knobs for manual degaussing, balance, and DC level.  The DC level requires constant attention, even in short time spans, which is why the AM503B is such a pleasure to use with its AUTOBALANCE button.

The amount of drift over a 30 minute span is much improved in the AM503B by around a factor of 3 or 4 in these tests, as is the output stability for a minute or less.

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