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Accurate Low Amp Current Probe Advice

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MarkL:

--- Quote from: KungFuJosh on November 04, 2024, 09:49:08 pm ---
--- Quote from: MarkL on November 04, 2024, 06:45:15 pm ---They all have various drift, noise, and fidelity issues.  I use the AM503B the most because I like the auto balance, but it has processor clock noise in the output which can be annoying. 

--- End quote ---

Is there anything we can do to mitigate any of those issues? Currently (pun intended), the drift is the most annoying issue.

--- End quote ---
I don't know; I haven't spent time to figure out the source of drift.  And it could be multiple things.  It's why Tek added the auto-balance feature.  And DC measurements can magnetize the core and add an offset, so depending how accurate you want to be, it may be important to de-energize the circuit and check for a zero reading.

I will set up a quick test and plot the drift after a 30 minute warmup so you can compare and determine if your drift is out of line.

On the clock noise, I was considering a circuit that would pause the clock when making sensitive measurements, but it was easier to swap in the AM503 in those cases.

These probes aren't great for long term current measurements or in the range of a few ones or tens of mA.  I think it was mentioned earlier, but in those cases a differential preamp and shunt resistor would probably be better provided the preamp can sufficiently reject the CM range at the shunt.

KungFuJosh:

--- Quote from: MarkL on November 05, 2024, 03:22:49 pm ---These probes aren't great for long term current measurements or in the range of a few ones or tens of mA.  I think it was mentioned earlier, but in those cases a differential preamp and shunt resistor would probably be better provided the preamp can sufficiently reject the CM range at the shunt.

--- End quote ---

I got this primarily for entertainment/education. My most common range will likely be between 20mA to 40mA DCI. I don't need long term measurements, but I do want accuracy within a few minutes of zero/degauss.

Stability was better after a minimum of 60 minutes. However, my probe isn't great. The seller is allegedly sending a replacement.

I'll probably spend more time messing with the probe/amp than actually using it. 😉

KungFuJosh:
I should have the replacement probe and the parts from Mouser tomorrow, and I will be doing the chip change and calibration.

Rico, who created that NVRAM replacement mod/documentation shared his spreadsheet with me, and said I can share it. Another member of the group also shared his version, and I've combined things from both for this new version (attached).

Rico's .BIN file for NVRAM comparison is also attached, along with the PDF instructions.

Thanks,
Josh

KungFuJosh:
The replacement probe arrived today, and it's much better. Without any warmup time, I tested it and it was right on for 100mA test. Drift after 10 minutes of warmup time is still within 4mA (2mA on screen). I'll test more later after proper warmup.

The previous probe was never this accurate or stable, even after an hour warmup. The replacement is in much better shape physically too.

Thanks,
Josh

KungFuJosh:
Here it is after 25 and 30 min warmup time.

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