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Accurate Low Amp Current Probe Advice

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Martin72:
Battery does not have to be.
For small currents I used a 1k resistor, for larger currents 100 Ohm, and then adjusted the voltage and current with the power supply unit to achieve the desired values.
See link in reply #152

KungFuJosh:
Speaking of batteries, the battery in my AM503B is weak (.6V, should be ~1.5V), so I removed it to see what it is (390 or SR1130SW). Removing it cleared the NVRAM and reset the amp to factory default (355 error after power on).

You had mentioned before that the AM503 normally starts up with ref coupling, but mine had always defaulted to DC, never ref on start. After reinserting the dying battery, now it starts up as you said in ref coupling. I wonder what other weird things were set. 🤷 Anyway, I ordered a new battery.

Thanks,
Josh

KungFuJosh:
This probe has some serious drift. Note the timestamps and scale offset. This was after over 30 minutes warmup time. I'll let it warm up longer and see if it ever stabilizes at all.

Thanks,
Josh

KungFuJosh:
Here are those tests. They're not terrible, but none are dead on. I degaussed multiple times throughout the process to try and get it closer.

I also had one of my SDMs in circuit to verify current level was accurate within at least 1 or 2 decimal places.

Thanks for the tests, now I think I understand how the current amplifier works with regards to the scope scale. 10mA/div is 1:1 with 10mV/div on scope, and each variation changes the ratio.

This needed over an hour to get to the mediocre stability it had. I'm assuming that's more the probe's fault than the amplifiers. Am I also correct to assume the amplifier seems good, but the probe is not great?

Thanks,
Josh

MarkL:
If you lost your NVRAM, you also lost your gain calibration constants.

Take a look here:

  https://w140.com/tekwiki/wiki/AM503B

And specifically the documents:

  AM503B/5030, A6300 067-0271-00 Verification and Adjustment Kit Manual 070-9352-01
  AM503B/AM5030 Calibration Adapters

You might want to go through the verification procedure to at least determine if the inaccuracy is due to the amp or the probe.

For the AM503B, calibration requires temporary installation of the GPIB option card (as found in the AM5030), a TM5000 mainframe, and a computer with GPIB.  Fortunately, someone has reverse-engineered the contents of the NVRAM, and it's possible to just write the new cal values directly to the NVRAM:

  https://groups.io/g/TekScopes/files/AM503B%20Calibration%20without%20GPIB/AM503B_AM5030%20NVRAM%20Replacement_V01.pdf

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