Author Topic: Accurate Low Amp Current Probe Advice  (Read 17958 times)

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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #175 on: November 05, 2024, 03:22:49 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. 

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

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #176 on: November 05, 2024, 03:55:04 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.

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. 😉
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #177 on: November 06, 2024, 09:45:56 pm »
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
« Last Edit: November 06, 2024, 09:48:21 pm by KungFuJosh »
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #178 on: November 07, 2024, 03:09:06 pm »
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
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #179 on: November 07, 2024, 03:31:26 pm »
Here it is after 25 and 30 min warmup time.
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #180 on: November 07, 2024, 04:09:43 pm »
Some more tests...

5mA and below was less stable, but not bad. I think that's mostly because what I'm seeing is +/-1mA if I wait long enough.

After 12 minutes, 10mA was showing 11mA. That's significantly better. The previous probe would drift down down down and fall off the screen in a few minutes. 🙄
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #181 on: November 07, 2024, 05:06:52 pm »
Here's a quick stability test. Not terrible! 😉
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #182 on: November 07, 2024, 05:58:14 pm »
Upgrading the NVRAM chip was easy using a $10 CH341A programmer.

I used Rico's BIN file and cleared the data and wrote that to the chip. I dunno if that was really necessary, but I figured it was worth it to test the programmer before inserting the new chip.

I don't have the adapters needed to do an actual calibration, but thankfully, it's pretty accurate already. I'll eventually see about finding or making the needed adapters so I can calibrate it for fun.

I installed the new chip and powered up the 503B, and it worked as normal with the factory default warning. Then I removed the chip, and wrote the serial number to it and reinstalled it in the 503B. No more default settings warning.

I attached the 341A programmer software since I had trouble finding a virus free version. This one was clean.

Thanks,
Josh
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #183 on: November 07, 2024, 09:34:58 pm »
The Tektronix current clamp is still one of the best you can get, especially if it's used and therefore cheaper, because new it costs a fortune.
It's all the more annoying that you made such a bad purchase, I was luckier.

 
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #184 on: November 07, 2024, 09:42:27 pm »
The Tektronix current clamp is still one of the best you can get, especially if it's used and therefore cheaper, because new it costs a fortune.
It's all the more annoying that you made such a bad purchase, I was luckier.

Were you though? 😉

My crap probe got replaced with a good probe for no extra fee. Now I'm messing with the amplifier to see how new lower ESR filter caps affect it.

The new probe measurements looked good, right?
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #185 on: November 07, 2024, 10:10:18 pm »
They do.

Quote
Were you though?

My mixed system (A6302 from the US, AM503A from the Czech Republic, TM502 from Germany) works without any problems. 8)

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #186 on: November 07, 2024, 10:43:53 pm »
My mixed system (A6302 from the US, AM503A from the Czech Republic, TM502 from Germany) works without any problems. 8)

Have you tried a stability test over time?

All my quick tests looked good, and the stability test was decent with the newer probe. After I replace the larger filter caps I'm going to try again after a nice warmup. As I've said before, this is for entertainment and education. I could have left it alone and it would be fine for measuring. 😉
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #187 on: November 08, 2024, 01:55:05 am »
Ya know, I thought replacing some caps might help, but I wasn't expecting this result. 😎

So far, I only replaced 4 caps (2 each):
50V 2000uF Old: 60mΩ ESR    New: 13mΩ ESR
50V 1000uF Old: 120mΩ ESR    New: 11mΩ ESR

That looks slightly better than the test in reply #181.

Thanks,
Josh
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Online Kean

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Re: Accurate Low Amp Current Probe Advice
« Reply #188 on: November 08, 2024, 02:27:26 am »
You can see the quantization in the blue trace, so the red trace surely has different averaging applied on the scope.
 

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #189 on: November 08, 2024, 03:00:01 am »
You can see the quantization in the blue trace, so the red trace surely has different averaging applied on the scope.

The red trace is an SDM3055X-E. The blue trace is the SDS2404X HD. The settings are identical between both stability tests, except I think the 2nd test sampled more frequently (log rate in TC).

I attached both tests to this reply to make it clearer.
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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #190 on: November 08, 2024, 03:45:21 am »
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.
« Last Edit: November 08, 2024, 03:50:15 am by MarkL »
 

Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #191 on: November 08, 2024, 05:08:48 am »
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.
 

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #192 on: November 08, 2024, 12:30:41 pm »
Here's another drift run

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
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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #193 on: November 08, 2024, 03:38:12 pm »
Here's another drift run
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.
...
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.

 
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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #194 on: November 08, 2024, 05:13:36 pm »
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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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #195 on: November 08, 2024, 06:21:44 pm »
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 main focus of the thread was for the probe, which I settled on the A6302. It's good to see data with different amps with it...and I'm glad I got the 503B, even if it did need some work. 😉
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #196 on: November 08, 2024, 09:23:19 pm »
Stability Test 3 Setup:
SPD3303X at 3.05V with a 100Ω CMF resistor.
SDM3055X-E in-circuit for current reference.
A6302 on negative lead of PSU.
AM503B connected to SDM3065X via 50Ω termination.

PP (min and max values): 235.5826µV over ~50 minutes.

Thanks,
Josh
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Online Kean

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Re: Accurate Low Amp Current Probe Advice
« Reply #197 on: November 08, 2024, 09:29:32 pm »
The red trace is an SDM3055X-E. The blue trace is the SDS2404X HD. The settings are identical between both stability tests, except I think the 2nd test sampled more frequently (log rate in TC).

Ah, sorry.  I recognized the screenshot as being TestController, but it didn't register that the measurements were from two different devices.  I was thinking you were showing before and after.
 
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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #198 on: November 08, 2024, 09:38:40 pm »
Stability Test 3 Setup:
SPD3303X at 3.05V with a 100Ω CMF resistor.
SDM3055X-E in-circuit for current reference.
A6302 on negative lead of PSU.
AM503B connected to SDM3065X via 50Ω termination.

PP (min and max values): 235.5826µV over ~50 minutes.

Thanks,
Josh
So... If you're still using 10mA/div to be comparable with previous tests, that's a drift of 0.235mA.  Half of what I was getting.  Not bad!

Edit: Ah, right.  The numbers say it was 10mA/div.  Great!
 
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #199 on: November 08, 2024, 09:43:44 pm »
A good result that I am tempted to reproduce.
 
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