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Accurate Low Amp Current Probe Advice

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KungFuJosh:
Here's the manual for the TM502A & its PSU: https://w140.com/tekwiki/images/e/ee/070-6502-00.pdf

KungFuJosh:
For the PSU caps, I'm leaning towards these:

MAL205658472E3

LGU1E183MELB

Thanks,
Josh

MarkL:

--- Quote from: KungFuJosh on November 10, 2024, 11:09:41 pm ---
--- Quote from: MarkL on November 10, 2024, 11:02:58 pm ---Was your measurement technique consistent in your comparison?  You had swapped in the DMM in place of the scope at some point.

I would expect the large filter caps to affect noise and not so much the slowly changing DC drift.

--- End quote ---
Yeah, I left everything setup exactly the same. Nothing was moved.

The initial change from 4.5mV to 0.2mV was done using the scope. Test 3 with the DMMs was 0.236mV over a longer period (see attached, and note the names displayed for each device). The only change was the 4 larger caps in the 503B at the time.

--- End quote ---
As I mentioned previously, the 4.5mV Pk-Pk measurement looks more like random noise to me, which is completely within reason as being corrected by the new caps.  I think it's difficult to conclude anything about slower moving, and possibly much smaller, DC drift because of the randomness of the measurement.

Perhaps the measurement selected was capturing higher frequency noise, or line-related noise not on a cycle boundary (like measuring RMS on the whole screen with only a cycle or two visible).  And all of these things depend on the sample rate and possibly capture size (don't know - I don't have a Siglent scope).

I would pick a slow sweep rate (like 100ms/div), hi-res (aka box car averaging), and no trigger (free-run).  Then ask the scope to read the RMS of the entire screen or capture buffer (whatever is possible on the Siglents).  You could also ask the scope for the statistical average of a number of RMS measurements as a different way to do averaging.

But still a DMM is a better way to measure this drift.  The DMM "before" cap replacement measurement is not available for a direct comparison.

MarkL:

--- Quote from: nctnico on November 10, 2024, 11:09:15 pm ---...
I probably missed this but are there any schematics available? The only cause for slow drift I can think of where it comes to electrolytics would be leakage current. But this would mean the large capacitors would be part of some kind of (slow) servo mechanism. Maybe to cancel DC magnetic field in the probe head? The schematics should tell what is what.

--- End quote ---
I'm not aware of any schematics either.  The board layout is definitely different between the A and B, but a quick glance says that some of the components are the same (processor, attenuator, output amplifier, etc.).

The four big filter caps in the A schematic are involved in supply filtering, and are located in the rear near the backplane connector as would be expected.  In the B, they are also in the same area, although the layout differs a little.  And the B uses 2200uF instead of 1000uF.  They are almost certainly performing power filtering.

My guess is that there was a lot of power line hum showing up in the output which was resolved by the new caps.

KungFuJosh:

--- Quote from: MarkL on November 11, 2024, 12:12:21 am ---As I mentioned previously, the 4.5mV Pk-Pk measurement looks more like random noise to me, which is completely within reason as being corrected by the new caps.  I think it's difficult to conclude anything about slower moving, and possibly much smaller, DC drift because of the randomness of the measurement.

Perhaps the measurement selected was capturing higher frequency noise, or line-related noise not on a cycle boundary (like measuring RMS on the whole screen with only a cycle or two visible).  And all of these things depend on the sample rate and possibly capture size (don't know - I don't have a Siglent scope).

I would pick a slow sweep rate (like 100ms/div), hi-res (aka box car averaging), and no trigger (free-run).  Then ask the scope to read the RMS of the entire screen or capture buffer (whatever is possible on the Siglents).  You could also ask the scope for the statistical average of a number of RMS measurements as a different way to do averaging.

But still a DMM is a better way to measure this drift.  The DMM "before" cap replacement measurement is not available for a direct comparison.

--- End quote ---

Regarding the scope version of the tests, nothing was changed except the caps. It could very well have been AC garbage causing the higher PP, which is why I tried that. Whether or not that's affecting the drift directly, I dunno. 4.5mV PP noise certainly could mask a much smaller drift that was hiding in there.

The only thing that makes me wonder about that though is that before changing the caps, if I waited long enough, the signal would drift right off the scope screen. 🤷

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