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Hantek CC-65 AC/DC Current Probe Teardown and Testing
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Bob Sava:

--- Quote from: mickab on November 14, 2020, 08:51:27 am ---Indeed this is a typo error for the OPA195. ;)

In first place U4 was an OPA2350 but the response wasn't great due to the poor capability of driving capacitive load.
C15 represent the C-Load and is needed for noise attenuation.

Good observations Mechatrommer ;) the OPA2350 can't work at 9V but the regulator of the clamp is set at 7V.
I think we can replace all op amp with OPA2192. This will need some simulation and testing but it is possible. ;)
Tell me if you're interested.  :D

--- End quote ---

If capacitive load on U4-1 is an issue - is resistor R18 different than 1k?
Noy:
So whats best now?
Every opamp OPA2192 or mixed with OPA2350?
newbieswe:
Does anyone have a parts list for the caps used in the modded schematic?
I'm about to order but don't know which size specs to choose.
If anyone have a complete parts list, that would help a lot.
Noy:
All 0603 ones. The caps you need are all so small that you can choose X7R or They are only available as C0G.. So you can use >25V types..
All OPAMPS are MSOP..
toli:
Glad to have found this thread. I've ordered a unit yesterday, and was looking for the schematic in the hope of extending the BW further.

Looking at the schematic posted on page 5 of this thread, some things seem a bit odd. Did anyone else trace out to circuit to see this schematic is correct?

Most of all the output difference amplifier around U4 seems strange, but a few other things can be extended in BW too.
1 - R18, which in theory should help isolate the amplifier from the capacitive load, is wrapped inside the feedback loop which means it doesn't isolate the amp from the capacitive load, it can actually make stability worse. I would expect R18 to be located after the feedback point (or in other words, for R17 and its added bypass cap to connect to the output of U4 instead of after R18).
2 - C15 seems very high in value. R18 + C15 will create a low-pass-filter with a pole at 1.6KHz with the values shown. Since its inside the feedback loop, the opamp will try to compensate for it, but with the limited GBW of the opamp it will only be able to do this for a limited BW (and even that only with small signal amplitude). Since the measurement results people have posted seem to show fairly high BW compared to this 1.6KHz, the value of C15 is probably quite a bit lower than this. Perhaps its 100pF and its a typo only?
3 - 22pF in parallel to R17 will add a pole at ~280KHz, and I'd expect to see a matching capacitor to bypass R20 for a typical difference amplifier structure. This could perhaps explain some of the uneven response seen at >100Khz.
4 - The first stage around U3 also has C5/C6 which are 8.1pF (8.2?) following the mod, so in parallel with 11Kohm its a pole at ~1.8MHz. That is assuming an opamp with high enough GBW. At the low current range the gain at this stage is probably ~X20 so we need an opamp with GBW on the order of 36MHz to get to this 1.8MHz limit in this case. In the PDF file attached a few posts later I see these caps are 15pF, this will limit the BW further to ~1MHz, so a GBW of ~20MHz should suffice in this case.

A few thoughts (which I can't do anything about until I get my unit :)):
1 - My first guess would be that the value of C15 is off and its actually much lower in capacitance. Even then, it can probably be omitted completely after the mod. If we assume ~100pF load (cable + scope input) and R18 of 1K its a pole at 1.6MHz already, so adding additional capacitance will limit this further. R18 can probably be made smaller in value quite a bit (say 330R) to push out the pole its creating with the load capacitance, which would allow extended BW.
2 - The feedback for U4 will be taken (we can modify that fairly easily if it isn't the case) from output of U4 instead of after R18. In this case the 22pF in parallel to R17 can potentially be omitted completely, which would align R17 with R20 with no bypass and should result in a more even response at >100KHz and better CM rejection at the frequencies. If some capacitance is still needed there, we can scale down R16/17/19/20 to extend BW, which would make this stage less sensitive to parasitic capacitance if this is a problem. It will also reduce noise for this stage which can help in the high current range where first stage gain is reduced.
3 - This one is more of a trade-off. Currently the first stage seems to be operating with a gain of >X20 in the low current range (this is assuming the trimmer in the schematic is roughly in the center position), while the output stage has a gain of ~2.5 (for the low current range only, in the high current range the first stage gain is lower). This means the first stage needs to have a higher GBW as its more limiting in terms of BW. Moving more of the gain to the second stage can help extend BW further in the first stage which would improve overall BW. The down side here is that it will result in some noise degradation. A possible solution to this can be to move the gain switching to the second stage, and leave first stage with a constant gain of X5-X8 that will render noise of second stage non-dominant, but that's a bit more work.




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