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Hantek CC-65 AC/DC Current Probe Teardown and Testing
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toli:
I meant moving R18 outside the feedback loop (connecting R17 at the left side of R18 instead of the right side) would fix the stability issue.
For 20KHz this isnt a big problem probably, but for extended BW this is needed obviously.
dcac:
Oh I get it now, yeah for a higher bandwidth the current configuration is far from ideal.
toli:
Thanks dcac for your help so far, appreciate it  :-+
I will be sure to update here when I have a chance to play with my probe when it gets here in a few weeks time.

The parts used really makes you wonder why they aren't offering a higher spec unit for slightly more.
For instance, from a noise density standpoint, the amplifiers used there are the P272 if I understand correctly from the pictures online. These are 25nV/Hz, the resistors in the feedback loop of the first stage will give some extra noise but its not dominant compared to the opamps used. So if we take 2 such opamps for the instrumentation amp input we get ~35nV/rt(Hz). Lets round that up to 40nV/rt(Hz) with the feedback resistors + second stage + resistors infront of it which are ~1K in series to each sensor on each side. For the lower current range given the gain of both stages (x60 combined gain assumed) and translation to equivalent input signal this will result in ~24uA/rt(Hz). As a sanity check, assuming ~20KHz -3dB BW, single pole means noiseBW is ~30KHz. So 24uA/rt(Hz) * rt(30K) will give ~4.25mArms noise which seems reasonable from the info shared in this thread.
With the replacement opamps suggested here in this thread (OPA2350) the noise of the opamps is no longer dominant at 5nV/rt(Hz) per amplifier. So now the opamps and resistors will be more balanced, and the overall noise density will be ~X3 (10dB) lower than the original. If we modify the feedback network around U3 to use lower value resistors we can get this down further by another 2-3dB.
A similar argument could be made for the BW. These opamps have a GBW of >1MHz (the stock units), so its possible to get well over 20KHz from this unit. Splitting the gain evenly across the stages could achieve ~150KHz BW at the cost of noise (I didn't measure slew rate limitation though, so this might only suffice for limited amplitude). A better opamp would not cost much and will give higher BW with lower noise floor with minimal circuit modifications.

The main down-side to using higher spec opamps (BW and noise) would be the current consumption which is X2-X3 higher and will reduce battery life. But this one could have been easily solved in a higher price unit with an integrated battery of higher capacity. The higher integrated noise can also be circumvented by a BW limit switch, so that a much lower noise can be achieved for a limited BW.

All the noise numbers above are for high frequency. It does rise rapidly at low frequency, and at low frequency (<300Hz) the OPA2350 is actually noisier than the P272. As a possible alternative, I quite like the OPA2156. A quick look at it seems promising with lower noise across the band, sufficient GBW, sufficient max voltage rating, etc.

BTW, looking at the teardown photos, R18 doesn't seem like a resistor at all. Edit: yep, its a PTC indeed/EDIT. That would explain why its inside the feedback loop (to limit amplitude and timing distortion due to its non linearity) despite the fact it has an undesired effect on stability of this stage.
dcac:
Nice job identifying the PTC - that seems rather sophisticated choice for a device at this price point - I think I paid about 32 GBP for it a couple of years ago. Not that PTCs are really expensive but that they actually bothered to add that extra protection as supposed to just having a resistor there.

With regard to improving the probe I’ve made two mods to mine - I replaced C4 with a 1000uF/10V high quality cap - this have lower leakage and lower dielectric absorption so the zeroing process is and bit more distinct now and the larger value cap means less/slower drift. And then I lowered R24 to 1K to compensate for the increased charge time of C4 and to make zeroing slightly quicker again. But I never really measured the actual overall improvement except for the leakage in C4 which was significantly lower in the new cap and as it was an easy dropin mod in it went and then a 1.5K resistor in parallel with the 3.3K R24.
toli:
Replacing the cap is probably a good idea actually since its the dominant leakage source at this point, only problem is getting a cap with low enough leakage. This can be a bit of a trial and error perhaps. Did you find one that fits and has a good spec on the leakage or did you pick one based on measurements?
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