Products > Test Equipment

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

<< < (29/33) > >>

dcac:
It was just one I had in my caps box - I believe it's an Elna low ESR type so not really the specifically low leakage variants. I really only wanted to compare the leakage with the mounted 470uF/25V and that 1000uF/10V I found not only had double the capacitance but also considerable lower leakage and it was the same diameter 10mm but slightly taller 20mm vs 13.5mm but still no issue to fit it.

The drift now is less than 0.5mV after 5 minutes in high gain mode so often plenty of time to make the measurement but of course not stable enough for any logging purpose. And it's easier getting it to zero - with the original cap you had to hold the button for more than a couple of secs and then often press it again and again to really get it to zero - now it locks on much quicker.

dcac:
Updated Schematics - just an updated value for C15 and that R18 is a PTC - also added possible types for the HALL sensors (suggested by Zhao).

C15 is set to 2.7nF - I measured it earlier to 3nF but forgot to subtract 120pF for my test coax cable. And the fixed output coax on CC-65 is probably 100-150pF and is also in parallel with C15.

dcac:
I had a look at the noise performance and wanted to compare with what the spice simulation shows.

I measured CC-65 output noise in high gain mode to be about 2.5mV P-P but this is on a 100Mhz oscilloscope. My Fluke 45 shows it as about 0.5mV RMS. But I noticed how sensitive CC-65 is to EMI - just holding the probe in my hand I can get almost 3mV P-P 50Hz hum signal. Interestingly the front of the probe - or the actual clamp section - isn’t that sensitive to EMI so adding some shielding around the amplifier section should really be worthwhile if you plan to measure low level signals.

Simulating the noise in Micro-cap 12 I get about 133uV RMS output noise at 50KHz BW. This is with the Hall element simulated as generators with 400 ohms source resistance and with the TLC272 opamps.

If replace the opamps with TLC2272 which is an improved version that has lower noise at 9nV/sqrt(Hz) compared to 25nV/sqrt(Hz). I get 104uV RMS noise - so some improvement compared to 133uV RMS. But compared with the output noise I measured from CC-65 (roughly) 500uV RMS the overall improvement will probably not be noticeable as the Hall elements seems to contribute the majority of the noise them self.

And the simulated current consumption increased from 2.9mA to 3.3mA with the TLC2272

But keep in mind the measured noise is more a ball park value as the BW it represent isn’t well known. But from my previous experiences with Micro-cap noise simulations they seem to mimic the real world pretty well.

Below are RMS noise traces for TLC272 and TLC2272 - I only changed opamps at U3-1-2 as these stand for the majority of the amplification.

EDIT: There seem to be an error in the TLC272 simulation - there are four different spice models for these parts, it has to do with the supply voltage level and I don't think I selected correct model.

toli:
The noise density of 25nV/rtHz even if we assume this is the only source of noise (that is U3-1 and U3-2), we still get 35nV/rtHz. For a -3dB BW of >20KHz, and single pole response the noise-BW is 30KHz. The gain of the first stage would be (R22+R15)/(R6+VR1) (I've neglected the path via VR2 here). The second stage is R17/R16. So overall gain assuming VR1 is about half its max value would be (18.6/1.06)*(26.1/11) which is >40. I wrote it down explicitly just so that its easy to check my math in case there's an error.
We can calculate the total RMS noise as:
Input noise density * gain * sqrt(BW), and we use only the white noise density which is somewhat optimistic but reasonable. So we get 35e-9 * 40 * sqrt(30000) = ~0.25mV. Since we have a transfer gain of 10mA/mV this is ~2.5mArms.
This all assumed only U3 noise is relevant, and that its a white noise source. So in practice this will probably be higher than this value.
C5/C6 will add a pole, but C14 seems to offset this partially. This will therefore result in lower noise-BW, but with limited benefit, perhaps 20% at most. So that's not too far out of your simulations, about 3-4dB difference.

The TLC2272 does look significantly better than the original unit with good power consumption too. Another good option might be the OPA2196. It has lower noise (15nV/rtHz wide band noise), similar BW (2.5MHz GBW), lower offset (100uV max), can swing closer to the supply rails (might allow extending output range further in the sensitive range), and lower power consumption (140uA typical per channel). If extending BW to 100KHz is sufficient, it might be an excellent match for this application.
I've also given this some more thought in the mean-time while waiting for my unit to get here. My aim is to extend BW among other things, and I want to see how far I can effectively stretch it with minimal effort. Therefore I'm willing to pay for it with some power. My current plan is to go with the OPA1652 for U3, which has a few benefits in addition to noise density. As far as noise is concerned though, it should reduce noise density by about 10dB for the overall transfer function if my math is correct, this should be easy enough to measure on my typical audio measurement setup that covers this frequency range. I have a few other modifications planned, I will get to it when I get my probe and order the replacement parts for it. I will be sure to update here as well as post about it with some background on my blog like I typically do for such things.

Regarding sensitivity to EMI, I wonder at what frequency is it most sensitive? Perhaps at a few KHz and above? Looking at the schematic, the possible problem with this structure as I see it is that it ignores the reason we have the 2 sensors to begin with. By bypassing R13 with C11, the input signal to the instrumentation amplifier is no longer an average of the two sensors like it is at low frequency. This means we no longer attenuate ambient magnetic field by averaging the two sensors in reverse polarity. Therefore, if it was me, I'd remove this bypass capacitor and modify the amplifier transfer function to get around it if needed.

dcac:
In my previous post - there seem to be an error in the TLC272 simulation - there are four different spice models for these parts, it has to do with the supply voltage level and I don't think I selected correct model.

I'll post new simulations when I figured out which model is relevant for the CC65 circuit.

EDIT:
I think this should now be correct. The TLC272 error was quite substantial - simulated RMS noise is now 256uV compared to 133uV previously. Still though compared to the overall noise from the CC65 it would not make that much difference.

So Simulated RMS noise at 50KHz BW:
TLC272 = 256uV
TLC2272 = 108uV
OPA1652 = 67uV

Simulated current draw:
TLC272 = 2.9mA
TLC2272 = 3.3mA
OPA1652 = 5.0mA

Below are the traces for RMS noise and the Output frequency response - interestingly OPA1652 has slightly better reach even with the same capacitor values around the U3 lowpass filter.

TLC272 = Purple trace
TLC2272 = Blue trace
OPA1652 = Red trace

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod