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

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Maciej:
I would like to better understand supply / bias of Hall sensor.
At first, opamp U1-2 seems to have positive feedback thru R1, R2, or just circuit drawing mistake?
As I understand the Hall is current driven. Hall input resistance is in this case the current sensing resistance, diode D1 provides reference.
Input resistance of Hall changes exponentialy with falling temperature, if we look at here:

Would it be better to stabilise supply current with just another resistor?
I thing that bandwidth of U1-2 regulator should be nearly as low as temperature changes. It would help to cut off lot of noise. Just an idea.

toli:
R1/R2 provide negative feedback, the PNP device adds its gain which is inverting.

Making the loop slower for reduced noise is indeed a valid point (but the question is where the noise is coming from, at the input to the amplifier, or due to the components in the current path at which case the BW should actually be higher to clean it). In this case you are indeed right, its better to make it slower, which is exactly what I did in my probe via the new 1uF capacitor which reduced noise significantly. It provides local negative feedback, stabilizing the loop, but also making the BW far lower in the process which improved noise further.

I'm not sure I understood your question about the change of bias circuit with just a resistor, do you mean remove the diode completely for zero tempco? Generally speaking, depending on the range of temperature of use and the sensor used, the temp-co of the biasing circuit will need to be adjusted. In this case we are only guessing the sensor (and therefore tempco), but typically VH starts falling as you increase temperature of the sensor above room temp, so increasing the bias voltage to offset for this is a reasonable way of compensating for this.
This one has a reasonably readable figure for this: https://www.nicera.co.jp/wordpress/wp-content/uploads/2019/06/NHE520Fe.pdf so if we assume this data is more or less representative, it actually makes sense to use a positive tempco for the bias voltage to offset the reduced signal. That is assuming we are at or above room temp.

Maciej:
The biasing circuit looks like this:

with R3 and R4 as Hall sensors. Voltage input is here stabilised at 2*Vref
Spatially and thermally both sensors are preety distanced.
Looking at ambient temperature curve for voltage driving "Vc Const" I see no chance to compensate it with linear Vref tempco.


Both sensor candidates NHE520 and HW101A look very similar is voltage mode. I cannot compare current mode "Ic const" because graphs are not to scale.
BTW. When using P-N junction tempco it is recommended to use BJT junction rather than standard true diode.

toli:
Using the diode is a simple way to improve temperature stability over a limited temperature range where the signal drops with a given slope (above room temp if we trust the figures from the sensors we think are of interest). It needn't be perfect, simply better than nothing. Is using a constant voltage instead of one with this tempco better for the temperature range in question? No idea, to know for sure we need to know the exact spec of the sensor used, or to measure tempco ourselves.
Seems like some sort of such correction as in this one is included in other similar probes too, so without measuring this to know better, I think it makes sense to leave this tempco there.

HardyKefes:
Hello, does anyone know whether the ESI 695 current probe is more accurate than the Hantek cc65?
Also, How is the TECPEL CA-60 AC/DC compared to the Hantek?

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