# Why is my current measuring with the DRV425 wrong?

I want to get the current through a conductor by measuring the magnetic field of a conductor by using a DRV425. I use the DRV425 Evaluation Board with the original settings (R1 = 100 Ohms) for my research purposes.

I use a 12 Ohms resistor and connect the resistor with the output of my signal generator. The signal generator generates a square signal with the following signal parameter:

• High level: 5 V
• Low level: 0 V
• Frequency: 1 Hz

I use some tape to fix the wire on top of the DRV425. The thickness of the wire is around 2 mm, so the distance between the center of the wire and the chip is around 1 mm.

The green arrow indicates the flow direction of the current and the black line indicates the sensitivity axis of the sensor. Then angle of the wire and the sensitivity axis is around 90° (can´t rotate the green arrow that fine, so the angle of the arrow isn´t the same as the angle of the wire).

I also measure the output signal of the signal generator and the output voltage of the DRV425 with an oscilloscope.

As you can see from the screenshot I measure the following voltages with the sensor:

• Low: 1.461V
• High: 1.358 V

Now I calculate the H and B of the conductor with r = 1 mm during the high phase of the signal from the signal generator. For this I measure the current with a multi meter

$I = 158 mA$

$H = \frac{I}{2\cdot \pi \cdot r}$

$H = 25.146 \frac{A}{m}$

$B = H \cdot \mu _{0}$

$B=31.59 \mu T$

Next I calculated the B with the equation from the DRV425 evaluation board manual:

$B = \frac{V_{Out}}{4\cdot 12.2\cdot 100\Omega }$

I have to use the difference between the high and the low voltage of the signal for VOut, because I want to calculate the resulting field based on the current difference when I turn on the signal. So my difference is

$\Delta V=1.461 V - 1.358V = 0.103 V$

Which results in a B field of

$B=\frac{0.103V}{4\cdot 12\cdot 100\Omega }=21 \mu T$

So I have a difference of 10µT which I can not explain. Where is my mistake?.

Update:

Based on the answer of Andy aka I replace the calculation of the current with a direct current measurement with a multi meter.

I also check the distance tolerance and it seems that the results are equal when I increase the distance by 0,5 mm. So a possible answer is that my distance measurement wasn´t exact enough and I haven´t consider the tolerances enough. So the next idea would be to use a thinner conductor to replace the thick (and flexible) isolation of the current silicone wire.

• Please see my updated question. I have added the sensitivity axis of the sensor. – Kampi 2 days ago