Wrong Current with NI9203

[Yabudabi]

Hi

I am using the NI 9264 module as a voltage supply and the NI 9203 module to measure the current in my circuit.

In my circuit, I just have a resistor with known resistance. The goal is to calculate the resistance using R=U/I, with U the applied voltage and I the measured current.

For a resistor of 10 Ohm, I measure the following currents:

I_1(1V) = 6.6mA, I_2(2V) = 8.6mA, I_3(3V) = 8.6mA, such that I get R_1=151 Ohm, R_2 = 226 Ohm, R_3 = 347 Ohm.

For a resistor of 1.5 kOhm, I measure the following currents:

I_4(1V) = 0.6mA, I_5(2V) = 1.2mA, I_6(3V) = 1.8mA, such that I get R_4=1.64 kOhm, R_5 = 1.64 kOhm, R_6 = 1.64 kOhm.

I made sure that both modules are connected to the same ground.

Does anyone have an idea where this difference (or wrong current) comes from?

Thank you very much!

[Antoniooh]

NI-9264 is not a power supply.

From the specs:

Current drive for this module is  +-16mA all channels maximum; +-4mA per channel typical

[santo_13]

If you're using only one channel, it 9264 can source up to +/-4mA without sag in the voltage. This translates into maximum resistance you can measure up to 2.5k ohm (10V / 4mA).

[Yabudabi]

Thank you very much for your answer, I really appreciate that!

Your answer would explain the weird behaviour for the 10 Ohm resistor. However, I still have an offset of 140 Ohm for a 1.5 kOhm resistor.

Do you have an idea where this comes from?

[santo_13 · Accepted solution]

NI-9203 has a current shunt internally to measure the current. This value comes to 138 ohms (listed in the datasheet).

Since 9203 and your resistor are in series, your test voltage is always applied across your resistor + 138 ohm.

[Yabudabi]

You are right, thanks very much!