Problems measuring with USB6229, 6218, 6343

[tlyou]

Hi, 

We have been trying to measure the voltage of the following circuit using a DAQ. The MOSFETs (PSMN1R1-30PL) in the circuit are controlled by an Arduino UNO. The voltage from the bench power supply is 30V. We have been trying with a few DAQs (USB 6229, USB6218 BNC, USB6343 BNC) to measure the 4 voltages but none of them were able to deliver accurate measurements and the problems seem to lie in the 0.1 Ohm resistors most of the times. We've been stuck at this for a while and have tried different configurations but we're still getting inaccurate measurements. It should be 2.45 mV across the 0.1 Ohm resistor but the DAQ usually gives 5 - 10 mV or sometimes even 100 mV depending on how to we set things up. Does anyone have experience with such a problem?

We were wondering if it is just a configuration problem. We are now using USB6218 BNC. What should be the correct setting for measuring a circuit like this (measurement mode, scan rate, FS/GS, etc.)? How should the circuit be connected to AI GND?

Thanks.

[Henrik_Volkers]

Looks like all your DAQs are multiplexed input ones. quite common failure to overlook the figure 1 in the specifications that show the settletime error.

I would add some buffer amplifier .. INAs build for highside current measurments ...

And have a look at your GND potential (30V, Arduino, how it's connected to the DAQ)  is the arduino needed, if you have a AO on the DAQ?

Do you really have R1-3 in series? why? 

As a test: connect only one channel , read 300ms at highest samplerate ,  look at the data, do a FFT ..   know your enemies 😉    humm, spikes, SMPS,,   

[tlyou]

Thank you for the reply!

We did overlook figure 1 in the specifications but I'm not sure if that matters as the resistors we use are quite small. We've tried lower the sampling rate as well but that didn't work.

We've added some INAs across the 0.1 Ohm resistors along with low pass RC filters and the measurements are now much better. Seems that is the solution. We have only tested with the 0.1 Ohm resistors though due to not enough INAs available. We will test the circuit more thoroughly when the ordered INAs arrive.

Arduino is needed because we want to eventually run the system without DAQ. The Arduino runs some closed loop control algorithm which I think would take some time to adapt it to the control program for DAQ.

The reason we have R1-3 in series is because this is an already simplified test circuit to figure out the problem with the DAQs. There will be an actuator with variable but unknown resistance connected in parallel to R2 and R3 in the real setup. The resistors are arranged in this way so that we can calculate the voltage and current through those actuators by knowing the voltage across R1 and R2.

Thank you for the ideas for testing. Never thought of things like FFT as I'm not familiar with electronics. Will look into them.