negative voltage bias USB-6009

[igor c]

I'm measuring the hard-drive's 12v supply. A .1ohm shunt resistor is inserted in the power line to measure differential voltage. A multimeter shows about .05V while DAQ is all over the place in a negative range. I'm using a USB-6009. Has anyone run into this problem?

[igor c]

Also tried a .5ohm resistor. The differences between hard drive states (spinning/shut down) is more pronounced, but the negative offset still exists.

[hoffiz]

Hi,

First the USB-6009 has a maximum range of 10 Volts so make sure your are not trying to drive it with more than that, 6009 Manual, for more information specially page 26. Second suggestion would be to confirm your connections the wires from the shunt resistor should be connected to pins 2 and 4 for differential measurement on AI0. Last but not the least; check that you have not inverted the polarity doing the connection.  One of the best tests you can do is to measure the voltage from battery, doing the same set up as before.

I hope it helps

[igor c]

The shunt resistor is now a ~.5ohm one and it seems to have helped a bit. There is still a -40mV bias, though. Would this be due to the 10V limit?

i.

[hoffiz]

Hi,

The 40mV bias is not due to the 10 Volt limit, the way the card reacts to a voltage more than 10 volt from Ai to GND is by clipping the reading.

What I would do before continuing reading voltages with the USB-6009 is to simulate the circuit, so can draw a circuit of what you are doing? How is the shunt resistor connected to the 10 volts suppluy?

After we know what are the voltages and currents seen by the 6009 there are some options we can try. First option would be to built a simple voltage divider, if you take a look at this article we can use a simple voltage divider to scale down the 12 Volts supply: Basic Analog Circuits, then since we can consider this measurement as “floating signal”, this article suggestion “Field Wiring and Noise Considerations for Analog Signals”, should be able to improve your reading by adding the R like in table 1.

One point to notice is the difference here between your DMM and the USB-6009 is that your DMM probably has a high impedance input. Your USB-6009 analog input looks like the figure in this article Explanation of the Analog Input Circuitry on the 6008/6009.

Let me know if it helps