USB 6210 bias resistor overload for low voltage floating signal source ?

[mmiller]

I am trying to measure a floating signal source voltage that is between 200 and 500 mV using the USB-6210.  I am using a differential input configuration, and have wired it as described in "Field Wiring and Noise Considerations for Analog Signals" (https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider... with one resistor attached between the negative input and ground. 

I tested my setup with a 1.5 V battery, so I know that I have attached my wires and am using the data acquisition device correctly with the software. 

However, when I attempt to measure the experimental voltage, I get a saturated reading (negative full scale).  Can this be due to overloading with the resistor or does it mean something else?

[Seth_B.]

What size of resistor are you using?  We also generally recommend a bias resistor on both positive and negative inputs, otherwise the inputs are unbiased and it may affect readings.

[mmiller]

Hi Seth,

I'm using a 10 kOhm resistor.  Is saturation a symptom of only using one resistor rather than two? Your manual suggested that it should be possible to read the signal even without a resistor. 

[Seth_B.]

You can take readings without the resistor for grounded signals.  However, the issue is that floating sources are (obviously) floating and the difference between the positive and negative leads, when reading differentially, can float outside the measurement range, relative to ground.  This floating can result in one or both of the positive/negative leads floating past the ADC rail.  This appears as saturation at the positive or negative limits.

If you don't want the two resistors, you should use the RSE measurement type.  This doesn't have as good of common mode noise rejection, so you may see more noise.  Particularly if you have long wiring between your sensor and your device.  You can mitigate this by using twisted pair wiring.

[mmiller]

I will try the 2 resistor setup, but should I be using smaller resistors?

[Seth_B.]

Smaller resistors result in more current being directed through the resistor instead of the input.  We recommend bias resistors between 10kOhms and 100kOhms.  Go ahead and try the 10kOhm resistors.  You may need to increase the value if you still see too much of an impact.

[mmiller]

With the 2 resistor (10 k-Ohm, 1/4 W) setup, I am now measuring a 0 voltage difference for what I expect to be 400 mV.  Any ideas for what I could be doing wrong? I can still measure larger (>1 V) voltages. 

[Seth_B.]

Are you measuring in Differential or RSE mode?  And are the resistors between AI+ and AI GND and AI- and AI GND?