Connecting sensors with floating grounds and different output ranges to NI 9205

[sharmaa]

Thanks Seth,

The bias resistors are generally between 10k to 100k ohm. I have six ground referenced sensors and six floating signals. I presume, I would need six resistors for the the floating signals. Is there a NI document which tells me how to determine these resistors?

I think I have DC coupled signals (since I have DC voltage coming out of the sensors, that is what I read somewhere), so would it suffice if I tied only the '-' terminal of the sensor outputs to the COM port via the bias resistors?

Thanks.

[Seth_B. · Accepted solution]

The value of the resistor is generally dependent on the output impedance of your sensor.  In most cases, you want to start with a big resistor, like Kick.  Then, if you see an constant offset error that is out of spec, then try a smaller resistor.

The reason that you should use the same resistor value for both the + and - terminal is that you don't want an "impedance mismatch" between the channels.  Because your sensor will need to output enough current for both the channels on the 9205 and the resistor to ground, if only one channel is tied to ground then your sensor will end up with more current on the - terminal than the + terminal.  This extra current will appear as a voltage error.  The higher the voltage being output by your sensor, the worse the error will be.  This is what is called a "gain error".

The main points are:

1. Use a high value resistor (about 100k, though the exact value isn't as important) if your sensor has a low output impedance (<10ohms).  If it has a higher output impedance, use a smaller resistor.
2. Use the same resistor value on both the + and - terminal to minimize gain error due to an impedance mismatch in the channels.

Hope this helps!