Should I use AI SENSE or a dedicated instrumentation amplifier?

[Gustep]

Hello,

I am facing the following design decision while building a transimpedance (current to voltage conversion) preamp for my M-series DAQ card: When trying to measure a difference signal in the audio range (~ 1kHz) with a large common mode, what's the best way of eliminating the common mode? Should I use a dedicated instrumentation amplifier on my transimpedance preamp and acquire the resulting difference signal, or can I simply wire the raw signal (which still includes common mode) to AI, and the reference signal to AI SENSE, so that the PGIA on the DAQ card can perform the common mode rejection?

My understanding is that the PGIA on the DAQ card is already quite good, so adding an extra IA in front of it may not reduce the overall noise, but it certainly would add to the design complexity.








Message Edited by Gustep on 05-05-2008 02:35 AM

[Gustep]

I also wanted to add that I am doing a multi-channel measurement, where the common mode is identical on all channels. The common mode is also available as a relatively pure signal in my application. Hence it seems as if wiring the common mode signal to AI SENSE and using NRSE mode might be a good approach. Any thoughts on this?


Message Edited by Gustep on 05-05-2008 02:47 AM

[J_Thomas]

HI Gustep,

You pose a great question.  I am not sure that I have a good answer about which one would be better for your setup. Common-mode voltage is completely rejected (not measured) by an ideal differential measurement system. In general, a differential measurement system is preferable because it rejects not only ground loop-induced errors, but also the noise picked up in the environment to a certain degree. The single-ended configurations, on the other hand, provide twice as many measurement channels but can be justified if the magnitude of the induced errors is smaller than the required accuracy of the data.

In short, a differential system is probably ideal, but depending on your accuracy requirements NRSE may be good enough as it can also reject common mode voltage.

If you haven’t already, I would strongly recommend you read the following document:

Field Wiring and Noise Considerations for Analog Signals

It is one of the best documents I have seen that discuss the best measurement approachs for measuring different kinds of signals.