Hi,
I have some information as well as additional questions to better understand your application:
1. What are the voltage levels that your unidirectional microphone outputs? If these levels are low-level, it may be necessary to have external amplification to put the signal into the working range of your DAQ device. If the voltage levels are in a suitable working range, then a standard differential measurement should suffice. In the case of your PCI-6221, a differential measurement on ai0 would be done by making connections to ai0 (pin 68) and ai8 (pin 34).
2. If you are trying to reduce the amount of noise that can be introduced into your signal, I would definitely recommend using shielded wires and making them as short as possible. However, if you are wanting to filter out noise that has already been introduced, there are a couple of options that you have - hardware or software filtering. We have
signal conditioning products that allow you to implement hardware filters for noise such as AC line noise (60Hz). Another method you can use for filtering is through software by applying digital filters to your acquired data. If you are developing in LabVIEW, there are a number of VIs included that can allow you to do this. One in particular is the Filter Express VI.
Regards,
Andrew W
National Instruments
