How many ohms of resistor should I take in differential signaling in SCB-68

[raychu]

Dear all,

I want to do calibration with my hydrophones. The hydrophones are connected on SCB-68 board. I check the User Manual of SCB-68, and try to use differential connections. However, there is a paragraph like below:

for larger source impedances, this connection leaves the
differential signal path significantly off balance. Noise that couples
electrostatically onto the positive line does not couple onto the negative line
because it is connected to ground. This noise appears as a differential mode
signal instead of a common-mode signal, and thus appears in your data.
In this case, instead of directly connecting the negative line to AI GND,
connect the negative line to AI GND through a resistor that is about
100 times the equivalent source impedance. The resistor puts the signal
path nearly in balance, so that about the same amount of noise couples onto
both connections, yielding better rejection of electrostatically coupled
noise. This configuration does not load down the source (other than the very
high input impedance of the NI-PGIA).

How many ohms of resistor should I take? Or, How can I know the impedance of a hydrophone, even with amplifier?

Thank you.

[pallen]

Hello Raychu,

I don't have any real experience with hydrophones.  But I thought I'd try and give you a couple of ideas anyway.  

From what I've quickly read, most hydrophones are piezo electric devices.  These are typically very high impedance.  Most of them require a pre-amp to convert them to a lower impedance suitable for signal processing.  If you're passing the signal through an amplifier, then it's the output impedance of that amplifier that you need to know.  

http://www.etec.dk/hydrophones.html

http://en.wikipedia.org/wiki/Hydrophone

Where high impedance devices can be a real "gotcha" is when you're acquiring data from more than one channel.  Multiplexed inputs have a problem with these, and you can end up with "ghost" signals on your other channels. 

Without knowing the specs of your hardware, I can't make an accurate guess.  But we've used 10K resistors in this configuration with other devices.  But you will like want to look up the specs on your particular amplifier.