How do I make accurate high impedance measurements using NI USB DAQ 6343 in Labview

[jschwartz100]

Hi.  I am using a DAQ 6343 to make analog voltage measurements on a high impedance signal.  I am aware that low impedance is recommended, but this is what is present on my board.  I have tried measuring repeatedly from one A/D channel only, increasing the number of samples, decreasing the sampling rate, and I'm finding large variations in all of the measurements. 

     I'm wondering what is the best method for stabilizing the A/D's before making a measurement on the analog input channel? 

     Thank you in advance for your help. 

               Jodi

[DylanC · Accepted solution]

Hi jschwatz100,

Here is a good link on ghosting, which speaks about solutions for reading a signal from a high impedance source:

http://digital.ni.com/public.nsf/allkb/73CB0FB296814E2286256FFD00028DDF?OpenDocument

I would recommend building a simple voltage follower circuit as a buffer between your signal source and the DAQ. Here is a Wikipedia article on it:

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

I would also recommend keeping a 10:1 ratio for your Sample Rate:Buffer Size(N Samples in Continuous Mode) and lowering/raising both similarly.

DylanC

[jschwartz100]

Thank you.  We were able to redesign our hardware from 100k imput impedance to a lower input impedance. 

I did try reading from only one channel to get an idea of the settling time.  What worked best was to reduced the number of samples to 2.  I put a delay of 500 ms and only retained the second sample.  This seemed to help with settling. 

   Jodi