DC offset problems with ContAcq&SelectableSave VI

Question

I am using the ContAcq&SelectableSave VI from the ni.com examples page to acquire and save a continuous analog signal.  I want to sample the channel every second, however, when I run the VI the waveform chart displays the signal between 0.2 and 4 volts when it should be -2 to 2 volts. 
I am using LabView 7 Express with a PCI-6034E card.
I have set the signal generator to 1Hz and set the VI as follows:
scan rate: 1Hz
scans to read at a time: 10
buffer: 20
Is there a reason for this? Because when I sample a 40Hz signal at 200 Hz it seems to work as expected.
I am intending to use this VI to continuously save analog data over a long period of time such as a number of weeks is this a suitable vi for this task?   
cheers
Pauldoc

Answer 1

Pauldoc,
i better first of all write what i understood from your post:
you want to acquire analog data with a PCI-6034E continuously. the
signal you want to acquire is a periodically signal with no offset and
a voltage between -2 and 2 Volts (e.g sine, triangular). one period
takes 1s, the frequency is 1Hz.
you use the correct VI, but your setup for the VI is scan rate 1Hz. it
is configured to be buffered (hardware timed, 10 samples at a time)
which is correct so you should get one array of samples every 10s.
now the problem is that you mess up with a fundamental for analog input: NYQUIST. please refer to the following link .
therefore NI suggests to sample analog signals with 10 times the
frequency you want to acquire at maximum, so for your application it
would be 10Hz.
Norbert B.
NI Germany
Message Edited by Norbert B on 10-11-2005 03:52 AM
CEO: What exactly is stopping us from doing this?
Expert: Geometry
Marketing Manager: Just ignore it.