erroneous lab-pc+ readings

[Shamash]

I'm attempting to read an analog signal using a lab-pc+ card, but I get some crazy results. When I am applying 0 volts to all channels I get 0 volts on ch0, -.7 volts on ch1, -1.4 volts on ch2, and -2.0 volts on ch3. When I apply 3.8 V directly to pin 1 ONLY, I read 3.8 volts on ch0, 1 V on ch1, 2 V on ch2, and 3 V on ch3. Is this a setup problem, or do I have a bad board?

[L Aguila]

Shamash,

First, check what type of signal you are trying to read, either grounded or floating. Then, in what mode are you trying to read that signal?

For example, if you are trying to read any signal in differential mode, any value that is read in the rest of the channels is OK, except in the channel where you read the signal, of course. However, you will imperatively need bias resistors if that signal is a floating one. Check chapter three of the following User Manual for the board for further details.Finally, make sure you configure the board properly in MAX.

http://digital.ni.com/manuals.nsf/websearch/53215E9F6E1D0EA78625665E006359B4?OpenDocument&node=132100_US

Hope this helps.

L Aguila
Applications Engineer
National Instruments

[Filipe A.]

Hello;

If you don't have any valid signal connected to any other channel but only the 0 Volts on channel 0, meaning that the other channels inputs are floating, you must disregard those values, since the ADC is not being referenced to any point.

To prove that theory, connect the other channels to GND. You will see that those channels will show 0 Volts as well as the channel 0 input value.

Hope this helps.
Filipe A.
Applications Engineer
National Instruments