dc offset while acquiring signal using PCI 7030/6040E

[RPKumar]

I am acquiring analog signal from a signal generator using Labview 6.1 RT and PCI 7030/6040E.The accessory used is SCB 68.No signal conditioning accessory is used.The acquired channel is showing a DC offset which gradually increases and reaches a maximum level of 10V. The sig. generator o/p is not having any dc offset.What could be the problem ?

[Filipe A.]

Hello;

The first thing you should double check is if the signal is connected to the DAQ boards, accordainly with the input mode configured.
Make sure you have the signal connected in differential mode, and also have the input mode of the DAQ device configured as differential mode too.

On more thing you can check is if the SCB-68's jumpers are set to by pass the CJC circuitry. You can check the jumpers positions on the SCB-68 Users Manual that you can download from www.ni.com.

Regards
Filipe A.
Applications Engineer
National Instruments

[RPKumar]

I have tried like this.When connected in differential mode it didn't work.But when given in Non referenced Single ended mode, it works.Now the problem is that, when there is no input, there is a 50 Hz pickup in the channels.How can this be avoided ? I haven't connected any biasing resistors because,from the manual I understand that ,when a ground referenced source is connected to the board in NRSE mode, no biasing resistors are required. How the pickup can be avoided ?