Lost precision in a DAQ Vi.

[steve_ac]

We are measuring a high value resistance using a slow DAQ Vi in LabView 6.1 and it's fine, but when We try to measure the same resistance using a fast DAQ vi (buffered) the value is changed about a 30%. We keep on mind that there is a capacitance value in the lines and we remove that element. We need to know how improve the data adquisition without lost the precision? Thank you.

[RIO-Todd]

Does the value increase or decrease by 30%? If the value is increasing then you might be running into charge injection. Charge injection is caused by sampling at high rates with a high-impedance source. The documents linked below discuss this phenomenon and give some solutions. I would look into decreasing the source-impedance by using a unity-gain buffer circuit as explained in the documents below. I hope this helps.

Is Your Data Inaccurate Because of Instrumentation Amplifier Settling Time?
http://zone.ni.com/devzone/devzoneweb.nsf/Opendoc?openagent&F7B07C1AEFAC727B8625686500693572

Using a Unity Gain Buffer (Voltage Follower) with a DAQ Device
http://zone.ni.com/devzone/conceptd.nsf/webmain/CD57A73721E0612586256BAE0055CDD9?opendocument