USB 6008 floating voltage

[heang]

I have encountered problem of using usb 6008 to measure voltage from LM60.
Before connecting this LM60 to USB 6008, i supply dc voltage to test the calibrated USB 6008 from NI. it works well. However, once i connected LM60, at room temperature, it gives value of 1.4 v. i am doubt if it is floating voltage. I knew that USB 6008 cannot be calibrated by user, anyway, can anyone suggest me to overcome this 1.4v.?
i am terribly upset by the noise from LM60, i did capicitive load to filter noise, it works well. what my question is whenever the sensor is not connected(open circuit), it gives 1.4 v,(RSE), once connected to LM60, it gives value which is not accurate with the temperature, may it be influenced by the floating voltage 1.4v?

i doubt if using scale, it cant work either, as it will give negative value perhaps.

pls advise. your urgent reply is much appreciate.
regards

[Chris_K_]

Hi heang

The 1.4V open circuit voltage that you are seeing is expected behavior for the USB-6008/6009.  Please see the following two Knowledge Base Articles:

http://digital.ni.com/public.nsf/allkb/C7E181E51E4299FC862570A700604141?OpenDocument

http://digital.ni.com/public.nsf/allkb/E687933C5694AB00862570BD00593CA3?OpenDocument

There are also other related problems reading LM35 Temperature sensors due to the low impedance of the 6008, and a common solution is to use a voltage follower (opamp) for the input to the 6008.  Please see the following forum for more information.

http://forums.ni.com/ni/board/message?board.id=250&message.id=21480&requireLogin=False

Chris_K