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DC drift using USB-1408FS



We are using a USB-1408FS to measure three signals in single-ended mode.  These signals are connected to pin 1, pin 4 and pin 5 and we are using pins 3 and 6 for ground connections.

We have been using this successfully in one lab on a test platform and have moved to another lab an have it connected to a prototype of the same system we were using in the lab.  The analog signals have a range of 0 - 1.8V.

We have three laptops that we can use - Asus, HP and Lenovo.   

When we start the DAQami software collected the analog signals, channel 0 (pin 1) is tuned where it should be - ~0mv, sometimes a bit negative.   Before anything in our system changes to cause the sensor signal to change, we observe a relatively slow rise in the baseline on channel 0 to between 300 and 400 mv.   At one time we could force this behavior by unplugging the Asus laptop from its AC adaptor at the barrel connector on the laptop.  We no longer had the rise occurring for about a day but ITS BACK! :)

Are we doing something wrong with the USB-1408FS?  We are just using it to capture signals for analysis.   But, this is effecting the control system.


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Disconnect all but one sensor. If the sensor is externally powered, disconnect the laptop power supply. Doing this should produce a suitable measurement. Next, reconnect the laptop power supply and check for drifting. If it does, you could be looking at a ground loop. If it doesn't, connect another sensor and test for drifting. For minor ground loops, differential inputs are preferred over single-ended.

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