DC drift using USB-1408FS

[WalterC]

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.

 

[JRys]

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.

[WalterC]

So, the noise on this sensor signal is terrible when the laptop is plugged into its power supply that's plugged into the AC mains.   By plugged into its power supply, I mean the 2.1mm barrel plug is plugged into the side of the laptop.  It doesn't matter if the supply is plugged into the AC mains.   That is causing some of the problem and I found another problem in one of our other modules that I fixed and that addressed much of the drift.  However, we still have some.   Everything is still in single-ended mode.  This sensor is being read on ACH0.  ACH1 is active but nothing is connected to it.  I have read that when they are active there can be some crosstalk as they are multiplexed in.   So, I have connected ACH1 to ground on the USB1408FS and will test that.  I've seen the unconnected analog channels float up to 1.4 before so it would make sense that if the adjacent channel is active and floating that ACH0 could be impacted.  Although I would assume that since ACH1 comes after ACH0, it might not.  But maybe I don't understand how crosstalk works in this situation.  Any thoughts will be appreciated.

[JRys]

The USB-1408FS noise specification for the 10-volt range is approximately +/- 10mV.  What are you getting, and what three sensors are you reading? Could you provide their datasheets? If the sensors are externally powered, do you use three separate power supplies or a single supply?

[WalterC]

There are actually just two sensors now.  These are proprietary sensors so I cannot supply a data sheet for them.   They are externally powered and each one has its own power supply.  The power is supplied from separate batteries.