~1.4V on analog input channels - USB1408FS

[WalterC]

We've been using the USB1408FS in single-ended input mode for some time now with no issues and then yesterday we blew a microcontroller.  Replaced it and then blew another one and never suspected the USB1408FS but when we measured the open voltage on channels 02, 03 and 04, they all had roughly 1.4VDC on them.  None of the other channels have this.  They are all at zero with nothing connected.  Any ideas what might be going on with the device?

 

[Fausto]

Hello @WalterC.

We recommend you short the inputs to unused channels if those channels are sampled, or to configure your software program to not sample from channel numbers which are open.  Most A/D daq devices have high input impedance front ends that, when left open, are likely to drift towards plus or minus full scale.  However, several MCC models (i.e. USB-1208LS, USB-1208FS, USB-1408FS, miniLAB 1008) have a resistive front end designed to divide down larger voltages and bias/shift from bipolar to unipolar, which results in a rather steady 1.4 volt reading when nothing is driving the input.  Ignore this value if sampling an open channel.

An additional bit of information is that these specific models work best for measurements, when connected to a low output impedance source.  If your signal has higher output impedance, such as a voltage divider or potentiometer output would have, inaccuracies will result.

Therefore, the 1.4 V reading is expected on an open channel.  If the other channels (not listed in your post) do not measure a 1.4 V reading, when left open, then verify various voltage levels connected to those channels.  Lastly, the USB-1408FS is no longer serviced by the factory, nor can a user service it in the field. 

Regards,

Fausto

[WalterC]

Thanks!  Just FYI, we borrowed a unit from a friend's lab, and we're seeing the same behaviour on those open lines on them.  

The signal comes in from the output of a unity gain buffer op amp circuit.  When that line is measured with a DVM, it's 0.497 V.  But when connected to the input on the USB-1408FS, the DAQami software reads 1.4V.

[Fausto]

In single-ended mode, if you test each input channel with your circuit signal, do they all read 1.4 V or only certain channels?

How many circuits do you connect to the input channels during your tests or what other signals do you have connected?

Using one circuit signal, please test each input channel in differential mode.