USB-1608G Single Ended - Noise and Measurement inconsistency

[scottadon]

Hi

I have 2 USB 1608G deployed in a remote acquisition project hundreds of miles away and so cannot immediately access them.  I am experiencing lots of noise problems and drifting of zero positions.   Since I have a third module here locally I decided to perform a simple test using a battery with the attached schematic.  What I see is that channel 0 has a clear 60Hz component and a 0.4 volt mean offset from channels 1,2, and 3. I had assumed this 60Hz component is coming from my laptop that the USB-1608G is plugged into, but it persists even when I remove the laptop power supply and run on battery.  I have similar problems with the devices deployed and so wondering if this is a design issue or there is a sensible resolution?  I have tried making jumpers for all AGND connections to battery negative terminal.  I have also tried tying the battery negative to the USB/Computer gnd/shield and no changes are seen (using a continuity check I can see that these connections are already made internally to the 1608 anyway):

    

[Fausto]

Hello @scottadon.

Your AI0 channel (CH0 terminal) appears to be an open channel and not making contact with the AAA battery, so it's behaving like an antenna.  Be sure to secure the wire with the screw terminal.  Touching the tops of the screw heads does not guarantee a proper connection.  Also, do not enable a channel for acquisition if it is not securely attached to a signal source or adjacent analog ground (AGND) terminal.

On the USB-1608G, disconnect all the wires from the AAA battery except for AI0 channel (CH0 terminal) to the positive terminal and AGND terminal to the negative terminal.  Disable all other analog input channels in DAQami except for AI0 channel.  Repeat the acquisition.  Afterward, move the battery's positive terminal wire over to AI1 channel (CH1 terminal) and repeat the acquisition, with only the AI1 channel enabled in DAQami.  How do the two channels compare?  Please reply with screen captures of the individual tests.  If the AI0 channel appears to be an open channel, then reply with the device's serial number; on the backside or as seen in InstaCal.

Regards,

Fausto

[scottadon]

HI Fausto

 

You are right, the input was floating, working now thank you. I will continue to investigate the problems with the remotes systems and post here as needed.  However, quick question... I have self-powered Current Transducers (current transformers with 0-10VDC output conditioning) connected to USB-1608G in singe ended mode and the measurements are unrealistically high.  I think this is because there is no common earth/gnd with the daq and the inputs, again, are floating and acting as an antenna.  So I have asked the team remotely to rewire for differential measurement.  When doing the differential measurement should we tie CH- to AGND?