USB-6210 voltage reading drops slowly with switch off

[Andrew Aji]

I am measuring constant 5V output from an adapter hooked up to the wall.  The positive wire goes through a switch to the positive AI terminal and the negative wire goes directly to the corresponding negative AI terminal (I basically want the analog voltage to reflect when the switch is on or off).  I have the DAQmx setup for differential configuration.  When the switch is on, it reflects 5V promptly, however when the switch turns off the voltage drops very slowly, taking 30 seconds or so to bottom out. Also it does not settle on 0V, it settles on about -600mV or so.

Is my configuration setup incorrect?  

Also when I hook up a multimeter to the circuit the DAQ measures perfectly, with the voltage dropping promply at the opening of the switch.  So possibly the setup is correct and the introduced impedance is what is needed?

[AnalogKid2DigitalMan · Accepted solution]

Andrew:

Your train of though is correct. The 6210 has an input impedance of over 10Gohms, so when you open the switch, the resitance/capitance factor will take a long time to bleed the voltage down and since the DAQ input is now an open circuit, it can float to just about any voltage level. You could put an arbitrary ristance across the input terminals of the DAQ (but not too low). Try something like 10K or 100K ohms.

Another alternative would be to use one of the digital inputs on the 6210 to monitor the switch. Since your supply is 5V, it is TTL compatible for the inputs.

-AK2DM