DAQ analog signal drain

[pistonbones]

We currently have a 9205 analog input module. I have multiple analog inputs (RSE) receiving signals from equipment outputs. Recently I discovered that if a piece of equipment is taken off line, creating an open ended signal to the DAQ module, the other channels are effected and display erroneousness values. If I short the open path, channel signal wire connected to the common ground, without the presence of voltage the display returns to normal operation.

I have thought about installing a switch, one side of the switch would connect in-line with the positive wire, the other end of the switch would connect through a resistor (drain) to the common GND on the module. Is there a way to utilize the labview environment to fix this problem without external components?

[johnsold · Accepted solution]

What you see is fairly common in multiplexed A/D converters.  The input capacitance of the disconnected channels charges to some voltage (unpredictable in general) and that voltage can create the effects you report. The only real solution is to connect all unused inputs to a low impedance source within their input common mode range.  Often shorting to ground is the easiest thing to do.  I do not know of any internal option for doing this.

Lynn