Hi,
I am using the LJTick-InBuff to make a differential potential measurement across source resistances between about 5 and 50 kOhm using two electrodes. I have tested out various resistances to GND on the negative channel of the measurement (from AIN1 to GND in this case) between 0.1 and 10 kOhm.
I am curious to know what is the largest possible resistance recommended between the negative channel (AIN1) to GND? My goal is to avoid reducing the input impedance of the differential potential measurement.
Thanks!
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So you are connecting a +/- differential signal pair to INA/INB on an LJTick-InBuff, and then also connecting a resistor from INB to GND? Presumably you are doing this to refer a floating system to T7-GND as discussed in the Differential Readings App Note. The idea is to define the signals so they are within the valid common-mode range of the T7:
https://labjack.com/support/datasheets/t7/appendix-a-3-2
Note that the typical analog input bias current is about 20 nA:
https://labjack.com/support/datasheets/t7/appendix-a-3
If your system is actually floating, then no current should flow through the resistor except the bias current, so if you use a 10 Mohm resistor and 20 nA flows through it that would put INB (and AIN1) at about 0.2 volts. So you can see that a very large resistor is fine.
If the voltage of your measurement points versus their ground is low enough (roughly +/-10V but see Appendix A-3-2 for exact limits), you can just connect your system ground to T7-GND and now the signal voltages are defined.