I am using my U6 pro to measure voltage and current of two different systems each with its own ground. I have made differential and single measurements even combined and I have always obtained interference in the values obtained. Each alone work but not together. There is some way to isolate ground from these two measurements. I connected a 100k resistor on the negative lead of the differential line and it did not work either.
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Is your drawing showing 1 system or 2 systems? It seems to show just 1 system, so it that drawing showing something the setup that works?
Are the 4 wires connected to the U6 going to analog inputs? Do you have any sort of ground reference or are things totally floating?
https://labjack.com/support/app-notes/differential-analog-inputs
Well, I meant two different measurements on a single system. The diagram shows 4 electrodes buried in the ground. A current is injected through the outer electrodes and the potential drop is measured by the central electrodes. The four cables are connected to analog inputs. Both the source, the one used to inject the current and the power supply of the DAQ (PC) are floating
What do you mean when you say "each alone work"? What exactly does work?
Can you add some approximate voltages to your diagram? Supply voltage, voltage at each stake, and voltage on either side of your 1 ohm shunt.
When I said it worked I meant that the measurements are accurate, only when I connect to the schematic the lines M-N or when I only connect the lines A-B, that leave the Shunt. The values are checked with a DMM.
In your new drawing it looks like you have a 150 VDC power supply, 1 ohm shunt, 1000mV dropped from B to A, and 500 mV dropped from N to M. Defining the negative of your power supply as 0, that would mean 149 volts are dropped across your shunt (149 amps?), B is at 1.0V, N is at 0.75V, M is at 0.25V, and A is at 0.0V. That does not seem right.
I will try to explain better what is happening. This is a system to measure soil resistivity. Through the electrodes A-B a current is injected through a DC source (50 V). It is required to measure the value of the flowing current through the ground. This is done using a Shunt Resistance of 1 Ohm and measured the voltage drop in that resistance (187 mv). This measurement is done in differential mode on analog inputs 0 and 1. If I do not do any other measurement. No interference occurs in the values obtained. And the value checked by a DMM is similar to that obtained in the DAQ. Likewise, I measure the voltage drop produced by the rsistivity of the soil. This is done on the M-N electrodes. Again if I measured with the DAQ, only this signal the values obtained (93 mv) are consistent with those checked by DMM. The problem starts when I try to measure the two signals at the same time by the DAQ, differential at the analog inputs 0 and 1 for the shunt and 2 and 3 for MN. The values obtained are not similar to those checked with DMM
So are you using a current source (set to 187 mA) or a voltage source (set to 50 volts)?
If it is a voltage source set to 50 volts, I'm guessing the "A-B" value on your drawing is wrong. If you have a 50V source, and are dropping 0.187V across the shunt, then B-A must be about 49.813V. Please confirm the voltages at A, M, N, and B, by connecting the negative of your DMM to the negative of your supply and connecting the positive of your DMM to those different points.