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A-3-2 Signal Range

The instrumentation amplifier in the T7 (see Figure 4-2) provides 4 different gains:  x1, x10, x100, and x1000.  These correspond to 4 input ranges:  ±10 volts, ±1 volts, ±0.1 volts, and ±0.01 volts.  The input ranges are straightforward for single-ended measurements, but can be a little tricky for differential measurements if neither channel (positive or negative) is at 0 volts.

The following figures show the approximate signal range of the T7 analog inputs at gains of x1 and x1000. "Input Common-Mode Voltage" or Vcm is (Vpos + Vneg)/2.

Keep in mind that the voltage of any input compared to GND should be within the Vm+ and Vm- rails by at least 1.5 volts, so if Vm is the typical ±13 volts, the signals should be within ±11.5 volts compared to GND.

Example #1: Say a differential signal is measured where Vpos is 10.05 volts compared to GND and Vneg is 9.95 volts compared to ground, and G=100. That means Vcm=10.0 volts, Vdiff=0.1 volts, and the expected Vout=10.0 volts. Figures for G=10 and G=100 are not available, but Vcm=10.0 volts and Vout=10.0 volts is not valid at G=1 or G=1000, so is certainly not valid for gains in between.

Example #2: Say a differential signal is measured where Vpos is 12.0 volts compared to GND and Vneg is 8.0 volts compared to ground, and G=1. That means Vcm=10.0 volts, Vdiff=4.0 volts, and the expected Vout=4.0 volts.  This looks almost okay in the G=1 figure below, but the voltage of Vpos compared to GND is too high so this is not valid.

Example #3: Say a single-ended signal is measured where Vpos is 10.0 volts compared to GND and G=1.; That means Vcm=5.0 volts, Vdiff=10.0 volts, and the expected Vout=10.0 volts. This is fine according to the figure below.