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A-3-2 Signal Range

T7 Signal Range

The instrumentation amplifier in the T7 (see Figure 4.2-2) provides 4 different gains:

  • x1 (RANGE is ±10 volts)
  • x10 (RANGE is ±1 volts)
  • x100 (RANGE is ±0.1 volts)
  • x1000 (RANGE is ±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 figures below show the approximate signal range of the T7 analog inputs at gains of x1 and x1000.

Input Common-Mode Voltage, known as Vcm, is:

Vcm = (Vpos + Vneg)/2

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. See Table A5-5 for more information on Vm+ and Vm-.

Example #1 - invalid because Vcm=10.0 with Vout=10.0 is invalid:

Suppose a differential signal is measured, where:

    • Vpos is 10.05 volts compared to GND
    • Vneg is 9.95 volts compared to GND
    • G=100 (RANGE=±0.1)

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 shown, but Vcm=10.0 volts and Vout=10.0 volts is not valid at G=1 or G=1000, so it is not valid for gains in between.

Example #2 - invalid because Vpos compared to GND is too high:

Suppose a differential signal is measured, where:

    • Vpos is 12.0 volts compared to GND
    • Vneg is 8.0 volts compared to GND
    • G=1 (RANGE=±10)

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 - valid:

Suppose a single-ended signal is measured, where:

    • Vpos is 10.0 volts compared to GND
    • G=1 (RANGE=±10)

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.