# 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.