8.3.3 Input Common-Mode Range

The linear input voltage range of the INA827 input circuitry extends from the negative supply voltage to 1 V below the positive supply, and maintains 88-dB (minimum) common-mode rejection throughout this range. The common-mode range for most common operating conditions is described in Figure 14 and Figure 35 through Figure 38. The INA827 can operate over a wide range of power supplies and V_REF configurations, thus making a comprehensive guide to common-mode range limits for all possible conditions impractical to provide.

The most commonly overlooked overload condition occurs when a circuit exceeds the output swing of A₁ and A₂, which are internal circuit nodes that cannot be measured. Calculating the expected voltages at the output of A₁ and A₂ (see Figure 57) provides a check for the most common overload conditions. The A₁ and A₂ designs are identical and the outputs can swing to within approximately 100 mV of the power-supply rails. For example, when the A₂ output is saturated, A₁ can continue to be in linear operation and responding to changes in the noninverting input voltage. This difference can give the appearance of linear operation but the output voltage is invalid.

A single-supply instrumentation amplifier has special design considerations. To achieve a common-mode range that extends to single-supply ground, the INA827 employs a current-feedback topology with PNP input transistors; see Figure 57. The matched PNP transistors (Q₁ and Q₂) shift the input voltages of both inputs up by a diode drop and (through the feedback network) shift the output of A₁ and A₂ by approximately +0.8 V. With both inputs and V_REF at single-supply ground (negative power supply), the output of A₁ and A₂ is well within the linear range, allowing differential measurements to be made at the GND level. As a result of this input level-shifting, the voltages at pins 2 and 3 are not equal to the respective input pin voltages (pins 1 and 4). For most applications, this inequality is not important because only the gain-setting resistor connects to these pins.