6 Detailed Description

6.1 Overview

The INA122 is a monolithic, precision instrumentation amplifier incorporating a two-op-amp design, providing

savings in power consumption and designed for portable instrumentation and data acquisition systems. An

external gain resistor (RG) sets the gain from 5V/V to 10000V/V.

Figure 6-1 shows a simplified circuit diagram of the INA122. The design of A1 and A2 are identical and

both internal outputs can swing to within approximately 100mV of the power supply rails, depending on load

conditions. When the output of A2 is saturated, A1 can still be in linear operation, responding to changes in the

noninverting input voltage. This can give the appearance of linear operation but the output voltage is invalid.

The most commonly overlooked overload condition occurs by attempting to exceed the output swing of A2,

an internal circuit node that cannot be measured. Calculating the expected voltages at output of A2 (see the

equation in Figure 6-1) provides a check for the most common overload conditions.

A single supply instrumentation amplifier has special design considerations. Using commonly available single

supply op amps to implement the two op amp topology cannot yield equivalent performance. For example,

consider the condition where both inputs of common single supply op amps are equal to 0V. The outputs of both

A1 and A2 must be 0V. But any small positive voltage applied to V+IN requires that A2 output must swing below

0V, which is not feasible without a negative power supply.

To achieve common-mode range that extends to single supply ground, the INA122 uses precision level-shifting

buffers on the inputs. This shifts both inputs by approximately 0.5V, and through the feedback network, shifts A2

output by approximately 0.6V. With both inputs and VREF at single supply, A2 output operates within linear range.

A positive V+IN causes A2 output to swing below 0.6V. As a result of the input level-shifting, the voltages at RG

pins (pins 1 and 8) are not equal to the respective input pin voltages (pins 2 and 3). For most applications, this is

not important because only the gain-setting resistor connects to RG pins.

6.2 Functional Block Diagram

Figure 6-1. INA122 Simplified Circuit Diagram

INA122

SBOS069A – OCTOBER 1997 – REVISED DECEMBER 2024

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