11 Instrumentation Amplifier

Figure 11-1 displays the instrumentation amplifier configuration. It has two input signals V1 and V2 and one output signal V_OUT. The output signal is directly proportional to the difference between the input signals. OP0 and OP1 op amps act as buffers for the V2-V1, differential input voltage, while OP2 acts as a differential amplifier, a combination of both inverting and non-inverting amplifiers. The gain, represented by the ratio between the output signal and the differential input signal, is given by the ratio between R2 and R1.

The output voltage V_OUT is given by the formula:

$V_{O U T} = (V_{2} - V_{1}) \times G a i n$

The gain is given by the wiper position for the resistor ladders for OPAMP0 and OPAMP2 according to Table 1.

Table 11-1. Gain Selection for Instrumentation Amplifier
GAIN	OP0RESMUX.MUXWIP	OP2RESMUX.MUXWIP
1/15	`0x7`	`0x0`
1/7	`0x6`	`0x1`
1/3	`0x5`	`0x2`
1	`0x3`	`0x3`
3	`0x2`	`0x5`
7	`0x1`	`0x6`
15	`0x0`	`0x7`

As observed, a three op amp instrumentation amplifier takes a differential signal as input and provides a single-ended output signal while ensuring a high input and a low output impedance. Hence it can fit as a signal conditioning element between a differential output sensor and an ADC, which would allow for further processing and decision making.