In this configuration, the gain of the circuit is set via the value of the resistor labeled RG. Looking at the input stage, consisting of the two operational amplifiers, any common-mode signal is only amplified by unity gain, regardless of the differential gain (set by RG) in the first two amplifiers. Hence, this circuitry can accommodate a wide common-mode range (limited by the headroom of the first two amplifiers), regardless of the gain. This is an advantage over the two op amp INA previously discussed. The difference amplifier will then remove any common-mode components. Similar to the previous architectures that have been discussed, the common-mode-rejection performance depends on the resistor ratio matching, as shown below:
Where: Rt = total mismatch of the resistor pairs
Due to the fact that the common-mode component always sees unity gain, the common-mode rejection of the three op amp instrumentation amplifier will increase proportionally with the amount of differential gain.
Several monolithic instrumentation amplifiers are based on this circuit concept. A monolithic solution offers very well matched amplifiers, and the ability to use trimmed resistors results in good common-mode rejection and gain accuracy. In more recent times, monolithic instrumentation amplifiers have made additional improvements to this basic architecture. Current-mode topologies, for example, eliminate the need for precision resistor matching in order to achieve high common-mode rejection. In any case, a discrete solution, using operational amplifiers and discrete components, will typically be more costly and result in degraded performance.
INA and Op Amp Specifications
As previously mentioned, operational amplifiers and instrumentation amplifiers are related, and as we have illustrated, op amps can be used to construct INAs. Due to this similarity, there are some specifications that are common to both operational and instrumentation amplifiers. However, there are also specifications that are unique to INAs, due to the specific functionality of such a device. Two important specifications for measurement applications that are common between op amps and INAs are input bias current and input offset voltage/offset voltage drift.
Input bias current is the amount of current flow into the inputs of the amplifier that is required to bias the input transistors. The magnitude of this current can vary from µA down to pA, and is strongly dependent upon the architecture of the amplifier-input circuitry. This parameter becomes extremely important when connecting a high-impedance sensor to the input of an amplifier. As the bias current flows through this high impedance, a voltage drop occurs across the impedance, resulting in a voltage error. Whether the circuit contains an operational amplifier or an instrumentation amplifier, bias current can play a critical role in the overall error budget of the circuitry.
Another important amplifier specification common to both operational and instrumentation amplifiers is input offset voltage. As the name implies, this specification is the amplifier’s voltage difference between the inverting and non-inverting inputs. This voltage offset is dependent on the topology of the amplifier, and can range from microvolts to millivolts in magnitude. Like all electrical components, amplifiers will change behavior over temperature. This is certainly true of the amplifier’s voltage offset. The voltage offset is a source of error, and, as the offset drifts over temperature, this error becomes correlated to the temperature. Even a high-precision amplifier will be susceptible to temperature drift. This error source can be minimized by selecting a low-drift amplifier—such as an amplifier with a zero-drift topology—or by implementing periodic system calibrations to calibrate out the offset and drift.