Introduction To Instrumentation Amplifiers
The Operational Amplifier (op-amp) is similar and related to the Instrumentation Amplifier (INA). Both are based on the same building block. However, the INA behaves somewhat differently and is a specialized version of the op-amp, configured to perform special functions.
Many op-amp applications involve the use of a feedback loop, e.g. inverting, non-inverting gain-controlled amplifier, voltage follower (buffer), active filters, etc. The figure below shows an example of a voltage follower. If R1 and R2 do not exist, the op-amp is configured as a voltage follower.
Other op amp applications involve using open-loop configurations, such as a comparator configuration. The accompanying image shows an open-loop op-amp used as a comparator where VDD and VSS are the positive and negative supplies of the op-amp.
For more details about op-amp configurations, please check Introduction to Operational Amplifiers.
INA vs Op-Amp
The major difference is the lack of an external feedback loop, as shown in the accompanying image:
The feedback in INA is designed to be internal. Due to this, the INA configuration is usually limited to one or two external resistors. Because a closed-loop op amp with feedback can be configured to control its gain using external feedback resistors, the common-mode signal at the inputs will also be amplified by the closed-loop gain. The result is that the dynamic range of the output is limited. Just like op-amp, an INA amplifies the difference between the two inputs of the INA (see accompanying image). INAs are used to provide differential gain while offering a high common-mode rejection ratio, i.e., eliminating unwanted signals, such as interference and noise. Therefore, INAs are often found in sensor applications that require low system noise and high resolution.
INA Features & Applications
In most cases, INAs are used for:
- providing differential gain
- common-mode rejection
The accompanying image shows an example of an INA application that is used as a remote voltage sensor. It performs the basic function of an INA which amplifies the difference between Vsen+ and Vsen- while rejecting common-mode noise. INA applications can also be found in the pressure sensor, strain gauge (Wheatstone bridge), remote thermocouple, and remote Resistance Temperature Detectors (RTDs) where a current source or voltage divider produces a voltage from the RTD.
