The Operational Amplifier (Op Amp) is similar and related to the Instrumentation Amplifier (INA) since they are based on the same building block. However, the INA behaves somewhat differently and is simply 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 figure below 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 visit the Introduction to Operational Amplifiers (Op Amps) page.
INA vs Op Amp
The major difference between an INA and an op amp is the lack of an external feedback loop as shown in the figure below:
The feedback in an INA's is designed to be internal. Due to this reason, 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 figure below). INAs are used to provide differential gain, while offering high common mode rejection ratio, i.e., eliminating unwanted signals, such as interference and noise. For this reason, 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 figure below 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 pressure sensor, strain gauge (wheatsone bridge), remote thermocouple, and remote RTD (Resistance Temperature Detectors) where a current source or voltage divider produces a voltage from the RTD.