An instrumentation amplifier (In-Amp) is a device that amplifies the difference between its two input terminals while rejecting common-mode signals and produces an output voltage with respect to a reference terminal. In-Amps provide the important function of extracting small signals from the output of transducers, where noisy signals must be rejected. In-Amps are usually deployed in applications such as strain and weight measurement using load cells or strain gauges, as well as temperature measurement where RTD, Thermocouples or thermistors are used. In-amps are widely used in medical applications where bio-signals such as ECG, EMG and EEG as well as blood pressure are being measured.
Although In-Amps provide huge benefits to the end solution design, there are some key attributes that need to be considered to ensure correct operation. One of these crops up when using low voltage supplies or single supply voltages to power an In-Amp. In-Amps typically have inaccessible internal nodes which can saturate and result in an incorrect output depending on the particular configuration which includes the common-mode voltage, gain, and reference pin voltage. The diamond plot is a common way of illustrating the valid input and output operating ranges of an In-Amp.
So how do you ensure that the requirements of your design will always be met by the chosen In-Amp?
To ensure that the chosen configuration is optimized and always within the operating limits of the device, the In-Amp Diamond Plot tool from Analog Devices can help with this. The Diamond plot tool provides key information regarding the correct operating range for the chosen In-Amp based on the design requirements of
- Input range
- Desired output swing
- Reference voltage
- In-Amp Gain
- Power supply rails
To see the Diamond Plot tool in action take a look at this excellent video available on analog devices’ website or Analog Devices’ youtube here. While there, why not take a look there our additional education videos on this topic and many more on Analog Device’s YouTube Channel.
Another great resource is the AN-1401 Application Note titled Instrumentation Amplifier Common-Mode Range: The Diamond Plot, this note gives an excellent overview, of the in-amp diamond plot.
For even more details on instrumentation amplifiers from the fundamentals to how they operate and examples of how to use and where they apply then take a look at A Designer's Guide to Instrumentation Amplifiers, 3RD Edition available in the Education Library (here) on Analog Device’s Website.
If you have a question in relation to any of the low power In-Amps highlighted in the Precision Low Power signal chains why not ask a question here and we’ll help you work through your design challenges.