What is an Instrumentation Amplifier?

Many pressure sensors require amplification or close control over performance, in order to achieve the exact requirements for a specific application. In such cases it is common to use a simple strain-gauge sensor. These come in many different configurations, but a standard Wheatstone bridge arrangement is available from most manufacturers.

To interface this type of sensor to a standard input card is not always as simple as it seems. The major problem is that the signal output is elevated up on a DC level and the output change is very small (in the millivolt range). In addition, the zero offset can be a negative as well as a positive signal. Lastly, effects on the power supply will change the output from the sensor (ratiometric).

Diagram of Wheatstone Bridge

The amplifier circuit of choice is the instrumentation amplifier or differential input amplifier, and is one of the most versatile signal processing components available. It is commonly used for precision amplification of differential DC or AC signals as well as rejecting large values of common-mode noise.

Basic problems to avoid

In order to achieve a zero output, it is necessary to supply the IC with a positive and negative supply. Even now, most ICs still have a basic zero offset.

All pressure sensors have a zero offset (±5mV); this can be both positive and negative. Ensure that the amplifier is able to register the difference and perhaps even introduce a zero offset adjustment.

If single-ended supplies are used, we recommend moving the zero output up to a positive output, eg 1V.

The input impedance on the amplifier should ideally be 10 to 100 times larger than the sensor resistance in order to avoid bias current interference.

Finally, if you intend to have adjustable amplification we recommend doing this on a single-ended amplifier, which can be added after the instrumentation amplifier.

For full technical details read the Wheatstone bridge principle.

Read more: Index to all of our Technical Notes on Pressure

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