What are the features of the AD8237 instrumentation amplifier?

The AD8237 is a micropower, zero-drift, rail-to-rail input/output instrumentation amplifier. Two resistors set the gain from 1x to 1000x.

The following two points are different from general instrumentation amplifiers.

The first is to have a reference (REF) terminal with a special architecture.

Second, by employing an indirect current feedback architecture, an ideal diamond plot can be achieved at high gain settings.

These two features simplify circuit design using the AD8237 instrumentation amplifier.

REF terminal with special architecture

All instrumentation amplifiers have a REF terminal, and it is a convenient terminal that determines the output voltage based on this terminal.

In a typical instrumentation amplifier, the REF pin must be driven at low impedance, so it is usually buffered with an operational amplifier or the like after resistive voltage division to make it low impedance [Figure 1-(B)].

If you take the method of driving with resistance division as shown in Fig. 1-(A), the balance of the subtractor circuit will be lost due to this voltage division resistance, and as a result, the common mode noise rejection ratio of the instrumentation amplifier will decrease, and the gain accuracy will decrease. You need to be careful as it will degrade.

The AD8237 REF pin has a special architecture. Therefore, even if the potential of the REF pin is determined by resistance division, the performance is not impaired.

If the gain is high, it is possible to directly connect the semi-fixed resistor for adjustment. This reduces the number of buffer op amps required for the REF pin from the instrumentation amplifier circuit.

Also, the offset voltage of the AD8237 is very small, and offset adjustment is also possible here.

diamond plot

Figure 2 shows the achievable output voltage (VOUT) for a given input common mode voltage (VCM). Because the shape is hexagonal, Analog Devices calls this diagram a diamond plot.

Instrumentation amplifiers appear to be independent of the input common-mode voltage, but when the input common-mode voltage approaches the supply voltage, they saturate the internal amplifier, even though the input and output voltages themselves are within range.

For simple op amp circuits, this input common-mode voltage range and output voltage are the only limits on headroom. However, instrumentation amplifiers use a combination of two or three op amps, and each combination of input range, output range, and headroom limitations such as internal nodes must be considered.

The range that takes these into account should be used in the white area represented by the diamond plot and labeled VALID RANGE in Figure 2. Especially in low-supply and single-supply applications, the problem becomes more difficult as the diamond plot is much smaller and the operating range is limited.

AD8237 Diamond Plot

The AD8237 is one of the few in-amps that can match the ideal diamond plot in most configurations.

For large gain settings (G=100), such as the graph conditions in Figure 3, the diamond plot for the AD8237 is a simple square.

With this diamond plot characteristic, the AD8237 can fully amplify very small signals with common-mode voltages equal to or slightly above the supply voltage.

Summary

In circuit design using an instrumentation amplifier, the use of the AD8237 makes it possible to reduce the size of the circuit because it can be configured without using a buffer op amp for the REF pin.

In addition, by adopting an indirect current feedback architecture, an ideal diamond plot can be realized at a large gain setting, which has the advantage of being able to use a wide output voltage (VOUT) range with respect to the input common mode voltage (VCM). .

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