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 that it has a reference (REF) terminal with a special architecture.

Second, the indirect current feedback architecture allows for an ideal diamond plot at higher 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 terminal must be driven with low impedance, so it is usually buffered with an operational amplifier or similar device after resistive voltage division to achieve low impedance [Figure 1-(B)].

If a resistor divider is used for driving the instrumentation amplifier as shown in Figure 1-(A), the voltage divider resistors will disrupt the balance of the subtractor circuit, resulting in a decrease in the common-mode noise rejection ratio of the instrumentation amplifier and a decrease in gain accuracy.

The AD8237 REF pin has a special architecture that allows the REF pin potential to be determined by a resistor divider without compromising performance.

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.

In a simple op amp circuit, the headroom is limited only by the input common-mode voltage range and output voltage, but an instrumentation amplifier combines two or three op amps, so the combined headroom limits of each individual input range, output range, and internal nodes must be taken into account.

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.

At a high gain setting (G = 100), as in the graph conditions in Figure 3, the AD8237's diamond plot becomes a simple rectangle.

This diamond plot characteristic allows the AD8237 to fully amplify small signals even in the presence of common-mode voltages equal to or slightly exceeding the supply voltages.

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, the use of an indirect current feedback architecture allows for the realization of an ideal diamond plot at high gain settings, which has the advantage of enabling designs using a wide range of output voltage (VOUT) relative to the input common-mode voltage (VCM).

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