AD623ARZ-R7 datasheet(15/24 Pages) AD

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AD623

Rev. D | Page 15 of 24

THEORY OF OPERATION

The AD623 is an instrumentation amplifier based on a modified

classic 3-op-amp approach, to assure single or dual supply

operation even at common-mode voltages at the negative

supply rail. Low voltage offsets, input and output, as well as

absolute gain accuracy, and one external resistor to set the

gain, make the AD623 one of the most versatile instrumentation

amplifiers in its class.

The input signal is applied to PNP transistors acting as voltage

buffers and providing a common-mode signal to the input

amplifiers (see Figure 41). An absolute value 50 kΩ resistor in

each amplifier feedback assures gain programmability.

The differential output is

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

kΩ

100

The differential voltage is then converted to a single-ended

voltage using the output amplifier, which also rejects any

common-mode signal at the output of the input amplifiers.

Because the amplifiers can swing to either supply rail, as well as

have their common-mode range extended to below the negative

supply rail, the range over which the AD623 can operate is further

enhanced (see Figure 20 and Figure 21).

The output voltage at Pin 6 is measured with respect to the

potential at Pin 5. The impedance of the reference pin is 100 kΩ;

therefore, in applications requiring V/I conversion, a small

resistor between Pin 5 and Pin 6 is all that is needed.

POSITIVE SUPPLY

INVERTING

NEGATIVE SUPPLY

NONINVERTING

50kΩ

OTUPUT

REF

GAIN

Figure 41. Simplified Schematic

Note that the bandwidth of the in-amp decreases as gain is

increased. This occurs because the internal op-amps are the

standard voltage feedback design. At unity gain, the output

amplifier limits the bandwidth.

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