AD623
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 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
7
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.
INVERTING
2
4
50kΩ
50kΩ
50kΩ
50kΩ
50kΩ
50kΩ
1
GAIN
8
OTUPUT
6
The differential output is
REF
5
⎛
⎜
⎝
100 kꢀ ⎞
⎜
⎟
VC
VO = 1+
7
4
⎟
RG
⎠
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.
NONINVERTING
3
NEGATIVE SUPPLY
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).
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.
Rev. D | Page 15 of 24
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