Datasheet
Table Of Contents
- Features
- Applications
- General Description
- Functional Block Diagram
- Revision History
- Specifications
- Absolute Maximum Ratings
- Pin Configuration and Function Descriptions
- Typical Performance Characteristics
- Theory of Operation
- Applications Information
- Evaluation Board
- Outline Dimensions
Data Sheet AD623
Rev. G | Page 23 of 32
THEORY OF OPERATION
The AD623 is an instrumentation amplifier based on a modified
classic 3-op-amp approach to ensure single- or dual-supply
operation even at common-mode voltages at the negative supply
rail. Low voltage offsets (input and output), absolute gain
accuracy, and one external resistor to set the gain make the
AD623 a versatile instrumentation amplifier.
The input signal is applied to positive-negative-positive (PNP)
transistors acting as voltage buffers and providing a common-
mode signal to the input amplifiers (see Figure 70). An absolute
value 50 kΩ resistor in each amplifier feedback ensures gain
programmability.
The differential output is
C
G
O
V
R
V
+
=
kΩ100
1
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 30, Figure 31, Figure 32, and Figure 33).
The output voltage at Pin 6 (OUTPUT) is measured with
respect to the potential at Pin 5 (REF). The impedance of the REF
pin is 100 kΩ. Therefore, in applications requiring voltage
conversion, a small resistor between Pin 5 (REF) and Pin 6
(OUTPUT) is all that is needed.
+V
S
+V
S
–V
S
7
4
–IN
+IN
2
3
7
–V
S
4
50kΩ
50kΩ 50kΩ
50kΩ
50kΩ
50kΩ
OUTPUT
REF
6
5
8
1
R
G
–R
G
+R
G
00778-041
Figure 70. Simplified Schematic
Because of the voltage feedback topology of the internal op
amps, the bandwidth of the instrumentation amplifier decreases
with increasing gain. At unity gain, the output amplifier limits
the bandwidth.