AD621 Analog Devices, Inc., AD621 Datasheet - Page 10

• Manufacturer Part Number: AD621
• Description: Low Drift, Low Power Instrumentation Amplifier
• Manufacturer: Analog Devices, Inc.

AD621
THEORY OF OPERATION
The AD621 is a monolithic instrumentation amplifier based on
a modification of the classic three op amp circuit. Careful layout
of the chip, with particular attention to thermal symmetry builds
in tight matching and tracking of critical components, thus
preserving the high level of performance inherent in this circuit,
at a low price.
On chip gain resistors are pretrimmed for gains of 10 and 100.
The AD621 is preset to a gain of 10. A single external jumper
(between Pins 1 and 8) is all that is needed to select a gain of
100. Special design techniques assure a low gain TC of 5 ppm/°C
max, even at a gain of 100.
Figure 3 is a simplified schematic of the AD621. The input
transistors Q1 and Q2 provide a single differential-pair bipolar
input for high precision, yet offer 10× lower Input Bias Current,
thanks to Superβeta processing. Feedback through the Q1-A1-R1
loop and the Q2-A2-R2 loop maintains constant collector cur-
rent of the input devices Q1 and Q2, thereby impressing the
input voltage across the gain-setting resistor, RG, which equals
–IN
2
400
R3
I1
PRECISION BRIDGE TRANSDUCER
Q1
R = 350
R = 350
20 A
C1
555.6
R1 25k
G = 100
–
R6
A1
5555.6
1
+V
–V
10V
V
R5
B
7
S
4
S
8
G = 100
R2
A2
R = 350
R = 350
20 A
25k
–
C2
Q2
I2
10k
10k
400
R4
3
–
+
10k
10k
+IN
A3
SUPPLY CURRENT = 1.3mA MAX
AD621A MONOLITHIC
INSTRUMENTATION
AMPLIFIER, G = 100
5
OUTPUT
AD621A
REF
+
–
6
REFERENCE
R5 at a gain of 10 or the parallel combination of R5 and R6 at a
gain of 100.
This creates a differential gain from the inputs to the A1/A2
outputs given by G = (R1 + R2) / RG + 1. The unity-gain
subtracter A3 removes any common-mode signal, yielding a
single-ended output referred to the REF pin potential.
The value of RG also determines the transconductance of the
preamp stage. As RG is reduced for larger gains, the transcon-
ductance increases asymptotically to that of the input transistors.
This has three important advantages: (a) Open-loop gain is
boosted for increasing programmed gain, thus reducing gain-
related errors. (b) The gain-bandwidth product (determined by
C1, C2 and the preamp transconductance) increases with pro-
grammed gain, thus optimizing frequency response. (c) The
input voltage noise is reduced to a value of 9 nV/√Hz, deter-
mined mainly by the collector current and base resistance of the
input devices.
Make vs. Buy: A Typical Bridge Application Error Budget
The AD621 offers improved performance over discrete three op
amp IA designs, along with smaller size, fewer components and
10 times lower supply current. In the typical application, shown
in Figure 4, a gain of 100 is required to amplify a bridge output of
20 mV full scale over the industrial temperature range of –40°C to
+85°C. The error budget table below shows how to calculate
the effect various error sources have on circuit accuracy.
Regardless of the system it is being used in, the AD621 provides
greater accuracy, and at low power and price. In simple systems,
absolute accuracy and drift errors are by far the most significant
contributors to error. In more complex systems with an intelligent
processor, an autogain/autozero cycle will remove all absolute
accuracy and drift errors leaving only the resolution errors of
gain nonlinearity and noise, thus allowing full 14-bit accuracy.
Note that for the discrete circuit, the OP07 specifications for
input voltage offset and noise have been multiplied by 2. This is
because a three op amp type in amp has two op amps at its inputs,
both contributing to the overall input error.
100k
0.02% RESISTOR MATCH, 3PPM/ C TRACKING
DISCRETE 1% RESISTOR, 100PPM/ C TRACKING
SUPPLY CURRENT = 15mA MAX
3 OP AMP, IN AMP, G = 100
10k
OP07D
OP07D
+
–
10k
–
+
10k
10k
10k
OP07D
+
–
10k