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參數(shù)資料
| 型號： | AD621S |
| 廠商： | Analog Devices, Inc. |
| 元件分類： | 運動控制電子 |
| 英文描述： | Low Drift, Low Power Instrumentation Amplifier |
| 中文描述： | 低漂移，低功耗儀表放大器 |
| 文件頁數(shù)： | 10/16頁 |
| 文件大?。?/td> | 663K |
| 代理商： | AD621S |
AD621
REV. A
–10–
V
B
–V
S
A1
A2
A3
C2
Q1
Q2
25k
R3
400
10k
10k
I2
I1
10k
REF
10k
+IN
– IN
20
μ
A
20
μ
A
R4
400
OUTPUT
C1
25k
5555.6
5R6
G=100
G=100
+V
S
1
8
4
2
3
6
5
7
R1
R2
R5
Figure 29. Simplified Schematic of 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 pre-
serving 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 29 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 Super
β
eta 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
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 sub-
tracter 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 transis-
tors. This has three important advantages: (a) Open-loop gain is
boosted for increasing programmed gain, thus reducing gain-re-
lated 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 in-
put voltage noise is reduced to a value of 9 nV/
√
Hz
, determined
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 30, a gain of 100 is required to amplify a bridge out-
put 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 intelli-
gent processor, an auto-gain/auto-zero cycle will remove all ab-
solute 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 in-
put 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 in-
puts, both contributing to the overall input error.
R = 350
+10V
PRECISION BRIDGE TRANSDUCER
AD621A MONOLITHIC
INSTRUMENTATION
AMPLIFIER, G=100
3 OP-AMP IN-AMP, G=100
*0.02% RESISTOR MATCH, 3PPM/
°
C TRACKING
**DISCRETE 1% RESISTOR, 100PPM/
°
C TRACKING
SUPPLY CURRENT = 15mA MAX
100
**
10k
*
10k
**
10k
*
10k
*
10k
**
10k
*
SUPPLY CURRENT = 1.3mA MAX
OP07D
OP07D
OP07D
AD621A
REFERENCE
R = 350
R = 350
R = 350
Figure 30. Make vs. Buy
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