AD620
Note that for the homebrew 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.
Regardless of the system in which it is being used, the AD620
provides greater accuracy 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.
10V
10kΩ*
10kΩ*
OP07D
R
G
AD620A
499
Ω
R = 350
R = 350
Ω
Ω
R = 350Ω
10k
10k
Ω
Ω
**
**
REFERENCE
100Ω**
OP07D
R = 350
Ω
AD620A MONOLITHIC
INSTRUMENTATION
AMPLIFIER, G = 100
OP07D
10kΩ*
10kΩ*
SUPPLY CURRENT = 1.3mA MAX
"HOMEBREW" IN-AMP, G = 100
*0.02% RESISTOR MATCH, 3ppm/
°
C TRACKING
**DISCRETE 1% RESISTOR, 100ppm/
°
C TRACKING
SUPPLY CURRENT = 15mA MAX
PRECISION BRIDGE TRANSDUCER
Figure 39. Make vs. Buy
Table 3. Make vs. Buy Error Budget
Error, ppm of Full Scale
Error Source
AD620 Circuit Calculation
“Homebrew” Circuit Calculation
AD620
Homebrew
ABSOLUTE ACCURACY at TA = 25°C
Input Offset Voltage, µV
Output Offset Voltage, µV
Input Offset Current, nA
CMR, dB
125 µV/20 mV
1000 µV/100 mV/20 mV
2 nA ×350 Ω/20 mV
(150 µV × √2)/20 mV
((150 µV × 2)/100)/20 mV
(6 nA ×350 Ω)/20 mV
6,250
500
18
10,607
150
53
500
110 dB(3.16 ppm) ×5 V/20 mV
(0.02% Match × 5 V)/20 mV/100
791
Total Absolute Error
7,559
11,310
DRIFT TO 85°C
Gain Drift, ppm/°C
Input Offset Voltage Drift, µV/°C
Output Offset Voltage Drift, µV/°C
(50 ppm + 10 ppm) ×60°C
1 µV/°C × 60°C/20 mV
15 µV/°C × 60°C/100 mV/20 mV
100 ppm/°C Track × 60°C
(2.5 µV/°C × √2 × 60°C)/20 mV
(2.5 µV/°C × 2 × 60°C)/100 mV/20 mV
3,600
3,000
450
6,000
10,607
150
Total Drift Error
7,050
16,757
RESOLUTION
Gain Nonlinearity, ppm of Full Scale
Typ 0.1 Hz to 10 Hz Voltage Noise, µV p-p
40 ppm
0.28 µV p-p/20 mV
40 ppm
40
14
54
40
27
67
(0.38 µV p-p × √2)/20 mV
Total Resolution Error
Grand Total Error
14,663
28,134
G = 100, VS = 15 V.
(All errors are min/max and referred to input.)
Rev. G | Page 14 of 20
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