AD620
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
10k
Ω
*
10k
Ω
*
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.
R = 350
Ω
R = 350
Ω
R
G
499
Ω
OP07D
AD620A
REFERENCE
100
Ω
**
10k
Ω
**
10k
Ω
**
OP07D
R = 350
Ω
R = 350
Ω
00775-0-039
PRECISION BRIDGE TRANSDUCER
Figure 39. Make vs. Buy
Table 3. Make vs. Buy Error Budget
Error Source
ABSOLUTE ACCURACY at T
A
= 25°C
Input Offset Voltage, µV
Output Offset Voltage, µV
Input Offset Current, nA
CMR, dB
AD620 Circuit Calculation
125 µV/20 mV
1000 µV/100 mV/20 mV
2 nA ×350 Ω/20 mV
110 dB(3.16 ppm) ×5 V/20 mV
“Homebrew” Circuit Calculation
(150 µV × √2)/20 mV
((150 µV × 2)/100)/20 mV
(6 nA ×350 Ω)/20 mV
(0.02% Match × 5 V)/20 mV/100
Total Absolute Error
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
Total Drift Error
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
(0.38 µV p-p × √2)/20 mV
Total Resolution Error
Grand Total Error
Error, ppm of Full Scale
AD620
Homebrew
6,250
500
18
791
7,559
3,600
3,000
450
7,050
40
14
54
14,663
10,607
150
53
500
11,310
6,000
10,607
150
16,757
40
27
67
28,134
G = 100, V
S
= ±15 V
.
(All errors are min/max and referred to input.)
Rev. G | Page 14 of 20
00775-0-041
SUPPLY CURRENT = 1.3mA MAX
00775-0-040
AD620A MONOLITHIC
INSTRUMENTATION
AMPLIFIER, G = 100
OP07D
10k
Ω
*
10k
Ω
*
"HOMEBREW" IN-AMP, G = 100
*0.02% RESISTOR MATCH, 3ppm/
°
C TRACKING
**DISCRETE 1% RESISTOR, 100ppm/
°
C TRACKING
SUPPLY CURRENT = 15mA MAX
AD [ ANALOG DEVICES ]
