Datasheet
AD620
Rev. H | Page 13 of 20
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
removes 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 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
10V
PRECISION BRIDGE TRANSDUCE
R
R = 350
Ω
R = 350
Ω
R = 350
Ω
00775-0-039
AD620A MONOLITHIC
INSTRUMENTATION
AMPLIFIER, G = 100
SUPPLY CURRENT = 1.3mA MAX
AD620A
R
G
499
Ω
REFERENCE
00775-0-040
Figure 37. Make vs. Buy
"HOMEBREW" IN-AMP, G = 100
*0.02% RESISTOR MATCH, 3ppm/
°
C TRACKING
**DISCRETE 1% RESISTOR, 100ppm/
°
C TRACKING
SUPPLY CURRENT = 15mA MAX
100
Ω
**
10k
Ω
*
10k
Ω
**
10k
Ω
*
10k
Ω
*
10k
Ω
**
10k
Ω
*
OP07D
OP07D
OP07D
00775-0-041
Table 4. Make vs. Buy Error Budget
Error, ppm of Full Scale
Error Source AD620 Circuit Calculation “Homebrew” Circuit Calculation AD620 Homebrew
ABSOLUTE ACCURACY at T
A
= 25°C
Input Offset Voltage, μV 125 μV/20 mV (150 μV × √2)/20 mV 6,250 10,607
Output Offset Voltage, μV 1000 μV/100 mV/20 mV ((150 μV × 2)/100)/20 mV 500 150
Input Offset Current, nA 2 nA ×350 Ω/20 mV (6 nA ×350 Ω)/20 mV 18 53
CMR, dB 110 dB(3.16 ppm) ×5 V/20 mV (0.02% Match × 5 V)/20 mV/100 791 500
Total Absolute Error 7,559 11,310
DRIFT TO 85°C
Gain Drift, ppm/°C (50 ppm + 10 ppm) ×60°C 100 ppm/°C Track × 60°C 3,600 6,000
Input Offset Voltage Drift, μV/°C 1 μV/°C × 60°C/20 mV (2.5 μV/°C × √2 × 60°C)/20 mV 3,000 10,607
Output Offset Voltage Drift, μV/°C 15 μV/°C × 60°C/100 mV/20 mV (2.5 μV/°C × 2 × 60°C)/100 mV/20 mV 450 150
Total Drift Error 7,050 16,757
RESOLUTION
Gain Nonlinearity, ppm of Full Scale 40 ppm 40 ppm 40 40
Typ 0.1 Hz to 10 Hz Voltage Noise, μV p-p 0.28 μV p-p/20 mV (0.38 μV p-p × √2)/20 mV 14 27
Total Resolution Error 54 67
Grand Total Error 14,663 28,134
G = 100, V
S
= ±15 V.
(All errors are min/max and referred to input.)