Ad620 – Analog Devices AD620 User Manual
Page 11
AD620
REV. E
–11–
Make vs. Buy: A Typical Bridge Application Error Budget
The AD620 offers improved performance over “homebrew”
three op amp IA designs, along with smaller size, fewer compo-
nents and 10
× lower supply current. In the typical application,
shown in Figure 34, 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 in which it is being used, the AD620
provides greater accuracy, and at low power and price. In simple
R = 350
⍀
+10V
PRECISION BRIDGE TRANSDUCER
AD620A MONOLITHIC
INSTRUMENTATION
AMPLIFIER, G = 100
“HOMEBREW” 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
AD620A
R
G
499
⍀
REFERENCE
R = 350
⍀
R = 350
⍀
R = 350
⍀
Figure 34. Make vs. Buy
Table I. Make vs. Buy Error Budget
AD620 Circuit
“Homebrew” Circuit
Error, ppm of Full Scale
Error Source
Calculation
Calculation
AD620
Homebrew
ABSOLUTE ACCURACY at T
A
= +25
°
C
Input Offset Voltage,
µ
V
125
µ
V/20 mV
(150
µ
V
×
√
2)/20 mV
1
6,250
10,607
Output Offset Voltage,
µ
V
1000
µ
V/100/20 mV
((150
µ
V
× 2)/100)/20 mV
14,
500
10,
150
Input Offset Current, nA
2 nA
× 350
Ω
/20 mV
(6 nA
× 350
Ω
)/20 mV
14,1
18
14,1
53
CMR, dB
110 dB
→3.16 ppm, × 5 V/20 mV (0.02% Match × 5 V)/20 mV/100
14,
791
10,
500
Total Absolute Error
1
7,558
11,310
DRIFT TO +85
°
C
Gain Drift, ppm/
°
C
(50 ppm + 10 ppm)
× 60
°
C
100 ppm/
°
C Track
× 60
°
C
1
3,600
1
6,000
Input Offset Voltage Drift,
µ
V/
°
C
1
µ
V/
°
C
× 60
°
C/20 mV
(2.5
µ
V/
°
C
Ч
√
2
Ч 60
°
C)/20 mV
1
3,000
10,607
Output Offset Voltage Drift,
µ
V/
°
C
15
µ
V/
°
C
× 60
°
C/100/20 mV
(2.5
µ
V/
°
C
Ч 2 Ч 60
°
C)/100/20 mV
14,
450
10,
150
Total Drift Error
1
7,050
16,757
RESOLUTION
Gain Nonlinearity, ppm of Full Scale
40 ppm
40 ppm
14,1
40
10,1
40
Typ 0.1 Hz–10 Hz Voltage Noise,
µ
V p-p 0.28
µ
V p-p/20 mV
(0.38
µ
V p-p
×
√
2)/20 mV
141,
14
13,1
27
Total Resolution Error
14,1
54
101,
67
Grand Total Error
14,662
28,134
G = 100, V
S
=
±
15 V.
(All errors are min/max and referred to input.)
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 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.