AD620B AD [Analog Devices], AD620B Datasheet - Page 11

• Manufacturer Part Number: AD620B
• Description: Low Cost, Low Power Instrumentation Amplifier
• Manufacturer: AD [Analog Devices]

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
Error Source
ABSOLUTE ACCURACY at T
DRIFT TO +85 C
RESOLUTION
G = 100, V
(All errors are min/max and referred to input.)
REV. E
Input Offset Voltage, V
Output Offset Voltage, V
Input Offset Current, nA
CMR, dB
Gain Drift, ppm/ C
Input Offset Voltage Drift, V/ C
Output Offset Voltage Drift, V/ C
Gain Nonlinearity, ppm of Full Scale
Typ 0.1 Hz–10 Hz Voltage Noise, V p-p 0.28 V p-p/20 mV
PRECISION BRIDGE TRANSDUCER
R = 350
R = 350
S
= 15 V.
+10V
R = 350
R = 350
A
= +25 C
AD620 Circuit
Calculation
125 V/20 mV
1000 V/100/20 mV
2 nA × 350 /20 mV
110 dB→3.16 ppm, × 5 V/20 mV (0.02% Match × 5 V)/20 mV/100
(50 ppm + 10 ppm) × 60 C
1 V/ C × 60 C/20 mV
15 V/ C × 60 C/100/20 mV
40 ppm
499
SUPPLY CURRENT = 1.3mA MAX
R
G
Table I. Make vs. Buy Error Budget
AD620A MONOLITHIC
INSTRUMENTATION
AMPLIFIER, G = 100
Figure 34. Make vs. Buy
AD620A
REFERENCE
–11–
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.
“Homebrew” Circuit
Calculation
(150 V × 2)/20 mV
((150 V × 2)/100)/20 mV
(6 nA × 350 )/20 mV
Total Absolute Error
100 ppm/ C Track × 60 C
(2.5 V/ C × 2 × 60 C)/20 mV
(2.5 V/ C × 2 × 60 C)/100/20 mV
Total Drift Error
40 ppm
(0.38 V p-p × 2)/20 mV
Total Resolution Error
Grand Total Error
100 **
**DISCRETE 1% RESISTOR, 100PPM/ C TRACKING
*0.02% RESISTOR MATCH, 3PPM/ C TRACKING
“HOMEBREW” IN-AMP, G = 100
SUPPLY CURRENT = 15mA MAX
OP07D
OP07D
10k **
10k **
10k *
10k *
Error, ppm of Full Scale
AD620
16,250
14,500
14,118
14,791
17,558
13,600
13,000
14,450
17,050
14,140
141,14
14,154
14,662
OP07D
10k *
10k *
AD620
Homebrew
10,607
10,150
14,153
10,500
11,310
16,000
10,607
10,150
16,757
10,140
13,127
101,67
28,134