AD620

Rev. G | Page 14 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 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.

R = 350

Ω

10V

PRECISION BRIDGE TRANSDUCER

R = 350

Ω

R = 350

Ω

R = 350

Ω

AD620A MONOLITHIC

INSTRUMENTATION

AMPLIFIER, G = 100

SUPPLY CURRENT = 1.3mA MAX

AD620A

499

Ω

REFERENCE

Figure 39. 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

Table 3. Make vs. Buy Error Budget

Error, ppm of Full Scale

Error Source

AD620 Circuit Calculation

“Homebrew” Circuit Calculation

AD620

Homebrew

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

.

(All errors are min/max and referred to input.)