AD622

Rev. D | Page 10 of 16

The AD622 provides greater accuracy at lower cost. The higher

cost of the homebrew circuit is dominated in this case by the

matched resistor network. One could also realize a homebrew

design using cheaper discrete resistors that are either trimmed

or hand selected to give high common-mode rejection. This

level of common-mode rejection, however, degrades significantly

over temperature due to the drift mismatch of the discrete

resistors.

Note that for the homebrew circuit, the LT1013 specification for

noise has been multiplied by √2. This is because a two op amp

type instrumentation amplifier has two op amps at its inputs,

both contributing to the overall noise.

Table 4. Make vs. Buy Error Budget

Total Error in ppm

Relative to 1 V FS

Error Source

AD622 Circuit Calculation

Homebrew Circuit Calculation

AD622

Homebrew

Total RTI Offset Voltage, μV

250 μV + 1500 μV/10

800 μV × 2

400

1600

Input Offset Current, nA

2.5 nA × 1 kΩ

15 nA × 1 kΩ

2.5

15

CMR, dB

86 dB

→50 ppm × 0.5 V

(0.1% Match × 0.5 V)/10 V

25

50

Total Absolute Error

427.5

1665

DRIFT TO 85°C

Gain Drift, ppm/°C

(50 ppm + 5 ppm) × 60°C

(50 ppm)/°C × 60°C

3300

3000

Total RTI Offset Voltage, μV/°C

(2 μV/°C + 15 μV/°C /10) × 60°C

9 μV/°C × 2 × 60°C

210

1080

Input Offset Current, pA/°C

2 pA/°C × 1 kΩ × 60°C

155 pA/°C × 1 kΩ × 60°C

0.12

9.3

Total Drift Error

3510.12

4089.3

RESOLUTION

Gain Nonlinearity, ppm of Full Scale

10 ppm

20 ppm

10

20

Typ 0.1 Hz to 10 Hz Voltage Noise, μV p-p

0.6 μV p-p

0.55 μV p-p × √2

0.6

0.778

Total Resolution Error

10.6

20.778

Grand Total Error

3948

5775