Datasheet

AD622 Data Sheet
Rev. E | 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 5. Make vs. Buy Error Budget
Error Source AD622 Circuit Calculation Homebrew Circuit Calculation
Total Error in ppm
Relative to 1 V FS
AD622 Homebrew
ABSOLUTE ACCURACY at T
A
= 25°C
Total RTI Offset Voltage, µV 125 µV + 1500 µV/10 800 µV × 2 275 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 302.5 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 (1 µV/°C + 15 µV/°C /10) × 60°C 9 µV/°C × 2 × 60°C 150 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 3450.12 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 3763 5775