AD627
The errors associated with each implementation (see Table 9ꢀ
show the integrated in-amp to be more precise at both ambient
and overtemperature. Note that the discrete implementation is
more expensive, primarily due to the relatively high cost of the
low drift precision resistor network.
MAKE vs. BUY: A TYPICAL APPLICATION ERROR
BUDGET
The example in Figure 4± serves as a good comparison between
the errors associated with an integrated and a discrete in-amp
implementation. A ±±00 mV signal from a resistive bridge
(common-mode voltage = 2.5 Vꢀ is amplified. This example
compares the resulting errors from a discrete two-op-amp
instrumentation amplifier and the AD627. The discrete
implementation uses a four-resistor precision network
(±% match, 50 ppm/°C trackingꢀ.
The input offset current of the discrete instrumentation amplifier
implementation is the difference in the bias currents of the two-
op amplifiers, not the offset currents of the individual op amps.
In addition, although the values of the resistor network are chosen
so that the inverting and noninverting inputs of each op amp
see the same impedance (about 350 Ωꢀ, the offset current of
each op amp adds another error that must be characterized.
+5V
+5V
+5V
350Ω
350Ω
350Ω
350Ω
LT10781SB
R
G
V
1/2
OUT
40.2kΩ
1%
+10ppm/°C
LT10781SB
AD627A
V
OUT
±100mV
1/2
+2.5V
3.15kΩ*
350Ω*
350Ω*
3.15kΩ*
+2.5V
AD627A GAIN = 9.98 (5+(200kΩ/R ))
HOMEBREW IN-AMP, G = +10
*1% RESISTOR MATCH, 50ppm/°C TRACKING
G
Figure 41. Make vs. Buy
Table 9. Make vs. Buy Error Budget
Total Error
AD627
(ppm)
Total Error
Homebrew
(ppm)
Error Source
AD627 Circuit Calculation
Homebrew Circuit Calculation
ABSOLUTE ACCURACY at TA = 25°C
Total RTI Offset Voltage, mV
Input Offset Current, nA
Internal Offset Current
(Homebrew Only)
(250 ꢀV + (1000 ꢀV/10))/100 mV
1 nA × 350 Ω/100 mV
Not applicable
(180 ꢀV × 2)/100 mV
20 nA × 350 Ω/100 mV
0.7 nA × 350 Ω/100 mV
3,500
3.5
3,600
70
2.45
CMRR, dB
Gain
(1% match × 2.5 V)/10/100 mV
1% match
3,531
25,000
10,000
38,672
77 dB→141 ppm × 2.5 V/100 mV
0.35% + 0.1%
13,500
20,535
Total Absolute Error
DRIFT TO 85°C
Gain Drift, ppm/°C
Total RTI Offset Voltage, mV/°C (3.0 ꢀV/°C + (10 ꢀV/°C/10)) ×
60°C/100 mV
(−75 + 10) ppm/°C × 60°C
50 ppm/°C × 60°C
(2 × 3.5 ꢀV/°C × 60°C)/100 mV
3,900
2,600
3,000
4,200
Input Offset Current, pA/°C
(16 pA/°C × 350 Ω × 60°C)/100 mV (33 pA/°C × 350 Ω × 60°C)/100 mV
3.5
7
Total Drift Error
Grand Total Error
6,504
27,039
7,207
45,879
Rev. D | Page 18 of 24
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