Datasheet

AD627 Data Sheet
Rev. E | Page 18 of 24
MAKE vs. BUY: A TYPICAL APPLICATION ERROR
BUDGET
The example in Figure 41 serves as a good comparison between
the errors associated with an integrated and a discrete in-amp
implementation. A ±100 mV signal from a resistive bridge
(common-mode voltage = 2.5 V) 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
(1% match, 50 ppm/°C tracking).
The errors associated with each implementation (see Table 9)
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.
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 Ω), the offset current of
each op amp adds another error that must be characterized.
V
OUT
V
OUT
+2.5V
+2.5V
+5V +5V
AD627A GAIN = 9.98 (5+(200kΩ/R
G
)) HOMEBREW IN-AMP, G = +10
*1% RESISTOR MATCH, 50ppm/°C TRACKING
R
G
40.2kΩ
1%
+10ppm/°C
AD627A
+5V
350Ω
350Ω
350Ω
350Ω
3.15kΩ* 350Ω*
350Ω* 3.15kΩ*
±100mV
1/2
LT10781SB
LT10781SB
1/2
00782-039
Figure 41. Make vs. Buy
Table 9. Make vs. Buy Error Budget
Error Source AD627 Circuit Calculation Homebrew Circuit Calculation
Total Error
AD627
(ppm)
Total Error
Homebrew
(ppm)
ABSOLUTE ACCURACY at T
A
= 25°C
Total RTI Offset Voltage, mV (250 μV + (1000 μV/10))/100 mV (180 μV × 2)/100 mV 3,500 3,600
Input Offset Current, nA 1 nA × 350 Ω/100 mV 20 nA × 350 Ω/100 mV 3.5 70
Internal Offset Current
(Homebrew Only)
Not applicable 0.7 nA × 350 Ω/100 mV 2.45
CMRR, dB
77 dB
→
141 ppm × 2.5 V/100 mV
(1% match × 2.5 V)/10/100 mV 3,531 25,000
Gain 0.35% + 1% 1% match 13,500 10,000
Total Absolute Error 20,535 38,672
DRIFT TO 85°C
Gain Drift, ppm/°C (−75 + 10) ppm/°C × 60°C 50 ppm/°C × 60°C 3,900 3,000
Total RTI Offset Voltage, mV/°C
(3.0 μV/°C + (10 μV/°C/10)) ×
60°C/100 mV
(2 × 3.5 μV/°C × 60°C)/100 mV
2,600
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 6,504 7,207
Grand Total Error 27,039 45,879