AD627
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.
+5V
350Ω
350Ω
+5V
+5V
LT10781SB
350Ω
350Ω
±100mV
R
G
40.2kΩ
1%
+10ppm/°C
AD627A
1/2
V
OUT
1/2
+2.5V
LT10781SB
V
OUT
AD627A GAIN = 9.98 (5+(200kΩ/R
G
))
HOMEBREW IN-AMP, G = +10
*1% RESISTOR MATCH, 50ppm/°C TRACKING
Figure 41. Make vs. Buy
Table 9. Make vs. Buy Error Budget
Total Error
AD627
(ppm)
3,500
3.5
Total Error
Homebrew
(ppm)
3,600
70
2.45
25,000
10,000
38,672
3,000
4,200
7
7,207
45,879
Error Source
ABSOLUTE ACCURACY at T
A
= 25°C
Total RTI Offset Voltage, mV
Input Offset Current, nA
Internal Offset Current
(Homebrew Only)
CMRR, dB
Gain
DRIFT TO 85°C
Gain Drift, ppm/°C
Total RTI Offset Voltage, mV/°C
Input Offset Current, pA/°C
AD627 Circuit Calculation
(250 μV + (1000 μV/10))/100 mV
1 nA × 350 Ω/100 mV
Not applicable
77 dB→141 ppm × 2.5 V/100 mV
0.35% + 0.1%
Homebrew Circuit Calculation
(180 μV × 2)/100 mV
20 nA × 350 Ω/100 mV
0.7 nA × 350 Ω/100 mV
(1% match × 2.5 V)/10/100 mV
1% match
Total Absolute Error
50 ppm/°C × 60°C
(2 × 3.5 μV/°C × 60°C)/100 mV
(33 pA/°C × 350 Ω × 60°C)/100 mV
Total Drift Error
Grand Total Error
3,531
13,500
20,535
3,900
2,600
3.5
6,504
27,039
(−75 + 10) ppm/°C × 60°C
(3.0 μV/°C + (10 μV/°C/10)) ×
60°C/100 mV
(16 pA/°C × 350 Ω × 60°C)/100 mV
Rev. D | Page 18 of 24
00782-039
+2.5V
3.15kΩ*
350Ω*
350Ω*
3.15kΩ*
AD [ ANALOG DEVICES ]
