AD627ARZ-R7 datasheet(19/25 Pages) AD

19 / 25 page

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.

VOUT

+2.5V

+5V

AD627A GAIN = 9.98 (5+(200kΩ/RG))

HOMEBREW IN-AMP, G = +10

*1% RESISTOR MATCH, 50ppm/°C TRACKING

40.2kΩ

+10ppm/°C

AD627A

+5V

350Ω

3.15kΩ*

350Ω*

3.15kΩ*

±100mV

1/2

LT10781SB

1/2

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 TA = 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

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

Total Drift Error

6,504

7,207

Grand Total Error

27,039

45,879

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