+

20 k

380 k

380 k

380 k

19 k

V− = −15 V

V+ = +15 V

4

7

2

3

1

6

5

(1)

INA149

www.ti.com

SBOS579A

– SEPTEMBER 2011 – REVISED NOVEMBER 2011

ERROR BUDGET ANALYSIS

The following error budget analysis demonstrates the importance of a high common-mode rejection ratio when

measuring small differential signals in the presence of high common-mode voltages. Figure 48 shows a typical

current measurement application.

Figure 48. Typical Current Measurement Application

error sources in this calculation are shown in relation to this full-scale voltage. The common-mode voltage in this

scenario is 265 V and the temperature range is from room temperature (+25

°C) to +85°C. Table 2 shows the

dominant error sources for the INA149 and a competitor device.

Table 2. Error Budget Analysis

ERROR (ppm of FS)

ERROR

SOURCE

INA149

COMPETIOR A

INA149

COMPETITOR A

Initial gain error

0.02% FS

0.05% FS

200

500

Offset voltage

1100

µV

1000

µV

110

100

Common mode

265 V/90 dB = 8380

µV

265 V/77 dB = 37432

µV

838

3743

Total acuracy error

1148

4343

Temperature drift

Gain

10 ppm/

°C × 60°C

10 ppm/

°C × 60°C

600

600

Offset voltage

10

µV/°C × 60°C

20

µV/°C × 60°C

60

120

Total drift error

660

720

Total error

1808

5063

If a smaller shunt resistor is used, the full-scale voltage drop is also smaller. A shunt resistor of 1

Ω causes a 1-V

voltage drop with a current of 1 A flowing through it. The error of 1808 ppm for a full-scale voltage of 10 V

becomes 18080 ppm (1.6%) for a full-scale voltage of only 1 V.

This example demonstrates that the dominate source of error, even over temperature, comes from the CMRR

specification of the devices. The common-mode error is 46% of the total error for the INA149 and 74% of the

total error for the competitor device.

Copyright

© 2011, Texas Instruments Incorporated

Submit Documentation Feedback

17

Product Folder Link(s): INA149