Datasheet

+
20 kΩ 380 kΩ
380 kΩ
380 kΩ
19 kΩ
V− = −15 V V+ = +15 V
4 7
2
3
1
6
5
V
OUT
R
S
= 10 Ω
V
CM
= 265 V
R
C
= 10 Ω
(1)
I
MAX
= 1 A
INA149
www.ti.com SBOS579B –SEPTEMBER 2011–REVISED JULY 2012
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.
(1) See the Measuring Current section for details about R
C
.
Figure 48. Typical Current Measurement Application
The maximum current through the shunt resistor (R
S
) is 1 A and generates a full-scale voltage drop of 10 V. All
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
Accuracy, T
A
= +25°C
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.
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