INA149AMDREP datasheet(17/26 Pages) TI1

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20 kΩ

380 kΩ

19 kΩ

V−

V+

VO = RS × IDUT+

Device

Under

Test

IDUT+

(+275 V max)

+VS

20 kΩ

380 kΩ

19 kΩ

V−

V+

(1)

IDUT−

−VS

(−275 V max)

VO = RS × IDUT−

(1)

INA149-EP

www.ti.com

SBOS608A – MARCH 2012 – REVISED APRIL 2012

MEASURING CURRENT

The INA149 can be used to measure a current by sensing the voltage drop across a series resistor, RS.

Figure 47 shows the INA149 used to measure the supply currents of a device under test.

The sense resistor imbalances the input resistor matching of the INA149, thus degrading its CMR. Also, the input

impedance of the INA149 loads RS, causing gain error in the voltage-to-current conversion. Both of these errors

can be easily corrected.

The CMR error can be corrected with the addition of a compensation resistor (RC), equal to the value of RS, as

shown in Figure 47. If RS is less than 5 Ω, degradation in the CMR is negligible and RC can be omitted. If RS is

larger than approximately 1 k

Ω, trimming RC may be required to achive greater than 84-dB CMR. This error is

caused by the INA149 input impedance mismatch.

Figure 47. Measuring Supply Currents of a Device Under Test

If RS is more than approximately 50 Ω, the gain error is greater than the 0.02% specification of the INA149. This

gain error can be corrected by slightly increasing the value of RS. The corrected value (RS') can be calculated by

RS' = RS × 380 kΩ/(380 kΩ – RS)

(3)

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