INA149AIDR datasheet(15/24 Pages) TI

15 / 24 page

20 k

380 k

19 k

V−

V+

VO = RS × IDUT+

Device

Under

Test

IDUT+

(+275 V max)

+VS

20 k

380 k

19 k

V−

V+

(1)

IDUT−

−VS

(−275 V max)

VO = RS × IDUT−

(1)

INA149

www.ti.com

SBOS579A

– SEPTEMBER 2011 – REVISED NOVEMBER 2011

MEASURING CURRENT

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

Figure 46 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 46. 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 90-dB CMR. This error is

caused by the INA149 input impedance mismatch.

Figure 46. 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)

Example: For a 1-V/mA transfer function, the nominal, uncorrected value for RS would be 1 kΩ. A slightly larger

value (RS' = 1002.6 Ω), compensates for the gain error as a result of loading.

The 380-k

Ω term in the equation for RS' has a tolerance of 25%, thus sense resistors above approximately 400 Ω

may require trimming to achive gain accuracy better than 0.02%.

Submit Documentation Feedback

Product Folder Link(s): INA149

INA149AIDR Datasheet(PDF) 15 Page - Texas Instruments

INA149AIDR Datasheet(HTML) 15 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL