Datasheet

+
20 kΩ 380 kΩ
380 kΩ
380 kΩ
19 kΩ
V− V+
4 7
2
3
1
6
5
V
O
= R
S
× I
DUT+
R
S
Device
Under
Test
I
DUT+
(+275 V max)
+V
S
+
20 kΩ 380 kΩ
380 kΩ
380 kΩ
19 kΩ
V− V+
4 7
2
3
1
6
5
R
S
R
C
(1)
I
DUT−
−V
S
(−275 V max)
V
O
= R
S
× I
DUT−
R
C
(1)
INA149
www.ti.com SBOS579B –SEPTEMBER 2011–REVISED JULY 2012
MEASURING CURRENT
The INA149 can be used to measure a current by sensing the voltage drop across a series resistor, R
S
.
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 R
S
, 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 (R
C
), equal to the value of R
S
, as
shown in Figure 46. If R
S
is less than 5 Ω, degradation in the CMR is negligible and R
C
can be omitted. If R
S
is
larger than approximately 1 kΩ, trimming R
C
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 R
S
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 R
S
. The corrected value (R
S
') can be calculated by
R
S
' = R
S
× 380 kΩ/(380 kΩ – R
S
) (3)
Example: For a 1-V/mA transfer function, the nominal, uncorrected value for R
S
would be 1 kΩ. A slightly larger
value (R
S
' = 1002.6 Ω), compensates for the gain error as a result of loading.
The 380-kΩ term in the equation for R
S
' has a tolerance of 25%, thus sense resistors above approximately 400 Ω
may require trimming to achive gain accuracy better than 0.02%.
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