Datasheet
INA138, INA168
6
SBOS122C
www.ti.com
FIGURE 1. Basic Circuit Connections.
Shunt
R
S
INA138
2
1
OUT
Q1
R
L
I
0
+
V
O
–
Load
R
G1
5kΩ
R
G2
5kΩ
V
IN+
V
IN–
3
4
I
S
V+
5
NOTE: (1) Maximum V
P
and V+ voltage is 60V with INA168.
V+ power can be common or
independent of load supply.
2.7 ≤ (V+) ≤ 36V
(1)
V
P
Load Power Supply
+2.7 to 36V
(1)
VOLTAGE GAIN EXACT R
L
(Ω) NEAREST 1% R
L
(Ω)
1 5k 4.99k
2 10k 10k
5 25k 24.9k
10 50k 49.9k
20 100k 100k
50 250k 249k
100 500k 499k
Some Analog-to-Digital (A/D) converters have input imped-
ances that will significantly affect measurement gain. The input
impedance of the A/D converter can be included as part of the
effective R
L
if its input can be modeled as a resistor to ground.
Alternatively, an op amp can be used to buffer the A/D converter
input. Figure 1 shows the recommended values of R
L
.
OUTPUT VOLTAGE RANGE
The output of the INA138 is a current, which is converted to
a voltage by the load resistor, R
L
. The output current remains
accurate within the compliance voltage range of the output
circuitry. The shunt voltage and the input common-mode and
power-supply voltages limit the maximum possible output
swing. The maximum output voltage compliance is limited by
the lower of the two equations below:
V
out
max
= (V+) – 0.7V – (V
IN+
– V
IN–
)(3)
or
V
out
max
= V
IN–
– 0.5V (4)
(whichever is lower)
BANDWIDTH
Measurement bandwidth is affected by the value of the load
resistor, R
L
. High gain produced by high values of R
L
will
yield a narrower measurement bandwidth (see Typical
Characteristics). For widest possible bandwidth, keep the
capacitive load on the output to a minimum. Reduction in
bandwidth due to capacitive load is shown in the Typical
Characteristics.
If bandwidth limiting (filtering) is desired, a capacitor can be
added to the output (see Figure 3). This will not cause
instability.
APPLICATIONS
The INA138 is designed for current shunt measurement
circuits, as shown in Figure 1, but its basic function is useful
in a wide range of circuitry. A creative engineer will find many
unforeseen uses in measurement and level shifting circuits.
A few ideas are illustrated in Figures 2 through 7.