Datasheet

INA138-Q1, INA168-Q1
SGLS174G –SEPTEMBER 2003–REVISED JANUARY 2013 www.ti.com
APPLICATION INFORMATION
Figure 9 illustrates the basic circuit diagram for both the INA138-Q1 and INA168-Q1. Load current I
S
is drawn
from supply V
S
through shunt resistor R
S
. The voltage drop V
S
in the shunt resistor is forced across R
G1
by the
internal op amp, causing current to flow into the collector of Q1. External resistor R
L
converts the output current
to a voltage, V
OUT
, at the OUT pin. The transfer function for the INA138-Q1 is:
I
O
= g
m
(V
IN+
− V
IN−
)
where g
m
= 200 μA/V.
In the circuit of Figure 9, the input voltage, (V
IN+
− V
IN−
), is equal to I
S
× R
S
and the output voltage, V
OUT
, is equal
to I
O
× R
L
. The transconductance, g
m
, of the INA138-Q1 is 200 μA/V. The complete transfer function for the
current measurement amplifier in this application is:
V
OUT
= (I
S
) (R
S
) (200 μA/V) (R
L
)
The maximum differential input voltage for accurate measurements is 0.5 V, which produces a 10 μA output
current. A differential input voltage of up to 2 V does not cause damage. Differential measurements (pins 3 and
4) must be unipolar with a more-positive voltage applied to pin 3. If a more-negative voltage is applied to pin 3,
the output current, I
O
, is zero, but it does not cause damage.
BASIC CONNECTION
Figure 9 shows the basic connection of the INA138-Q1. The input pins, V
IN+
and V
IN−
, should be connected as
closely as possible to the shunt resistor to minimize any resistance in series with the shunt resistance. The
output resistor, R
L
, is shown connected between pin 1 and ground. Best accuracy is achieved with the output
voltage measured directly across R
L
. This is especially important in high-current systems where load current
could flow in the ground connections, affecting the measurement accuracy.
No power-supply bypass capacitors are required for stability of the INA138-Q1. However, applications with noisy
or high-impedance power supplies may require decoupling capacitors to reject power-supply noise. Connect
bypass capacitors close to the device pins.
POWER SUPPLIES
The input circuitry of the INA138-Q1 can accurately measure beyond its power-supply voltage, V
+
. For example,
the V
+
power supply can be 5 V, whereas the load power supply voltage is up to 36 V (or 60 V with the INA168-
Q1). The output voltage range of the OUT terminal, however, is limited by the lesser of the two voltages (see
Output Voltage Range section).
SELECTING R
S
AND R
L
The value chosen for the shunt resistor, R
S
, depends on the application and is a compromise between small-
signal accuracy and maximum permissible voltage loss in the measurement line. High values of R
S
provide better
accuracy at lower currents by minimizing the effects of offset, while low values of R
S
minimize voltage loss in the
supply line. For most applications, the best performance is attained with an R
S
value that provides a full-scale
shunt voltage range of 50 mV to 100 mV. Maximum input voltage for accurate measurements is 500 mV.
R
L
is chosen to provide the desired full-scale output voltage. The output impedance of the INA138-Q1 OUT
terminal is very high, which permits using values of R
L
up to 500 kΩ with excellent accuracy. The input
impedance of any additional circuitry at the output should be much higher than the value of R
L
to avoid degrading
accuracy.
Some analog-to-digital (A/D) converters have input impedances that significantly affects 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 9 shows the
recommended values of R
L
.
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