Datasheet

INA138, INA168
5
SBOS122C
www.ti.com
TYPICAL CHARACTERISTICS (Cont.)
At T
A
= +25°C, V+ = 5V, V
IN+
= 12V, and R
L
= 125kΩ, unless otherwise noted.
STEP RESPONSE
10µs/div
200mV
G = 1
100mV
100mV
G = 1
0mV
50mV/div
STEP RESPONSE
10µs/div
G = 25
0V
G = 10
0V
1V/div
500mV/div
OPERATION
Figure 1 illustrates the basic circuit diagram for both the
INA138 and INA168. Load current I
S
is drawn from supply V
S
through shunt resistor R
S
. The voltage drop in shunt resistor
V
S
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 is:
I
O
= g
m
(V
IN+
– V
IN–
)(1)
where g
m
= 200µA/V.
In the circuit of Figure 1, 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 is
200µA/V. The complete transfer function for the current
measurement amplifier in this application is:
V
OUT
= (I
S
) (R
S
) (200µA/V) (R
L
)(2)
The maximum differential input voltage for accurate mea-
surements is 0.5V, which produces a 100µA output current.
A differential input voltage of up to 2V will 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
, will
be zero, but it will not cause damage.
BASIC CONNECTION
Figure 1 shows the basic connection of the INA138. 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. However, applications with noisy or high-
impedance power supplies may require decoupling capaci-
tors to reject power-supply noise. Connect bypass capacitors
close to the device pins.
POWER SUPPLIES
The input circuitry of the INA138 can accurately measure
beyond its power-supply voltage, V+. For example, the V+
power supply can be 5V, whereas the load power supply
voltage is up to +36V (or +60V with the INA168). 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 accu-
racy and maximum permissible voltage loss in the measure-
ment 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 applica-
tions, best performance is attained with an R
S
value that
provides a full-scale shunt voltage range of 50mV to 100mV.
Maximum input voltage for accurate measurements is 500mV.
R
L
is chosen to provide the desired full-scale output voltage.
The output impedance of the INA138 OUT terminal is very
high which permits using values of R
L
up to 500kΩ 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.