Datasheet

W
W
W
W
≤
≤
INA139-Q1
INA169-Q1
SGLS185E –SEPTEMBER 2003–REVISED MAY 2011 www.ti.com
APPLICATION INFORMATION
Figure 9 illustrates the basic circuit diagram for both the INA139 and INA169. 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 RG1 by the internal
operational amplifier, 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 INA139 is:
I
O
= g
m
(V
IN+
− V
IN−
) where gm = 1000 μA/V.
In the circuit of Figure 9, the input voltage (V
IN+
− V
IN−
) is equal to IS x RS and the output voltage (V
OUT
) is equal
to I
O
x R
L
. The transconductance (gm) of the INA139 is 1000 μA/V. The complete transfer function for the current
measurement amplifier in this application is:
V
OUT
= (I
S
) (R
S
) (1000 μA/V) (R
L
)
The maximum differential input voltage for accurate measurements is 0.5 V, which produces a 500-μA output
current. A differential input voltage of up to 2 V 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, IO, will be zero, but it will not cause damage.
Figure 9. Basic Circuit Connections
BASIC CONNECTION
Figure 9 shows the basic connection of the INA139. 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 INA139. 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.
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