Datasheet

INA170
6
SBOS193D
www.ti.com
OPERATION
Figure 1 shows the basic circuit diagram for the INA170.
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.
Without offset, the transfer function for the INA170 is:
I
O
= g
m
(V
IN
+
– V
IN
–
)(1)
where g
m
= 1000µA/V (2)
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 INA170 is
1000µA/V. The complete transfer function for the current
measurement amplifier in this application is:
V
OUT
= (I
S
) (R
S
) (1000µA/V) (R
L
)(3)
Applying a positive reference voltage to pin 3 causes a
current to flow through R
OS
, forcing output current I
O
to be
offset from zero. The transfer function then becomes:
V
VR
R
IRR
k
OUT
REF L
OS
SSL
=






±




•
••
1 Ω
(4)
The maximum differential input voltage for accurate mea-
surements is 0.5V, which produces a 500µA output current.
A differential input voltage of up to 2V will not cause
damage. Differential measurements (pins 1 and 2) can be
bipolar with a more-positive voltage applied to pin 2. If a
more-negative voltage is applied to pin 1, output current I
O
will decrease towards zero.
BASIC CONNECTION
Figure 1 shows the basic connection of the INA170. 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 6 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 INA170. However, applications with noisy or high
impedance power supplies may require de-coupling capaci-
tors to reject power-supply noise. Connect bypass capacitors
close to the device pins.
POWER SUPPLIES
The input circuitry of the INA170 can accurately measure
beyond its power-supply voltage, V+. For example, the V+
power supply can be 5V, while the load power-supply
voltage (INA170 input voltage) is up to +60V. However, the
output-voltage range of the OUT terminal (pin 6) is limited
by the supply.
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
FIGURE 1. Basic Circuit Connections.
Shunt
R
S
INA170
4
6
OUT
Q1
R
L
I
0
+
V
O
–
Load
R
G1
1kΩ
R
G2
1kΩ
V
IN
V
IN
2
1
I
S
V+
V
REF
3
8
5
V+ power can be
common or
independent of
load supply.
2.7 ≤ (V+) ≤ 40V
V
P
Load Power Supply
+2.7 to 60V
+ –
Q2
R
OS
VOLTAGE GAIN EXACT R
L
(Ω) NEAREST 1% R
L
(Ω)
11k 1k
22k 2k
5 5k 4.99k
10 10k 10k
20 20k 20k
50 50k 49k
100 100k 100k