Datasheet

INA170
7
SBOS193D
www.ti.com
currents by minimizing the effects of offset, while low values
of R
S
minimize voltage loss in the supply line. For most
applications, best performance is attained with an R
S
value
that provides a full-scale shunt voltage 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 INA170 Out terminal is very
high which permits using values of R
L
up to 100kΩ 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 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, as shown in Figure 2. See
Figure 1 for recommended values of R
L
.
OUTPUT VOLTAGE RANGE
The output of the INA170 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
–
)(5)
or
V
out max
= V
IN
–
– 0.5V (6)
(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 Char-
acteristic Curves). For widest possible bandwidth, keep the
capacitive load on the output to a minimum.
If bandwidth limiting (filtering) is desired, a capacitor can be
added to the output, as shown in Figure 3. This will not
cause instability.
APPLICATIONS
The INA170 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.
FIGURE 4. Offsetting the Output Voltage.
FIGURE 2. Buffering Output to Drive A/D Converter. FIGURE 3. Output Filter.
I
S
OPA340
INA170
2
1
4
6
Z
IN
R
L
Buffer of amp drives A/D converter
without affecting gain.
INA170
f
–3dB
=
1
2πR
L
C
L
V
O
f
–3dB
R
L
C
L
2
1
4
6
V
0
V
REF
R
L
6
Gain Set by R
L
Output Offset Current =
V
REF
R
OS
Output Offset Voltage = • R
L
V
REF
R
OS
INA170
2
1
45
3
I
S
R
OS