Datasheet

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INA139-Q1
INA169-Q1
www.ti.com SGLS185E –SEPTEMBER 2003–REVISED MAY 2011
POWER SUPPLIES
The input circuitry of the INA139 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 INA169).
However, the output voltage range of the OUT terminal is limited by the lesser of the two voltages (see the
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, 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 INA139 OUT terminal
is very high, which permits using values of R
L
up to 100 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 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 10.
See Figure 9 for recommended values of R
L
.
OUTPUT VOLTAGE RANGE
The output of the INA139 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
+
) − 0.7 V − (V
IN+
− V
IN−
)
or
V
OUT MAX
= V
IN−
− 0.5 V (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
yields a narrower measurement bandwidth (see the Typical Characteristics section). 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 11), which
will not cause instability.
Figure 10. Buffering Output to Drive the A/D Figure 11. Output Filter
Converter
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