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Texas Instruments Incorporated
Amplifiers: Op Amps
A dual-polarity, bidirectional current-
shunt monitor
By Thomas Kuehl
Senior Applications Engineer
Current-shunt-monitor ICs are an exten-
sion of the instrumentation amplifier
family of products. They provide an easy
method of monitoring circuit current and
possess similarities to the sensitive analog
voltmeter and external shunt resistor
commonly used to measure current in the
past. The analog voltmeter registered a
small voltage drop that developed when
current passed through a shunt resistor.
With a current-shunt monitor, the volt-
meter has been replaced by a specially
adapted instrumentation amplifier that
amplifies the voltage developed across
the shunt resistor. An output measure is
provided that is proportional to the cur-
rent through the resistor. Analog volt-
meters were commonly designed for a
full-scale voltage of 50 or 100 mV, and the
current-shunt monitor operates with
comparable input-voltage levels. Instead
of an analog meter-scale indication, the
current-shunt monitor provides a voltage
or current output level, or a digital output
code, that directly corresponds to the
measured current level.
A variety of current-shunt monitors are available that are
designed for high-side or low-side circuit connection, with
someofferingdifferentuserfunctions.Oftenthevoltage
ranges of the current-shunt-monitor supply and the
common-mode input are independent of each other. This
allows the current-shunt monitor to be operated from a
convenient supply-voltage level independent of the input
voltage. Many applications need only monitor current flow-
ing in one direction, and a current-shunt monitor such as
the INA138/168 provides this capability. A monitor intended
for single-direction, or unidirectional, current flow is
referred to as a unidirectional current-shunt monitor.
Otherapplicationsrequireabidirectionalcurrent-shunt
monitor where the circuit current can flow and be moni-
tored in either direction.
An example of a bidirectional current-shunt monitor is
the INA170. It is powered by a single supply voltage of
+2.7 to +40 V, while the input common-mode voltage
(CMV) may be any voltage between +2.7 and +60 V. The
input CMV is the external voltage that is applied to the
current-shunt-monitor input and provides current to the
output load. When the current through the shunt resistor
is zero, both inputs of the current-shunt monitor are ideally
Figure 1. The INA170 connected for bidirectional
current monitoring
V+
V Supply
8
R
1 k
INA170
G Ω
V IN+
2
–
+
+
I S
R
1 k
R S
G Ω
A1
Q1
V IN–
1
6
+
–
–
Output
V REF
3
+
Load
A2
Q2
R L
–
R OS
5
R OS
GND
4
at the same CMV potential; but when a current flows
through the shunt resistor, a differential voltage is devel-
oped and the inputs become separated by that amount.
This voltage difference is amplified by the gain factor of
the current-shunt monitor.
The bidirectional property allows the INA170 to monitor
current between two voltage potentials that are more posi-
tive or negative relative to each other. A simple illustration
of a bidirectional-current-flow system is a motor that draws
current from a battery when operating at a constant speed
or accelerating, but then acts as a generator, returning
current to the battery during deceleration. Figure 1 shows
the INA170 connected for bidirectional current monitoring.
Thus, two important current-shunt-monitor operating
parameters are the unidirectional or bidirectional input
voltage characteristic and the CMV range. The operational
CMV range often extends from near 0 V to a specified
maximum positive voltage, but some current-shunt moni-
tors include a negative voltage range as well. For example,
the INA193 through INA198 current-shunt-monitor family
provides a CMV range of –16 to +80 V. These devices are
unidirectional; so even if the input voltage is a negative
voltage, the output has to be more negative for current to
29
Analog Applications Journal
4Q 2008
High-Performance Analog Products
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Amplifiers: Op Amps
Texas Instruments Incorporated
Figure 2. Two INA193 ICs configured as a bidirectional current-shunt monitor
I F
I R
R S
V Supply
To Load
INA152
+5 V
+5 V
40 k Ω
40 k Ω
V IN+
V IN–
V+
V IN+
V IN–
V+
–
INA193
INA193
Output
OUT
OUT
+
+2.5-V
Ref
40 k Ω
40 k Ω
100 nF
Input CMV range =–16 to +80 V
1
2
3
6
5
4
REF3225
+5 V
470 nF
flow in the correct direction. Two of the unidirectional
current-shunt-monitor ICs may be interconnected to form
a bidirectional current-shunt monitor with a CMV range
that extends from –16 to +80 V. The addition of the INA152
instrumentation amplifier and +2.5-V reference completes
the circuit. A circuit schematic for the INA193 bidirectional
current-shunt monitor is shown in Figure 2.
Recently, a customer described an application where
monitoring a DC motor’s current was necessary. It was an
automotive application, and the system supply was available
to power a current-shunt monitor. The customer wanted
to know the current levels when the motor was running
normally in the forward direction and in reverse where a
negative, back EMF developed. The circuit shown in
Figure 2 is appropriate for this application; but the cus-
tomer wanted to keep the number of components and the
cost to a minimum, even if some precision had to be sacri-
ficed. This called for a different approach.
Earlier it was mentioned that current-shunt monitors are
an extension of instrumentation amplifiers. Also included
in the family are difference amplifiers, which consist of an
instrumentation-grade operational amplifier and four or
more precisely matched resistors. The difference amplifier
amplifies the difference in voltages applied to the two
inputs by a fixed gain. Gain and common-mode rejection
of the difference amplifier are optimized by precise laser
trimming of four thin-film resistors included on the inte-
grated circuit die. Difference amplifiers with a fixed gain
of 1:1, 10:1, and 100:1 V/V are commonly available; how-
ever, there is a unique product—the INA159—that has a
fixed gain setting of 0.2 V/V. Its primary role is to serve as
a level-translation amplifier between sensors having a
bipolar output voltage with a range of ±10 V, and a modern
analog-to-digital converter having a unipolar input range
of 0 to 5 V. The INA159 is a true difference amplifier that
can sense a differential voltage of either polarity. Its input
CMV range extends from –12.5 to +17.5 V when powered
from a single +5-V supply. These features allow the INA159
to be employed as a dual-polarity, bidirectional current
monitor. An additional instrumentation or operational
amplifier needs to be included after the INA159 to increase
the overall gain of the monitor circuit.
30
High-Performance Analog Products
4Q 2008
Analog Applications Journal
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Texas Instruments Incorporated
Amplifiers: Op Amps
Figure 3. INA159 dual-polarity, bidirectional current-shunt-monitor circuit
1 M Ω
2
4
–
4
2
5
–
OPA340
Output
3
6
6
R
0.1
S
Ω
INA159
+
1 µF
DC Motor
or Load
3
8
7
+
1
7
+5 V
1 µF
+5 V
Control
Motor Driver
10.1
k Ω
5.1
M Ω
+5 V
+
5 V
10 k
Midscale
Set
Ω
100 nF
(Optional)
1
2
3
6
5
4
REF3225
Input CMV range =–12.5 to +17.5 V
with +5-V supply
+5 V
470 nF
Figure 3 shows the circuit of the INA159 current-shunt-
monitorstage,followedbyanOPA340CMOSoperational-
amplifiergainstage.TheOPA340wasselectedbecauseits
near rail-to-rail input and output swing ranges allow the
circuit’s output voltage to approach 0 and 5 V at the
extremes.TheINA159gainis+0.2V/V,whiletheOPA340
gain is set to +100 V/V, for an overall circuit gain of +20 V/V.
AhigherOPA340closed-loopgaincouldbe
used to increase sensitivity, but the DC
errors and bandwidth would suffer. Also,
the shunt resistor R S , could be increased
from 0.1 W to a larger value. This would
increase the INA159 output, but the con-
sequences of the larger voltage drop and
higher resistor power dissipation should
be evaluated before doing so.
The output voltage delivered from the
INA159 current-shunt-monitor circuit is
centered at a level of about +2.5 V. This
is the approximate voltage level at the
OPA340outputwhennocurrentisflow-
ing through R S . Figure 4 shows an oscillo-
scope image of the INA159 current-shunt
monitor’s output response when the input
circuit is being driven by a 24-V PP , 1-kHz
sine wave. This input-voltage waveform is
recorded as the upper trace image. The
resistive load in the input circuit is 12 W,
resulting in peak current levels of ±1 A.
ThelowertraceisthatoftheOPA340
output voltage swinging from 0.5 to 4.5 V, indicating a peak
current of approximately ±1 A. A center-scale, manual
zeroingcircuitisincludedintheOPA340stage.Itcanbe
excluded if an exact 2.5-V center-scale voltage is not nec-
essary.TheINA159andOPA340combinedexhibitaband-
width of well over 100 kHz, making this circuit usable for a
wide variety of AC-current-monitoring applications.
Figure 4. Input/output waveforms of current-shunt-monitor circuit
Input Drive: 24 V
at 1 kHz with 12-
Ω
load
PP
C1
(5 V/div)
OPA340 Output Swing: 0.5 to 4.5 V
indicates ±1-A peak drive current.
(2 V/div)
C2
Timebase (500 µs/div)
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Analog Applications Journal
4Q 2008
High-Performance Analog Products
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Amplifiers: Op Amps
Texas Instruments Incorporated
Figure 5 shows oscilloscope images of the INA159
current-shunt monitor monitoring the current of a DC
pancake motor under load. The motor is being driven by a
slow 9-V PP , 0.1-Hz sine wave such that the armature direc-
tion follows the sine function, reversing direction every
half cycle. This input drive voltage, shown as the upper
trace, had to be adjusted to keep the motor from drawing
more than 1 A of current. The lower trace shows the
OPA340outputvoltagewherethepeakmotorcurrent
exceeds 1 A. Some evidence of clipping is seen at the
OPA340outputasoutput-voltage-swinglimits.Withproper
sizing of components, the circuit can be optimized for
monitoring this particular motor’s current levels. None-
theless, this illustrates the utility of the INA159 in a motor-
current-monitoring application.
Accuracy of the INA159 current-shunt monitor was
measured at a little better than 4.5%. Standard 1% resistors
were used, with no special selection being made. The cir-
cuitaccuracycouldbeimprovedbyreplacingtheOPA340
circuit with a precision, single-supply instrumentation
amplifier such as the INA326. However, the AC bandwidth
will decrease and the cost will be higher.
Related Web sites
www.ti.com/sc/device/ partnumber
Replace partnumber with INA138, INA152 , INA159,
Figure 5. Input/output waveforms of current-shunt-monitor
circuit of DC motor
Motor Drive: 9 V at 0.1 Hz
(5 V/div)
PP
C1
OPA340 Output Swing: Over 4.5 V
Clipping indicates over 1-A drive current.
(2 V/div).
C2
Timebase (5 s/div)
32
High-Performance Analog Products
4Q 2008
Analog Applications Journal
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