Current-Sense Amp Offers
Four-Quadrant Operation
By Alfredo H. Saab, Applications Engineering Manager,
and Tina Alikahi, Applications Engineer, Maxim Integrated
Products, Sunnyvale, Calif.
An op amp-based amplifier design overcomes
errors associated with resistive current sensing,
particularly those resulting from high commonmode voltages.
M
Current-Sense Challenges
any current-sensing ICs are available for
applications that require single-quadrant
operation, where positive common-mode
voltages are present and current flows in
only one direction. There are also ICs
that can handle current flow in both directions, known
as two-quadrant operation. But the choices become more
limited when the voltage and current ranges include positive and negative values, which require circuits capable of
four-quadrant operation.
One solution is a four-quadrant current-sense amplifier
design based on op amps and discrete components. This
amplifier design minimizes the effects of errors encountered
in resistor-based current sensing, particularly those associated with high common-mode voltages.
4.5 V to 76 V
I LOAD
VSENSE
ICHARGE
System load
and charger
Battery
RSENSE
RS-
The simplest way to measure an electrical current is
to insert a resistor in its path and sense the voltage drop.
Historically, this type of resistor is called a shunt, from the
time when all instruments were essentially voltmeters. In
these meters, the resistor would shunt the heavy current
away from the instrument, allowing the measurement to
be taken as a voltage.
The term more often used today in electronic systems
is “sense resistor,” though sometimes the term “burden
resistor” is used. The measurement is easy when one side
of the resistor is at the reference potential (ground), and the
measurement is called low-side current sensing.
When there is a common-mode potential (VCM) added
to the difference between the ends of the sense resistor
(high-side sensing), the measurement becomes harder as
the common-mode voltage is increased. The simplest form
of high-side sensing is where only positive common-mode
voltages are present and current flows in only one direction.
These conditions are described as a single-quadrant operation, which refers to a pair of coordinate axes with VCM and
I as the x and y coordinates.
Analog semiconductor companies offer a wide array of
current-sensing ICs for single-quadrant operation. These
devices are reasonably accurate and easy to use.
There also are ICs that can handle current flow in both
directions (two-quadrant operation). Fig. 1 illustrates a
typical application of such an IC. Both circuit types operate
with reasonable accuracy within a single-polarity (always
positive) range of VCM bounded by maximum and minimum
operating voltages.
The value of the minimum operating voltage can be
defined by the true minimum operating voltage below
which the circuit ceases to operate. But since the errors
grow rapidly as this true minimum voltage is approached,
the useful minimum voltage is usually a higher value. In
RS+
VCC
MAX4081T
REF1A
5V
REF1B
GND
OUT
Fig. 1. A current sensor designed for two-quadrant operation handles
current flow in both directions, but is restricted to common-mode
voltages of a single polarity.
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current sensing
either case, the minimum voltage rarely includes zero or
even gets close to zero.
When the current flow to be measured can be in either direction and the VCM can be of either polarity (the
four-quadrant case), the measurement becomes more
complicated. The complexity of the measurement increases
if the requirements for accuracy, precision and frequency
response are added to the design considerations. This type
of current sensing is required in many motor-control applications, magnetic-field correction systems, and ac or
ac + dc circuits.
Sources of Error
6/54 M6
Sensing current by way of a series resistor and a high-side
sensing IC introduces two errors. One error is the voltage
burden, which is the drop across the sense resistor in series
with the line voltage. This voltage drop
alters the voltage in the measured line.
R
VAB = VDIFF
I
A SENSE B
The other error is the current burden,
VCM
the small amount of current “stolen”
MAX5490
MAX5490
by the IC for its own operation. This
30
1
1
30
1N4148
current comes from the circuit in which
the current is being measured.
1 MF
The common-mode operating point
MAX4236
MAX4236
also
affects measurement accuracy,
A1
A2
R2
R1
1 MF
because the common-mode rejection
10 M7
10 M7
500 7-var
ratio (CMRR) of the sensing circuitry is
1N4148
not infinite. Since the current-carrying
+2.5 V
[+2.5 V Ref Voltage]
R3 331 k7
R4 331 k7
line voltage is a common-mode voltage,
[+2.5 V]
R5
there is an output-voltage uncertainty
T1
R6
5 M7
7
induced by the common-mode error
MAX4236
MAX4236
10 MF
A3
A4
OUT
that must be considered when estimating the error budget. As a consequence,
D1 VCC D2
Common
FS
the CMRR change with temperature is
MAX845
SD
also a concern.
10 MF
GND1
GND2
10 MF
-2.5 V
The sense resistor must always be in
a
four-terminal
type of configuration,
[Common]
R7
7
sometimes called a Kelvin connection.
In this configuration, there are two pairs
Fig. 2. The four-quadrant current sensor shown here can operate under common-mode
of terminals — one pair carries the curvoltages up to ±75 V, and has a current-sense voltage input range of ±80 mV for an output of
±2.5 V. (RSENSE is a four-terminal, Kelvin-type resistor. Resistors marked with ** are Caddock-type rent and the other carries the voltage
TF626R, 0.01%. Bracketed values are for single-supply operation. Output to circuits (ADCs, etc.) signal. One connection from each pair
converges with one of the other pairs at
must be taken between OUT and reference voltage.)
each resistor end.
The connection converging points
6#- 6
define the physical limits of the circuit
6#- 6
section length used for current sensing
6#- 6
the delimited section. If properly imple6#- 6
6#- 6
mented, the four-terminal technique
6#- 6
can exclude undesirable resistances
6#- 6
(contact resistances) that might exist
in the current path and the resistor
attachments, as well as the influence of
resistance in the voltage connections,
since they carry no current.
It is the resistance of the delimited
section that is the real sense-resistor
value, the factor that translates directly
the value of the current to be measured
into voltage, as:
VSENSE = RSENSE  IMEASURED .
As such, the resistor must have a very
4EMPERATURE ª#
small temperature coefficient, and its
Fig. 3. The amplifier’s dc CMRR behavior over temperature is critical to maintaining accuracy.
temperature increase due to dissipation
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current sensing
-20
-30
-40
VCM = 20 VPK-PK
CMRR (dB)
-50
-60
-70
-80
-90
-100
-110
-120
0.001
0.01
10
100
35
30
25
20
15
10
5
VCM = 20 VPK-PK
VDIFF = 4 mVPK-PK
0
0.001
0.01
0.1
1
10
100
1000
Frequency (kHz)
Fig. 5. A plot of gain versus frequency for the sensed voltage signal indicates the range
of frequencies over which the amplifier operates with a given acceptable error.
If a single op amp, standard instrumentation amplifier
configuration is used, the operational amplifier inputvoltage range (usually smaller than the output-voltage
swing) is the factor that limits the operating CMR. This
factor forces the use of a very small fractional gain, making
drift and noise problems worse.
With a two op amp instrumentation amplifier topology
(where both are configured as inverters), the op amp inputs
remain at the reference point — usually 0 V. So, the CMR
is limited only by the operational amplifier output swing,
allowing a higher CMR for a given supply voltage.
For all these amplifier types, the accuracy at the high
end of the useful CMR is set mostly by the CMRR. A better
CMRR will introduce a smaller uncertainty and, therefore,
One solution is to design a high-side current-sensing
circuit using discrete op amps. Naturally, this approach has
its challenges, too. One difficulty is the fact that most modern
high-performance op amps are low-voltage devices, with a
total supply voltage of 5 V or less. This fact complicates the circuit design when the common-mode range (CMR) is high.
The CMR of a high-side current-sense amplifier has three
values. The absolute maximum CMR voltage is the value
that, if exceeded, will destroy the amplifier. The operating
CMR is the range in which the amplifier will still process the
sense-resistor drop. And finally, the useful CMR is the range
that the current-sense resistor can swing through while the
amplifier still delivers an output within the error limits.
802PET22.indd 31
1
Fig. 4. Data on CMRR as a function of frequency is needed to calculate the uncertainty
when VCM is an ac voltage or contains ac components.
Four-Quadrant Operation
www.powerelectronics.com
0.1
Frequency (kHz)
Differntial gain (dB)
of its self-generated heat must be limited. This
is needed not only to maintain measurement
accuracy at a given value of current, but also
to keep the linearity of the measurement over
the current range. Power dissipation within
the resistor is low at low currents, but then
grows quickly with the square of the current.
Consequently, the value of the resistor might
change with the current value, resulting in a
nonlinear response if a large change in temperature is allowed.
In high-accuracy/precision measurements,
where discrete high-stability resistors are
used, no copper conductor should be present
within the bounds defined by the voltage contacts. That’s because the high-temperature coefficient of copper resistivity (0.39%/°C) will
deteriorate the desired sensor performance,
even if a good sense resistor is used.
Sense resistors also can introduce errors
in other ways. They can suffer from hysteresis, not returning to original values after
overloads. In addition, these components are
subject to long-term value drift (over months
or years) if mounted under mechanical stress,
which they release slowly while undergoing
temperature changes.
The designer must reduce all errors to
levels compatible with the expected design
accuracy by the choice of adequate circuits;
choice of sense resistor type, value and tolerance; good mechanical/thermal design; and
good electrical layout.
When the measurement problem includes a
bipolar range both for VCM and for the current
to be measured, or includes zero as part of the
VCM range, four-quadrant operation becomes
a necessity and the selection of available IC
solutions is limited. As the range for VCM is
increased, IC options become even more scarce
for a given accuracy limit.
31
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2/12/2008 9:38:40 AM
current sensing
“Step Forward, Raise Value”
Fuji Electric Device Technology
America, Inc.
Piscataway, NJ 08854, U.S.A.
Phone: 972-733-1700
Fax: 732-457-0042
For more Info. Visit
www.fujisemiconductor.com
better accuracy for a given common-mode voltage. The
CMRR is a function of the tolerance of the resistors used. As
the limits of the CMR are extended higher, the resistor tolerance required for a given accuracy becomes unbearably small.
Such precision resistors may not be available, and even when
they are, the cost of such resistors may be unbearably high.
The circuit in Fig. 2 alleviates these problems. First, the
circuit amplifies the sense-resistor voltage. Then, it applies
that signal to the amplifier that does the level translation
from the common-mode level of the current-carrying line to
the ground-referred level. In this way, the ratio of the desired
signal to the spurious (or uncertainties) is increased by the
gain of the first amplifier. This approach effectively increases
the accuracy of the measurement by the same factor.
Since the first amplifier reference point needs to move
with the common-mode level, it must be powered by an
isolated power source. The circuit in Fig. 2 can operate under
common-mode voltages up to ±75 V and has an input range
(voltage burden) of ±80 mV for an output of ±2.5 V.
The VOUT/VDIFF gain is 30, which is accurate within 0.1%.
The output-offset voltage at room temperature is less than
100 µV. The current burden is 5 MV or 10 µA total when
VCM is 50 V. The absolute maximum voltage (nonoperational)
for VCM is limited by the breakdown voltage of R1, R2 or T1,
whichever breaks first.
Amplifiers A1 and A2 make the first amplifier, powered
by an isolated power supply with the MAX845 highfrequency power-supply IC driving a low interwinding
capacitance transformer T1. A3 and A4 form the wide-CMR,
high-CMRR instrumentation amplifier.
Fig. 3 shows the amplifier’s dc CMRR behavior over
temperature, which is critical to maintaining accuracy.
The graph in this figure allows designers to determine the
amount of error introduced by the common-mode voltage
into the output voltage at each temperature.
For the amplifier described here, this error is adjusted to
zero at room temperature (by means of the 500-V potentiometer). The effect is attributed to changes in the ratios
R1/R3 and R2/R4 across the temperature range, and is about
2.5-mV change at the output from -10°C to 50°C.
Fig. 4 shows the same information as Fig. 3, but as a function of frequency. It presents the CMRR versus frequency
needed to calculate the uncertainty when VCM is an ac voltage, or contains ac components.
Fig. 5 is the gain as a function of frequency for the sensed
voltage signal. This plot shows the range of frequencies
(starting from dc) over which the amplifier is useful, within
a given acceptable error.
The frequency performance is important because it also
involves the phase shift behavior, which matters when the
current measurement is used to calculate power. In such
calculations, there are two considerations. First, there is
the frequency dependency of the absolute amplitude of the
current in the power formulas. But then, too, the cosine of
the phase angle weighs in as the so-called “power factor” in
the power calculations. PETech
32
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