Skew VOS in current-sense amplifiers
to improve current measurements
Measure input offset voltage (VOS) without limitations due to interaction with
minimum output voltage
By Prashanth Holenarsipur, Lead Product Definer, Maxim Integrated Products Inc.
Introduction
Certain applications require that you calibrate the input offset voltage (VOS) of a current-sense
amplifier to enhance the accuracy of its current measurements. This calibration, however, is
hampered by interactions between the amplifier’s minimum output voltage (VOL) and its input
VOS at zero current. The following discussion presents a simple method for “skewing” the input
VOS of a unidirectional current-sense amplifier. The method lets you measure input VOS without
the usual limitations due to VOL, and also improves the overall measurement accuracy.
Current-sense amplifiers are sophisticated ICs widely used in electronic equipment for real-time
monitoring of load currents. System controllers use the load information to implement powermanagement algorithms, for modifying the load-current characteristic itself, and also to
implement flexible overcurrent protection schemes. By applying a known input VOS to currentsense amplifiers on the production line, an equipment manufacturer can calibrate the VOS at zero
input current, and thereby achieve better measurement accuracy for the amplifiers.
Current-sense amplifiers magnify a small differential voltage while rejecting the input commonmode voltage. In this role they operate like traditional op-amp-based differential amplifiers. The
two amplifier architectures have an important difference, however: for current-sense amplifiers,
input common-mode voltages are allowed to exceed the power-supply voltage (VCC).
When a current-sense amplifier such as the MAX4080, Figure 1, operates with VCC = 5 V, it can
withstand an input common-mode voltage of 76 V.
Figure 1: Though powered with VCC = 5 V, this precision, unidirectional current-sense amplifier
can withstand common-mode voltages as high as 76 V.
That is just one of many current-sense amplifiers available from multiple suppliers, and all have
the ability to amplify small differential voltages over various ranges that exceed the powersupply rails. Their unique architecture avoids the limitations of common-mode rejection (CMR)
that arise from mismatched resistors. The MAX4080, for example, has a minimum DC CMRR of
100dB. In contrast, a traditional op-amp-based differential amplifier is hampered by CMRR. As
a consequence, its effective input VOS can be magnified through the signal chain.
Using calibration to improve precision
Current-sense amplifiers like the MAX4080 offer precision input offset voltages (VOS) of ±0.6 mV
maximum at 25°C and ±1.2 mV maximum over the range -40°C to +125°C, but many applications
demand even higher precision. Some, for instance, require further calibration of the input VOS.
Typically, this calibration is achieved by measuring VOS during production and storing it in
firmware. Using the stored data, you can then adjust VOS digitally, in real time, while the equipment
is in use in the field.
A preferred method of calibration, intended for the convenience of manufacturers, measures VOS
when the load current is zero (producing zero input differential voltage). You can then measure
the output VOS and subtract that voltage from all subsequent measurements.
Unfortunately, this method has a drawback. Because the VOL (output voltage low) and input VOS
interact, the output voltage doesn’t accurately reflect the input VOS at zero input sense voltage.
This interaction is characteristic of all single-supply amplifiers.
Consider a MAX4080T with gain of 20 and a (hypothetical) zero input VOS. One would expect to
measure a true zero at the amplifier’s output. Even at a zero input differential voltage with 10 μA
sink current, however, the amplifier is not guaranteed to produce an output voltage below 15 mV.
If these output voltage measurements were used directly for VOS calibration, the amplifier would
appear to have an input VOS of 15 mV/20 = 0.75 mV.
Similarly, if the MAX4080T has VOL = 0, then a positive input VOS produces a positive output
VOS as expected. However, a negative input VOS does not produce a positive output, because the
amplifier cannot generate output voltages below ground. Thus, with zero input differential
voltage, one cannot use the output voltage measurement directly to obtain the input VOS.
One of two methods is typically used during production to calibrate VOS out of the outputvoltage measurements. First, a bidirectional current-sense amplifier like the MAX4081 includes
a reference voltage of about 1.5 V, which effectively translates the output voltage measurements
by 1.5 V. As a result, the output for a zero-input differential voltage is 1.5 V ± VOS. This 1.5 V is
well above the amplifier’s VOL, so it doesn’t affect the error analysis.
You can then calculate VOS errors by measuring the difference between the output voltage and
the ideal 1.5 V input reference voltage. This method has a few drawbacks, however. The 0 V-to5 V input dynamic range of the device’s ADC is now reduced by 30%, to a span of 1.5 V to 5 V.
In addition, the method requires that you use a more expensive bidirectional current-sense
amplifier for unidirectional measurements. Finally, it’s not attractive to provide a low-drift 1.5 V
reference voltage, or to spend a second channel just to measure that voltage.
An alternative approach
As an alternative, you can employ a two-point measurement method that applies two known
values of differential input voltage (load current) to the current-sense amplifier. You then
calculate the input VOS graphically by extrapolating to a zero-sense voltage, using a straight-line
approximation based on the two voltage measurements. The VOS value obtained is then used for
calibration. This method has its own drawbacks: it requires two known and exact values of test
current, whose generation is inconvenient during production, especially for high-current
applications. It also increases test complexity. As a final note, it fails to achieve accurate
measurements for input differential voltages near zero, because VOL limitations cause an error at
small sense voltages.
Using input resistors to introduce VOS
There is yet another method for measuring the input VOS of a current-sense amplifier. Suitable
for use on the production line, it applies a zero input differential voltage and overcomes the
interactions between VOL and VOS.
Because all current-sense amplifiers have input bias currents, you should study carefully the use
of input resistors (as part of an input filter, for instance), because the resistors can introduce
unplanned gain and offset errors. In this case (Figure 2), the input resistors are intentionally
mismatched to introduce a substantial output VOS.
Figure 2: A 2-kΩ resistor in series with RS- injects a known VOS into this current-sense amplifier.
A 2 k resistor in series with RS-, for example, produces an input VOS that depends on the op
amp’s input bias current, which in turn ranges (according to process variations) from 5 μA
typical to 12 μA maximum. As a result, the amplifier VOS ranges from 10 mV to 24 mV.
This added VOS is multiplied by the gain of 20, producing an output offset from 200 mV (typical)
to 480 mV (maximum), which is adequate to override any VOL and VOS limitations in the basic
amplifier. Error in this VOS (induced by the input resistor) has a temperature dependence based
on both the input resistor’s drift characteristics (usually 100 ppm), and on the bias current
(negligible for this amplifier).
The resistor’s drift characteristic (+100 ppm) causes a +1% change in resistance value (i.e. +20 )
over a 100 C change. Thus, additional input VOS drift from the input resistor is about +0.1 mV
(+0.24 mV maximum across the process variation in bias current). This drift, however, is only 10%
of the input VOS one expects from this amplifier (±1.2 mV) in the absence of calibration.
A further decrease in the size of the input series resistor reduces the drift error. To account for
the 15 mV VOL and the ±1.2 mV input VOS over temperature, the additional input VOS must be a
minimum of 1.2 mV + 15 mV/20 = 1.95 mV ≈ 2 mV.
Table 1 shows the test results over temperature. This particular amplifier under test has
negligible drift in VOS, so all the measured drift in VOS is due to the input resistor and to biascurrent effects.
Table 1 Temperature-test results,
with and without input resistor
VOS
No Input Resistor
2k in Series with RS-
-40 C
-0.015 mV
9.69 mV
+25 C
0 mV
9.73 mV
+85 C
-0.005 mV
9.76 mV
+125 C
-0.01 mV
9.80 mV
About the author
Prashanth Holenarsipur is currently Lead Product Definer with the Amplifiers Group at Maxim
Integrated Products. Prior to joining Maxim, he was with GE Healthcare, where he helped design
data-acquisition circuits for CT and PET scanners. He holds Bachelors and Masters degrees in
Electrical Engineering from the Indian Institute of Technology (IIT), Bombay, and the
University of Minnesota, Minneapolis, respectively. He can be reached at prashanth@maximic.com.