Differential Overvoltage Protection Circuits for Current Sense Amplifiers

TECHNICAL ARTICLE
Emmanuel Adrados,
Paul Blanchard
Analog Devices, Inc.
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DIFFERENTIAL
OVERVOLTAGE PROTECTION
CIRCUITS FOR CURRENT
SENSE AMPLIFIERS
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Introduction
Harsh environments are a reality for many electrical systems used
in motor control or solenoid control applications. The electronics
that control motors and solenoids are by necessity in close proximity to the high currents and voltages used to create the physical
movement required by the end application. In addition to proximity,
these systems are often serviceable (for example, one might hire
a technician to change the controller board for a dishwasher solenoid), which leaves open the possibility of unintentional wiring
errors. The proximity to high currents and voltage, coupled with a
potential for incorrect wiring, necessitate a design that incorporates overvoltage protection.
To create efficient and safe systems, precision current sense
amplifiers monitor the currents in these applications. The precision
amplifier circuits need to be designed to protect from overvoltage
conditions, but these protection circuits may impact the accuracy
of the amplifier.
With proper circuit design, analysis, and verification, it is possible
to balance the trade-off between protection and accuracy. This
article discusses two common protection circuits and how the
implementation of these circuits affect the accuracy of the current
sense amplifiers.
12 V
Battery
+
–
Current Sense
Amplifier
Clamp
Diode
Solenoid
Figure 1. High-side current sensing in a solenoid control application.
Overvoltage Protection Circuit
Figure 2 shows the basic connection for overvoltage protection of a
current sense amplifier. When the differential input voltage exceeds the
maximum rated value for a given amplifier, the amplifier may begin to pull
current into the internal protection diodes. The additional series resistors,
R1 and R2, prevent large current flow to the internal protection diodes if
a large differential voltage signal is present between the input pins.
+5 V
R1
Current Sense Amplifiers
V1
Most current sense amplifiers are capable of handling high commonmode voltages (CMVs) but not high differential input voltages. In
certain applications, there are instances where the differential input
voltage at the shunt exceeds the specified maximum voltage of the
amplifier. This is common in industrial and automotive solenoid control
applications (Figure 1) where fault conditions caused by short circuits
may arise, exposing the current sense amplifier to a high differential
input voltage that may reach the same potential as the battery. This differential overvoltage can cause damage to the amplifier, especially if
there is no protection circuit present.
Visit analog.com
Output
Shunt
VS
–
R2
VCM
Figure 2. Basic overvoltage protection circuit.
VREF1
OUT
+
VREF2
GND
2
Differential Overvoltage Protection Circuits for Current Sense Amplifiers
Both the maximum rated voltage and the maximum input current tolerated
by a protection circuit vary from device-to-device. As a general rule of
thumb, limit the current passing through the internal differential protection
diodes to 3 mA unless there is a specification indicating a larger value is
acceptable. Given this value, calculate the values of R1 and R2 using the
following equation:
VIN_MAX − VRATED_MAX
R
= 3 mA
where: VIN_MAX is the expected maximum differential voltage.
VRATED_MAX is the maximum rated voltage (0.7 V). R is the total
series resistance (R1 + R2).
Agilent 3458A Meter
+
–
Agilent E3631A +5 V
Power Supply
Yokogawa
GS200
Precision
DC Source
VS
R1
+
–
+
10 Ω
R2
10 Ω
D1
D2
VREF1
OUT
VREF2
–
HAMEG
HMP 4030
GND
For example, if the expected maximum transient input voltage
is 10 V, the equation is
Figure 4. Test setup for evaluating the gain error, CMRR, and offset voltage.
10 V – 0.7 V
= 3 mA
R
If R = 3.1 kΩ, then based on Equation 1, R1 and R2 = 1.55 kΩ.
These values for R1 and R2 are significant, relative to the input
impedance of certain amplifiers, and can contribute a large error
to the overall system performance.
One way to reduce the value of R1 and R2 is to add external
protection diodes with higher current capabilities on the input
pins, as shown in Figure 3.
+5 V
V1
–
10 Ω
R2
10 Ω
VCM
+ IN
D1
D2
VREF1
OUT
VREF2
– IN
Gain Error
When series resistors are placed in series with the input of an amplifier,
they form a resistor divider with the differential input impedance of the
amplifier. The resistor divider introduces an attenuation that appears at
the circuit level as additional gain error. The additional gain error will be
larger for amplifiers with lower differential input impedances.
Table 1 shows the calculated, additional gain error, and the actual gain
error of the AD8210. The AD8418 was tested with and without the protection circuit as well. Table 2 shows the calculated additional gain error and
the actual gain error of the amplifier.
VS
R1
+
Both the AD8210 and the AD8418 were evaluated to measure the impact
of the additional series resistors on the gain error, CMRR, and offset voltage parameters of the devices.
GND
Figure 3. Overvoltage protection circuit with external input differential
protection diodes.
For example, when using the Digi-Key B0520LW-7-F Schottky diode,
which can handle up to 500 mA of forward current, the value of R is
reduced to 20 Ω.
Trade-Offs in System Performance
Adding series resistors to the input of the amplifier can degrade certain
performance parameters. In some amplifiers, R1 and R2 appear in series
with internal precision resistors. In other ampli­fiers, offset currents work
with the resistors to create offset voltages. The parameters more likely to
be affected are gain error, the common-mode rejection ratio (CMRR), and
offset voltage.
To examine the potential impact of the series resistance, measurements
were taken of two current sense amplifiers configured with protection resistors at the input pins. The test setup used for evaluating gain
error, CMRR, and offset voltage is shown in Figure 4. This setup uses
the Agilent E3631A power supply for providing the 5 V single supply to
the device, the Yokogawa GS200 precision dc source, for the differential
input voltage signal, the HAMEG HMP4030 for setting the CMV, and the
Agilent 3458A precision multimeter for measuring the output voltage of
the current sense amplifiers.
In the measured results, the AD8418 gain error shifts by 0.013% while
the AD8210 shifts by 0.497%. The input impedances of the AD8418 and
AD8210 are 150 kΩ and 2 kΩ, respectively, so it follows that the error
introduced in the AD8418 would be less than the AD8210.
Common-Mode Rejection Ratio
Because current sense amplifiers are usually exposed to envi­ronments
with high CMV, CMRR is one of the most important specifications. CMRR
assesses the ability of a device to reject high CMVs and attain optimal
accuracy and performance. It refers to a measure of change in output
voltage when equal voltage is applied at the two input terminals of
the amplifier. CMRR is defined as a ratio of the differential gain to the
common-mode gain and is usually specified in decibels.
Use the following equation to find the CMRR values for both amplifiers:
ADM
20 × Δ VCM
CMRR = A
=
Δ VOUT
CM
where: ADM is the differential gain of the AD8210 and the AD8418
(ADM = 20). ACM is the common-mode gain, ΔVOUT/ΔVCM.
When series resistor are in series with the input of an amplifier, the
mismatch of the series resistors is added to any mismatch of the internal
resistors, which will impact the CMRR.
The CMRR measurement results for the AD8210 and the AD8418 current
sense amplifiers are shown in Table 3 and Table 4, respectively.
The results indicate that the effect of the additional external series resistors is a reduction in the AD8418 CMRR and has a smaller effect on the
AD8210 CMRR. The AD8418 moved to 89 dB while the AD8210 remained
almost unchanged at 94 dB. The common-mode impedance of both
amplifiers is relatively high for fixed-gain devices with the AD8418 and
AD8210 at 750 kΩ and 5 MΩ, respectively.
Visit analog.com Table 1. AD8210 Gain Error
R1 (Ω)
0
10.2
Offset Voltage
R2 (Ω)
Additional
Gain Error (%)
Actual Gain
(V/V)
Actual Gain
Error (%)
0
10.2
0
0.497
19.9781
19.88089
–0.1095
–0.59705
Table 2. AD8418 Gain Error
R1 (Ω)
0
10.2
R2 (Ω)
Additional
Gain Error (%)
Actual Gain
(V/V)
Actual Gain
Error (%)
0
10.2
0
0.013
19.99815
19.9955
–0.00925
–0.0225
Table 3. AD8210 CMRR Performance at a Gain of 20
R1 (Ω)
0
10.2
R2 (Ω)
CMV = 0 V
and 4 V (dB)
CMV = 4 V
and 6 V (dB)
CMV = 4 V
and 65 V (dB)
CMV = 6 V
and 65 V (dB)
0
10.2
–92.77
–94.37
–104.96
–107.99
–121.49
–121.86
–123.35
–123.10
Table 4. AD8418 CMRR Performance at a Gain of 20
R1 (Ω)
0
10.2
R2 (Ω)
CMV = 0 V
and 35 V (dB)
CMV = 35 V
and 70 V (dB)
CMV = 0 V
and 70 V (dB)
0
10.2
–127.72
–88.89
–123.72
–104.35
–138.39
–93.05
Table 5. AD8210 Additional Offset Voltage Due to Input
Offset Current and External Impedances
R1 (Ω)
0
10.2
R2 (Ω)
VOUT (mV)
Additional Offset
Voltage (RTI) (μV)
0
10.2
5.598
5.938
0
17
Table 6. AD8418 Additional Offset Voltage Due to Input
Offset Current and External Impedances
R1 (Ω)
0
10.2
R2 (Ω)
VOUT (mV)
Additional Offset
Voltage (RTI) (mV)
0
10.2
–0.91
26.09
0
1.3
When bias currents pass through the external resistors, they produce
an error voltage in series with the intrinsic offset voltage of the device.
To compute this additional offset voltage error, multiply the input offset
current (IOS), which is the difference between the two input bias currents,
by the external impedance present on the input pins, as shown in the
following equation:
Offset Voltage = IOS × R
where: IOS is the input offset current. R is the additional
external impedance.
The increase in offset voltage based on measurements from the
AD8210 and the AD8418 current sense amplifiers are shown in
Table 5 and Table 6, respectively.
The results show that the increase in offset voltage in the AD8418 is
larger than the increase in offset voltage in the AD8210. This is caused by
the input offset current of the AD8418, which is around 100 μA.
Any additional impedances in series with the input pins (together) in
combination with the input offset current, create an additional offset
voltage error.
Conclusion
Implementing additional series resistors on the input pins is a simple
way to protect a current sense amplifier against overvoltage conditions.
The impact on performance measures such as gain error, CMRR, and
offset voltage is measurable and directly related to the magnitude of the
external resistors and the type of current sense amplifier used. If designed
properly, the circuit improves the application’s differential input voltage
ratings with a modest increase in component count and a minimal impact
on precision.
For more information on overvoltage protection for robust amplifiers,
see the Analog Dialogue article “Robust Amplifiers Provide Integrated
Overvoltage Protection.”
References
AD8210 Data sheet. Analog Devices Inc.
AD8418 Data sheet. Analog Devices Inc.
B0520LW Data sheet. Diodes Incorporated.
3
About the Authors
Emmanuel Adrados joined Analog Devices in
May 2011, following his graduation from the
Bicol State College of Applied Sciences and
Technology with a bachelor’s degree in
electronic engineering. He currently works
as a product applications engineer supporting
linear products. In his spare time, Emmanuel
enjoys playing chess and badminton.
He can be reached at [email protected]
Paul Blanchard is an applications engineer
at Analog Devices in the Instrumentation,
Aerospace, and Defense business unit,
located in Wilmington, MA. Paul started with
ADI in 2002 in the Advanced Linear Products
(ALP) Group covering instrumentation amplifiers and variable gain amplifiers. In 2009,
as part of the Linear Products Group (LPG),
he was primarily responsible for automotive
radar, current sensing, and AMR related applications. Currently,
as part of the Linear and Precision Technology (LPT) Group, he is
working on precision input signal conditioning (PISC) signal chain
technologies. Paul earned his bachelor’s and master’s degree in
electrical engineering from Worcester Polytechnic Institute.
He can be reached at [email protected]
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