Bidirectional Isolated High-Side Current Sense with 270 V Common-Mode Rejection
The AD629 amplifier accurately measures and buffers (G = 1) a
small differential input voltage and rejects large positive
common-mode voltages up to 270 V.
The dual AD8622 is used to amplify the output of the AD629 by a factor of 100. The AD8475 funnel amplifier attenuates the signal (G = 0.4), converts it from single-ended to differential, and level shifts the signal to satisfy the analog input voltage range of the AD7170 sigma-delta ADC.
Galvanic isolation is provided by the ADuM5402 quad channel isolator. This is not only for protection but also to isolate the
downstream circuitry from the high common-mode voltage. In addition to isolating the output data, the ADuM5402 digital isolator can supply isolated +5.0 V for the circuit.
The measurement result from the AD7170 is provided as a
digital code utilizing a simple 2-wire, SPI-compatible serial
This combination of parts provides an accurate high voltage
positive and negative rail current sense solution with a small
component count, low cost, and low power.
Figure 1. High Common-Mode Voltage Bidirectional Isolated Current Monitor (All Connections and Decoupling Not Shown)
The AD629, shown in Figure 2, is a difference amplifier
designed with internal thin film resistors allowing continuous
common-mode signals up to ±270 V with transient protection
to ±500 V. For REF(+) and REF(−) grounded, the signal on the
+IN terminal is attenuated by a factor of 20. The signal is then
amplified by a noise gain of 20, restoring the original amplitude
at the output.
Figure 2. AD629 High Common-Mode Voltage Difference Amplifier
The CMRR is 77 dB minimum @ 500 Hz for the AD629A, and 86 dB minimum @ 500 Hz for the AD629B.
In order to maintain the desired common-mode rejection, there
are several important conditions to meet. First, the ability of the part to reject these common-mode signals is determined by
the power supply voltage as shown in Figure 3. Failure to
implement dual supplies of a sufficient voltage will reduce the
Figure 3. AD629 Common-Mode Voltage Range vs. Power Supply Voltage
Secondly, the AD629 should only be operated in the unity gain mode using the internal matched thin film resistors. Changing the gain with external resistors will degrade the common-mode rejection due to mismatch errors.
The AD8622 is a CMOS low power, precision, dual, rail-to-rail
output op amp used primarily for amplifying the signal of
By cascading two inverting gain stages with a gain of –10, the
100 mV full-scale output of the AD629 is amplified by a factor
of 100 yielding a 10 V full-scale signal. These values can be
either positive or negative, depending on the direction of the
The dual supplies of the AD8622 allow both the input and
output signals to swing above and below ground as required to
measure bidirectional input currents.
In the final stage of the signal chain before conversion into a
digital word, the AD8622 output voltage is conditioned to fit the
analog input voltage range of the ADC.
The AD8475 "funnel amplifier," shown in Figure 4, provides two
optional attenuation factors (0.4 and 0.8). In addition, the signal
is converted into a differential one, and the common-mode
voltage at the output is determined by the voltage on the VOCM
pin. With a single 5 V supply, the analog input voltage range is
±12.5 V (for a single-ended input).
Figure 4. AD8475 Funnel Amplifier
As shown in Figure 1, the output common-mode voltage is set at 2.5 V by a resistor divider driven by the ADR435 reference output of 5 V.
The primary source of noise in the system is the output noise of
the AD629 of 15 μV p-p in the 0.1 Hz to 10 Hz bandwidth. For
a 100 mV full-scale signal, this corresponds to a noise-free code
The input noise of the AD8622 is only 0.2 μV p-p, which is negligible compared to the AD629. The output noise of the AD8475 is 2.5 μV p-p, which is also negligible at that point where the full-scale signal level is 4 V p-p.
Notice that the power supply voltage for the AD7170 is supplied
by the isolated power output (+5.0 VISO) of the ADuM5402 quad
The reference voltage for the AD7170 is supplied by the
ADR435 precision XFET® reference. The ADR435 has an initial
accuracy of ±0.12% (A grade), and a typical temperature
coefficient of 2 ppm/°C. The ADR435 has a wide operating
range (7.0 V to 18.0 V) and utilizes the +15.0 V rail for a
Although it is possible to operate both the AD7170 VDD and
REFIN(+) from the 5.0 V power supply, using a separate
reference provides better accuracy.
The input voltage to the AD7170 ADC is converted into an
offset binary code at the output of the ADC. The ADuM5402
provides the isolation for the DOUT data output, the SCLK
input, and the PDRST input. Although the isolator is optional, it is recommended to protect the downstream digital circuitry from the high common-mode voltage in the case of a fault condition.
The code is processed in the PC by using the SDP hardware
board and LabVIEW software.
Figure 5 shows a comparison between the code seen at the
output of the ADC recorded by LabVIEW and an ideal code
calculated based on a perfect system. The plots show how the
circuit achieves an end-point linearity error of less than 0.5%
over the entire input voltage range (−100 mV to +100 mV).
The offset error and gain error can be removed using software
calibration if desired.
Figure 5. Plot of Actual Code, Ideal Code, and % Error vs. Shunt Voltage
PCB Layout Considerations
In any circuit where accuracy is crucial, it is important to consider the power supply and ground return layout on the board. The PCB should isolate the digital and analog sections as much as possible. This PCB was constructed in a 4-layer stack up with large area ground plane layers and power plane polygons. See the MT-031 Tutorial for more discussion on layout and grounding and the MT-101 Tutorial for information on decoupling techniques.
The power supply to the AD7170 and ADuM5402 should be
decoupled with 10 μF and 0.1 μF capacitors to properly suppress
noise and reduce ripple. The capacitors should be placed as
close to the device as possible with the 0.1 μF capacitor having a
low ESR value. Ceramic capacitors are advised for all high
Care should be taken in considering the isolation gap between
the primary and secondary sides of the ADuM5402. The
EVAL-CN0240-SDPZ board maximizes this distance by pulling back any polygons or components on the top layer and aligning them with the pins on the ADuM5402.
Power supply lines should have as large a trace width as possible
to provide low impedance paths and reduce glitch effects on the
supply line. Clocks and other fast switching digital signals
should be shielded from other parts of the board by digital
A complete design support package for this circuit note,
including complete schematics and board layouts, can be
found at www.analog.com/CN0240-DesignSupport.
“High-Side Current Sensing: Difference Amplifier vs. Current
Sense Amplifier,” Analog Dialogue, January 2008, describes the
use of current sense and difference amplifiers. The article is
available at www.analog.com/HighSide_CurrentSensing.
The following URLs link to Analog Devices products useful in
solving the current sense problem:
WARNING! HIGH VOLTAGE. THIS CIRCUIT MAY CONTAIN LETHAL VOLTAGES. DO NOT OPERATE, EVALUATE, OR TEST THIS CIRCUIT, OR BOARD ASSEMBLY, UNLESS YOU ARE A TRAINED PROFESSIONAL, WHO IS QUALIFIED TO HANDLE HIGH VOLTAGE CIRCUITRY. BEFORE APPLYING POWER, YOU MUST BE FAMILIAR WITH THE CIRCUITRY AND ALL REQUIRED PRECAUTIONS FOR WORKING WITH HIGH VOLTAGE CIRCUITS.
This circuit uses the EVAL-CN0240-SDPZ circuit board and the EVAL-SDP-CB1Z System Demonstration Platform (SDP) evaluation board. The two boards have 120-pin mating connectors, allowing for the quick setup and evaluation of the circuit’s performance. The EVAL-CN0240-SDPZ board contains the circuit to be evaluated, as described in this note, and the SDP evaluation board is used with the CN0240 evaluation software to capture the data from the EVAL-CN0240-SDPZ circuit board.
- PC with a USB port and Windows® XP or Windows Vista® (32-bit), or Windows® 7 (32-bit)
- EVAL-CN0240-SDPZ circuit evaluation board
- EVAL-SDP-CB1Z SDP evaluation board
- CN0240 evaluation software
- Power supply: +6 V @ 1 A, or +6 V “wall wart”
- Dual power supply: ±15 V@ 10 mA
- Shunt resistor with maximum voltage of 100 mV at the maximum load current.
- Source voltage and electronic load
Load the evaluation software by placing the CN0218 evaluation
software disc in the CD drive of the PC. Using "My Computer,"
locate the drive that contains the evaluation software disc and
open the Readme file. Follow the instructions contained in the
Readme file for installing and using the evaluation software.
Functional Block Diagram
See Figure 1 of this circuit note for the circuit block diagram and the EVAL-CN0240-SDPZ-SCH pdf file for the circuit schematics. This file is contained in the CN0240 Design Support Package.
Connect the 120-pin connector on the EVAL-CN0240-SDPZ circuit board to the connector marked “CON A” on the EVAL-SDP-CB1Z evaluation (SDP) board. Nylon hardware should be used to firmly secure the two boards, using the holes provided at the ends of the 120-pin connectors.
Connect a shunt resistor (RSHUNT) across the J4 input terminals
with a load to ground as indicated in Figure 1. With power to
the supply off, connect a +6 V power supply to the pins marked
“+6 V” and “GND” on the board. If available, a +6 V "wall wart"
can be connected to the barrel connector on the board and used
in place of the +6 V power supply. Connect the USB cable
supplied with the SDP board to the USB port on the PC.
Note: Do not connect the USB cable to the mini USB connector
on the SDP board at this time.
It is important to connect the system ground and the PCB
isolated ground to guarantee correct voltage levels and
operation. Test point 31 and test point 32 give access to the
GND_ISO required to properly make this connection.
Lastly, before applying any high voltage to connector J4, be
certain the ±15 V supply (J5) is properly connected and has
been turned on. If this supply is not active, any high voltage
could potentially damage U2, the AD629, as well as several
other components on the PCB.
Apply power to the +6 V supply (or “wall wart”) connected to the EVAL-CN0240-SDPZ circuit board. Connect the ±15 V supplies to the EVAL-CN0240-SDPZ board U12 three-terminal screw connector. Launch the evaluation software and connect the USB cable from the PC to the USB mini-connector on the SDP board.
Once USB communications are established, the SDP board
can be used to send, receive, and capture serial data from the
EVAL-CN0240-SDPZ board. Data can be recorded for various
values of load current as the electronic load is stepped.
Information and details regarding how to use the evaluation
software for data capture can be found in the CN0240
evaluation software Readme file.
Information regarding the SDP board can be found in the SDP User Guide.
|AD629||High Common-Mode Voltage, Difference Amplifier||
|ADUM5402||Quad-Channel Isolators with Integrated DC/DC Converter (2/2 channel directionality)||
|ADR435||Ultralow Noise XFET® 5.0V Voltage Reference w/Current Sink and Source Capability||
|AD8622||Low Power, Low Noise, Low Bias Current, Precision Dual RRO Op Amp||
|AD7170||12-Bit Low Power Σ−Δ ADC||
|AD8475||Precision, Selectable Gain, Fully Differential Funnel Amp||