Circuit Note CN-0240 Devices Connected/Referenced High Common-Mode Voltage Difference AD629 Amplifier Precision, Low Noise, CMOS, Dual, Rail-to- AD8622 Rail Output Op Amp Circuits from the Lab reference circuits are engineered and tested for quick and easy system integration to help solve todays Precision, Selectable Gain, Fully Differential AD8475 analog, mixed-signal, and RF design challenges. For more Funnel Amplifier information and/or support, visit www.analog.com/CN0240. Quad-Channel Isolator with Integrated ADuM5402 DC-to-DC Converter 5 V, Low Noise, High Accuracy, XFET ADR435 Voltage Reference AD7170 12-Bit, Low Power - ADC Bidirectional Isolated High-Side Current Sense with 270 V Common-Mode Rejection EVALUATION AND DESIGN SUPPORT CIRCUIT FUNCTION AND BENEFITS Circuit Evaluation Boards This circuit, shown in Figure 1, monitors bidirectional current CN-0240 Circuit Evaluation Board (EVAL-CN0240-SDPZ) from sources with dc voltages of up to 270 V with less than 1% System Demonstration Platform (EVAL-SDP-CB1Z) linearity error. The load current passes through a shunt resistor, Design and Integration Files which is external to the circuit. The shunt resistor value is Schematics, Layout Files, Bill of Materials chosen so that the shunt voltage is approximately 100 mV at maximum load current. 15V 10k 0V TO 100mV + 15V + 10k 15V 1k AD629 R V SHUNT SHUNT REF 5V REF 1/2 1k REF+ ADR435 0V TO 10V AD8622 1/2 V SOURCE 0V TO + AD8622 1V 15V + V = 0V TO 100mV SHUNT 15V 5V LOAD 10k NOTE: SIGNAL VOLTAGES 5V SHOWN FOR POSITIVE SOURCE 1 2.5V CM V VDD1 5V ISO 2.5V TO 10k 4.5V VOCM ADuM5402 REFIN+ VDD + V V IA SCLK OA AIN+ +IN 0.4 V AD8475 AD7170 V PDSTR OB IB DOUT IN 0.4 AI N V V IC OC REF I N GND GND GND ISO 1 2.5V TO 0.5V Figure 1. High Common-Mode Voltage Bidirectional Isolated Current Monitor (All Connections and Decoupling Not Shown) Rev. 0 Circuits from the Lab circuits from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices Tel: 781.329.4700 www.analog.com be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) Fax: 781.461.3113 2012 Analog Devices, Inc. All rights reserved. 10154-001CN-0240 Circuit Note The AD629 amplifier accurately measures and buffers (G = 1) a the part to reject these common-mode signals is determined by small differential input voltage and rejects large positive the power supply voltage as shown in Figure 3. Failure to common-mode voltages up to 270 V. implement dual supplies of a sufficient voltage will reduce the common-mode rejection. The dual AD8622 is used to amplify the output of the AD629 by 400 a factor of 100. The AD8475 funnel amplifier attenuates the signal (G = 0.4), converts it from single-ended to differential, 360 T = +25C A and level shifts the signal to satisfy the analog input voltage 320 range of the AD7170 sigma-delta ADC. 280 T = +85C Galvanic isolation is provided by the ADuM5402 quad channel A 240 isolator. This is not only for protection but also to isolate the T = 40C A 200 downstream circuitry from the high common-mode voltage. In addition to isolating the output data, the ADuM5402 digital 160 isolator can supply isolated +5.0 V for the circuit. 120 The measurement result from the AD7170 is provided as a 80 digital code utilizing a simple 2-wire, SPI-compatible serial 40 interface. 0 This combination of parts provides an accurate high voltage 0 2 4 6 8 10 12 14 16 18 20 POWER SUPPLY VOLTAGE (V) positive and negative rail current sense solution with a small Figure 3. AD629 Common-Mode Voltage Range vs. Power Supply Voltage component count, low cost, and low power. Secondly, the AD629 should only be operated in the unity gain CIRCUIT DESCRIPTION mode using the internal matched thin film resistors. Changing The circuit is designed for a full-scale shunt voltage of 100 mV the gain with external resistors will degrade the common-mode at maximum load current I . Therefore, the value of the shunt MAX rejection due to mismatch errors. resistor is R = (500 mV)/(I ). SHUNT MAX The AD8622 is a CMOS low power, precision, dual, rail-to-rail The AD629, shown in Figure 2, is a difference amplifier output op amp used primarily for amplifying the signal of designed with internal thin film resistors allowing continuous interest. common-mode signals up to 270 V with transient protection By cascading two inverting gain stages with a gain of 10, the to 500 V. For REF(+) and REF() grounded, the signal on the 100 mV full-scale output of the AD629 is amplified by a factor +IN terminal is attenuated by a factor of 20. The signal is then of 100 yielding a 10 V full-scale signal. These values can be amplified by a noise gain of 20, restoring the original amplitude either positive or negative, depending on the direction of the at the output. current. 21. 1k 380k 8 NC REF() 1 The dual supplies of the AD8622 allow both the input and 380k output signals to swing above and below ground as required to 7 IN 2 +V S measure bidirectional input currents. 380k 6 +IN 3 OUTPUT In the final stage of the signal chain before conversion into a 20k 4 5 V REF(+) S digital word, the AD8622 output voltage is conditioned to fit the AD629 analog input voltage range of the ADC. NC = NO CONNECT The AD8475funnel amplifier shown in Figure 4, provides two Figure 2. AD629 High Common-Mode Voltage Difference Amplifier optional attenuation factors (0.4 and 0.8). In addition, the signal The CMRR is 77 dB minimum 500 Hz for the AD629A, and is converted into a differential one, and the common-mode 86 dB minimum 500 Hz for the AD629B. voltage at the output is determined by the voltage on the VOCM In order to maintain the desired common-mode rejection, there pin. With a single 5 V supply, the analog input voltage range is are several important conditions to meet. First, the ability of 12.5 V (for a single-ended input). Rev. 0 Page 2 of 5 10154-002 COMMON-MODE VOLTAGE (V) 10154-003