Circuit Note CN-0218 Devices Connected/Referenced High Common Mode Voltage, Current AD8212 Shunt Monitor Precision, Low Noise, CMOS, Rail-to-Rail Circuits from the Lab reference circuits are engineered and AD8605 Input/Output Op Amp tested for quick and easy system integration to help solve todays analog, mixed-signal, and RF design challenges. For more Quad-Channel Isolator with ADuM5402 information and/or support, visit www.analog.com/CN0218. Integrated DC-to-DC Converter 2.5V, Low Noise, High Accuracy, Band Gap ADR381 Voltage Reference AD7171 16-Bit Low Power - ADC 500 V Common-Mode Voltage Current Monitor EVALUATION AND DESIGN SUPPORT CIRCUIT FUNCTION AND BENEFITS Circuit Evaluation Boards The circuit shown in Figure 1 monitors current in systems with CN-0218 Circuit Evaluation Board (EVAL-CN0218-SDPZ) high positive common-mode dc voltages of up to +500 V with System Demonstration Platform (EVAL-SDP-CB1Z) less than 0.2% error. The load current passes through a shunt Design and Integration Files resistor, which is external to the circuit. The shunt resistor value Schematics, Layout Files, Bill of Materials is chosen so that the shunt voltage is approximately 500 mV at maximum load current. V SHUNT + R SHUNT + WARNING HIGH VOLTAGE. THIS CIRCUIT MAY CONTAIN LETHAL VOLTAGES. DO NOT +500V I LOAD OPERATE, EVALUATE OR TEST THIS CIRCUIT, SOURCE LOAD OR BOARD ASSEMBLY, UNLESS YOU ARE A TRAINED PROFESSIONAL, WHO IS QUALIFIED TO HANDLE HIGH VOLTAGE CIRCUITRY. BEFORE APPLYING POWER, YOU MUST BE GND FAMILIAR WITH THE CIRCUITRY AND ALL ISO REQUIRED PRECAUTIONS FOR WORKING WITH V+ VSENSE HIGH VOLTAGE CIRCUITS. ALPHA AD8212 5V Q1: 2SA1807TLP, +3.3V ISO 600V, 1A, BJT PNP IOUT Q1 COM BIAS VIN 0.1F +2.5V ADR381 VOUT +3.3V R2 GND 100. 0k 1% +3.3V V V ISO ISO DD1 R2 GND ISO 100. 0k ADuM5402 REFIN(+) VDD 1% SCLK V V OA IA AD8605 AIN+ I BIAS R1 R2 AD7171 PDRBT V V OB IB 4. 99k 100. 0k AIN 1% DOUT V V IC OC REFIN() GND GND ISO R2 GND GND GND ISO ISO 1 100. 0k 1% R2 GND GND GND 100. 0k ISO ISO 1 1% NOTE: ADuM5402 PROVIDES GALVANIC ISOLATION FOR GND ISO DIGITAL SIGNALS AND ISOLATED POWER FOR CIRCUIT. Figure 1. High Common-Mode Voltage Current Monitor (All Connections and Decoupling Not Shown) Rev. B 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 20112013 Analog Devices, Inc. All rights reserved. 09918-001CN-0218 Circuit Note The AD8212 accurately amplifies a small differential input times the COM (Pin 2), which is the most negative of all the voltage in the presence of large positive common-mode voltages terminals, is always 5 V less than the supply voltage (V+). greater than 500 V when used in conjunction with an external In this mode of operation, the supply current (I ) of the AD8212 BIAS PNP transistor. circuit is based entirely on the supply range and the chosen value of Galvanic isolation is provided by the ADuM5402 quad channel the RBIAS resistor. For example, for V+ = 500 V, and RBIAS = 500 k , isolator. This is not only for protection but to isolate the IBIAS = (500 V 5 V)/RBIAS = 990 A, downstream circuitry from the high common-mode voltage. In In this high voltage mode, IBIAS should be between 200 A and addition to isolating the output data, the ADuM5402 digital 1 mA. This ensures the bias circuit is active, allowing proper isolator can also supply isolated +3.3 V for the circuit. operation of the device. The measurement result from the AD7171 is provided as a Note that the 500 k bias resistor (5 R2) is made up of five digital code utilizing a simple 2-wire, SPI-compatible serial individual 100 k resistors. This is to provide protection from interface. resistor voltage breakdown. Additional breakdown protection This combination of parts provides an accurate high voltage can be added by eliminating the ground plane immediately positive rail current sense solution with a small component under the resistor string. count, low cost, and low power. The load current flowing through the external shunt resistor CIRCUIT DESCRIPTION produces a voltage at the input terminals of the AD8212. Internal amplifier A1 responds by causing transistor Q1 to conduct the The circuit is designed for a full-scale shunt voltage of 500 mV necessary current through resistor R1 to equalize the potential at maximum load current IMAX. Therefore, the value of the shunt at both the inverting and noninverting inputs of the amplifier A1. resistor is RSHUNT = (500 mV)/(IMAX). The AD8212 process has a breakdown voltage limitation of The current through the emitter of transistor Q1 (IOUT) is 65 V. For this reason, the common-mode voltage must remain proportional to the input voltage (VSENSE) and, therefore, the below 65 V. By utilizing an external PNP BJT transistor, the load current (ILOAD) through the shunt resistor (RSHUNT). The output current (I ) is converted to a voltage by using an common-mode voltage range can be extended to greater than OUT external resistor, the value of which is dependent on the input- 500 V, depending on the breakdown voltage of the transistor. to-output gain desired in the application. V+ R SHUNT The transfer function for the AD8212 is: 1 8 IOUT = gm VSENSE AD8212 VSENSE = ILOAD RSHUNT R1 R2 VOUT = IOUT ROUT VOUT = (VSENSE ROUT)/1000 A1 gm = 1000 A/V The input sense voltage has a fixed range of 0 V to 500 mV. The output voltage range can be scaled according to the value of R . A Q1 OUT 1 mV change in V produces a 1 mA change in I , which, when SENSE OUT 5V passed through a 1 k resistor, causes a 1 mV change in VOUT. In the circuit of Figure 1, the load resistor is 4.99 k , thereby OUTPUT CURRENT providing a gain of 5. A full-scale input voltage of 500 mV COMPENSATION BIAS CIRCUIT produces a 2.5 V output, which corresponds to the full-scale input range of the AD7171 ADC. 5 2 3 6 The AD8212 output is intended to drive high impedance nodes. Q2 Therefore, if interfacing with a converter, it is recommended the I VOUT BIAS output voltage across ROUT be buffered so that the gain of the R R OUT BIAS AD8212 is not affected. Notice that the power supply voltage for the ADR381 and the AD7171 are supplied by the isolated power output (+3.3 VISO) of Figure 2. AD8212 High Voltage Operation Using an External PNP Transistor the ADuM5402 quad isolator. There is no dedicated power supply for the AD8212. Instead, it The reference voltage for the AD7171 is supplied by the ADR381 creates a 5 V supply by essentially floating itself off the 500 V precision band gap reference. The ADR381 has an initial accuracy common-mode voltage by utilizing an internal 5 V series of 0.24% and a typical temperature coefficient of 5 ppm/C. regulator as shown in Figure 2. This regulator ensures that at all Rev. B Page 2 of 6 LOAD 09918-002