Circuit Note CN-0188 Devices Connected/Referenced Micropower, Zero-Drift, Rail-to-Rail Input ADA4051-2 and Output, Dual Op Amp Circuits from the Lab reference circuits are engineered and tested for quick and easy system integration to help solve todays AD7171 Low Power, 16-Bit, Sigma-Delta ADC analog, mixed-signal, and RF design challenges. For more 2.5 V, Low Noise, High Accuracy, Band ADR381 information and/or support, visit www.analog.com/CN0188. Gap Voltage Reference Quad-Channel Isolator with ADuM5402 Integrated DC-to-DC Converter Low Cost, Level Shifted Low Side Current Monitor for Negative High Voltage Rails EVALUATION AND DESIGN SUPPORT CIRCUIT FUNCTION AND BENEFITS Circuit Evaluation Boards The circuit shown in Figure 1 monitors current in individual CN-0188 Circuit Evaluation Board (EVAL-CN0188-SDPZ) channels of 48 V to better than 1% accuracy. The load current System Demonstration Platform (EVAL-SDP-CB1Z) passes through a shunt resistor, which is external to the circuit. Design and Integration Files The shunt resistor value is chosen so that the shunt voltage is Schematics, Layout Files, Bill of Materials approximately 50 mV at maximum load current. +3.3V +3.3V ISO 0.1F VIN +2.5V V V ISO DD1 10F VOUT ADR381 ADuM5402 REFIN(+) VDD GND R7 SCLK V V AIN+ OA IA R1 4.99k AD7171 24.9k PDRST V V 1% OB IB AIN DOUT GND V V REFIN() GND ISO IC OC GND GND ISO 1 U1: ADA4051-2 GND ISO R6 1k D2 GND GND ISO 1 U1A U1B Q1 NOTE: OPTIONAL ADuM5402 PROVIDES GALVANIC ISOLATION FOR DIGITAL SIGNALS 0.1F R3 AND ISOLATED POWER FOR CIRCUIT 48.7k , 1% D1 D1: MMSZ5232B-7-F 5.6V D2: MBRA120ET3 R2 R4 R5 1k 1k 4.99k Q1: ZXMN7A11GTA, 70V N-CH MOSFET 1% 1% 1% R SHUNT 48V + V SHUNT SOURCE LOAD I LOAD + GND ISO Figure 1. Low Side Current Monitor for Negative High Voltage Rails (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 2011 Analog Devices, Inc. All rights reserved. 09570-001CN-0188 Circuit Note The measurement result from the AD7171 is provided as a reference provides better accuracy. A 2.5 V reference is chosen digital code utilizing a simple 2-wire, SPI-compatible serial to provide sufficient headroom. interface. The entire circuit operates on a single +3.3 V supply. The input voltage to the AD7171 ADC is converted into an Optional galvanic isolation is provided by the ADuM5402 quad offset binary code at the output of the ADC. The ADuM5402 channel isolator. In addition to isolating the output data, the provides the isolation for the DOUT data output, the SCLK ADuM5402 digital isolator can also supply isolated +3.3 V for input, and the PDRST input. the circuit. The ADuM5402 is not required for normal circuit The code is processed in the PC by using the SDP hardware operation unless galvanic isolation is needed. board and LabVIEW software. This combination of parts provides a accurate high voltage The graph in Figure 2 shows how the circuit tested achieves an negative rail current sense solution with a small component error of 0.3% over the entire input voltage range (0 mV to 50 mV). count, low cost, and low power. The accuracy of the measure- A comparison is made between the code seen at the output of ment is primarily determined by resistor tolerances and the the ADC, recorded by LabVIEW, and an ideal code calculated accuracy of the band gap reference, and is typically better than 1%. based on a perfect system. CIRCUIT DESCRIPTION The circuit is designed for a full-scale shunt voltage of 50 mV at 1.0 maximum load current I . Therefore, the value of the shunt 65536 MAX 0.8 62768 resistor is R = (50 mV)/(I ). SHUNT MAX INPUT OUTPUT 0.6 Thegroun for the op amp stage is connected to the 57768 0.4 commonmode source voltage (48 V). The voltage for the op 0.2 52768 amp stage is supplied by thefloatin 5.6 V zener diode, which 0 is biased at a current of approximately 2 mA. This eliminates 47768 0.2 the need for a separate power supply. The circuit will operate ADC CODE ERROR (%) 0.4 42768 with a source voltage from 60 V to 10 V with no 0.6 modifications. 37768 0.8 The shunt voltage is amplified by a factor of 49.7 using U1A, 32768 1.0 where G = 1 + R3/R2. The zero-drift ADA4051-2 has a low 0 5 10 15 20 25 30 35 40 45 50 SHUNT VOLTAGE (mV) offset voltage (15 V maximum) and does not contribute Figure 2. Plot of Output and Error vs. Shunt Voltage significant error to the measurement. A full-scale shunt voltage of 50 mV produces a full-scale output voltage from U1A of 2.485 V (referenced to the common-mode source voltage). In order to calculate this ideal code, there are several An N-channel MOSFET transistor with a large VDS breakdown assumptions which must be made about the performance of the (70 V) inside the feedback loop of U1B applies the output system. First, the op amp gain stage must multiply the input voltage of U1A across resistor R5, and the resulting current signal by exactly 49.7. Depending on resistor tolerances (1%), flows through R6 and R7. The full-scale voltage from U1A of this value will vary by 2% worst case. Secondly, the current sink 2.485 V produces a full-scale current of 0.498 mA, which resistor (R5) and the ADC input resistor (R7) are assumed to be generates a full-scale voltage of 2.485 V across resistor R7. The identical. In the circuit, these particular resistors have a voltage across R7 is applied to AIN of the ADC. Resistor R6 tolerance of 1%. Since they are the same value, the matching and the Schottky diode D2 provide input protection for the will probably be better than 1%. Resistors with tighter AD7171 in the event the MOSFET shorts out. tolerances can be used, which will increase the accuracy and Notice that the power supply voltage for the ADR381, the the cost of the circuit. AD7171, and the floating zener diode is supplied by the isolated Several items have been implemented on the PCB, which are power output (+3.3 VISO) of the ADuM5402 quad isolator. not crucial to the function or performance of the circuit but are The reference voltage for the AD7171 is supplied by the required to ensure user and hardware safety. As an example, if ADR381 precision band gap reference. The ADR381 has an Q1 breaks down or shorts out, the ADC, SDP board, user, and initial accuracy of 0.24% and a typical temperature coefficient users PC are all at risk due to the large negative voltage of 5 ppm/C. potential. The safety items included are passive elements R6, D2, which protect the AD7171, and the ADuM5402 quad- Although it is possible to operate both the AD7171 VDD and channel digital isolator, which protects the circuits on the SDP REFIN(+) from the 3.3 V power supply, using a separate board, as well as the user s PC. Rev. B Page 2 of 5 ADC CODE ERROR (%) 09570-002