Circuit Note CN-0280 Devices Connected/Referenced Circuits from the Lab™ reference circuits are engineered and tested for quick and easy system integration to help solve today’s analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0280. AD7401A Isolated Sigma-Delta Modulator AD8639 Auto-Zero, Rail-to-Rail Output Dual Op Amp ADuM6000 Isolated, 5kV, DC-to-DC Converter ADM8829 Switched-Capacitor Voltage Inverter ADP121 150mA, Low Quiescent Current, CMOS Linear Regulator ADP7104 20V, 500 mA, Low Noise, CMOS LDO ADP7182 −30V, 200mA, Low Noise, Negative Linear Regulator Robust Completely Isolated Current Sense Circuit with Isolated Power Supply for Solar Photovoltaic Converters EVALUATION AND DESIGN SUPPORT Design and Integration Files Schematics, Layout Files, Bill of Materials CIRCUIT FUNCTION AND BENEFITS The circuit in Figure 1 is a completely isolated current sensor with an isolated power source. The circuit is highly robust and can be mounted close to the sense resistor for accurate measurements and minimum noise pickup. The output is a single 16 MHz bit stream from a sigma-delta modulator that is processed by a DSP using a sinc3 digital filter. The circuit is ideal for monitoring the ac current in solar photovoltaic (PV) converters where the peak ac voltage can be several hundred volts, and the current can vary between a few mA and 25 A. CIRCUIT DESCRIPTION The circuit uses a 1mΩ sense resistor to measure peak current up to ±25 A using a dual AD8639 low offset amplifier. The gain of the amplifier is set to 10 to take advantage of the full scale range of the AD7401A sigma delta modulator. Higher currents can be measured (up to ±50 A or ±100 A) by simply lowering the gain of the AD8639 accordingly to ensure that the full scale input range of the AD7401A is used to the maximum advantage. With a typical offset voltage of only 3 μV, drift of 0.01 μV/°C, and noise of 1.2 μV p-p (0.1 Hz to 10 Hz), the AD8639 is well suited for applications in which dc error sources must be minimized. The solar panel application benefits greatly from nearly zero drift over the operating temperature ranges. Many systems can take advantage of the rail-to-rail output swing provided by the AD8639 to maximize signal-to-noise ratio (SNR). A guard ring is used around the inputs of the current measurement circuit to prevent any leakages from entering this sensitive low voltage area. The BAT54 Schottky diodes protect the inputs of the AD8639 from transient over-voltages and ESD. The single pole RC filter (102 Ω, 1 nF) has a differential mode bandwidth of 1.56 MHz and reduces the wideband noise at the input of the AD7401A. The sigma-delta modulator requires a clock input from an external source like a DSP processor or FPGA. The clock frequency can range from 5 MHz to 20 MHz, and a frequency of 16 MHz was used for the circuit in Figure 1.The highly robust single bit stream output of the modulator can be processed directly by a sinc3 filter where the data can be converted to an ADC word. The ±25 A current through the 1 mΩ resistor creates a voltage of ±25 mV. This is then amplified by the AD8639 to ±250mV and input to the AD7401A. The differential input of the AD7401A acts as the difference amplifier in the traditional three op amp instrumentation amplifier configuration. 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 suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices 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) One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2013 Analog Devices, Inc. All rights reserved. CN-0280 Circuit Note 13kΩ 40kΩ +2.5V_ISO −2.5V_ISO 5V_ISO +2.5V_ISO BAT54S 0.1µF ±25A 51Ω 1/2 AD8639 10Ω VDD1 VDD1 10Ω VDD2 GND 51Ω MDAT 1nF CLOCK 100Ω AD7401A 91kΩ 27kΩ 82pF 51Ω 1/2 –2.5V_ISO 47kΩ VIN VOUT GND2 ADP7104 GND2 GND −2.5V_ISO +2.5V_ISO +2.5V_ISO 5V VISO IN ADP121-2.5 0.1µF 10µF VISO GND 10kΩ 40kΩ 10kΩ 10µF ADP7182 GND OUT 10µF 0.1µF 47µF RCSEL ADuM6000 IN ADM8829 RCIN 0.1µF 10µF CAP+ CAP− GND 4.7µF VDD1 VSEL −5V_ISO ADJ VOUT VIN VDD1 GNDISO GND1 GNDISO GND1 AGND_ISO 10857-001 OUT −2.5V_ISO 3.3V 5V 5V_ISO 10µF DSP ADJ VIN− GND1 BAT54S DATA MCLKIN RF AD8639 10µF ADP7104 ±250mV 91kΩ RG 10kΩ VIN VOUT 47µF VIN+ RF 1mΩ SENSE RESISTOR 6V TO 20V ADJ 5V DGND Figure 1. Isolated Current Sense Circuit (Simplified Schematic: All Connections and Decoupling Not Shown) Both ac and dc information can be analyzed using the AD740x devices, thus not only monitoring ac performance but also any dc injection that may be present in the system. DC injection is important in solar applications because too much dc current injected into the grid may saturate any transformers in its path; therefore the dc current must be limited to the low milliampere range. A key advantage using the AD740x devices is that they can be very close to the actual ac current path, while the DSP or FPGA can be a distance away or even on another board in the system. This increases the accuracy of the overall system by minimizing the effects of EMI/RFI. Table 1. Maximum Continuous Working Voltage1 for AD7401A Parameter AC Voltage, Bipolar Waveform AC Voltage, Unipolar Waveform Max 565 Unit V peak 891 V peak DC Voltage 891 V 1 Constraint 50-year minimum lifetime Maximum CSA/VDE approved working voltage Maximum CSA/VDE approved working voltage Refers to continuous voltage magnitude imposed across the isolation barrier. See the AD7401A data sheet for more details. Safety is accomplished using the isolation barrier of a 20 µm polyimide. Further information on this and the various approvals can be obtained from the relevant datasheets. The AD7401A can operate to voltages up to 891 V unipolar range or 565 V bipolar range, across its isolation barrier as shown in Table 1. Rev. 0 | Page 2 of 7 CN-0280 SOLAR PANEL DC-TO-DC CONVERSION DC-TO-AC CONVERSION ~ = The ADuM6000 is a 5 V isolated dc-dc converter which operates using an internal 625 kHz PWM to drive the 5 V dc power across the isolation barrier. This is rectified on the isolated side of the barrier and filtered. AC CURRENT MEASURMENT USING SHUNT DC LINK = = AD7401A The AD8639 op amp supplies are regulated to ±2.5 V for better noise performance. The +2.5 V is supplied by the low noise ADP121 low dropout regulator, which is driven from the +5 V isolated supply. 2-WIRE CONNECTION SOLAR PANEL DC-TO-DC CONVERSION Current transformers offer alternative methods of isolation known as galvanic isolation. This document describes the typical performance of a current measurement module designed by Analog Devices using the AD7401A and the ADuM6000 devices. Solar Photovoltaic (PV) Inverter System Application A solar PV inverter converts power from a solar panel and transfers this power to the utility grid efficiently. Power from the solar panel, which is essentially a dc current source, is converted to ac current and fed onto the utility grid in phase with the frequency of the grid, and to a very high efficiency level (95% to 98%). The conversion can take one or more stages as shown in Figure 2. The first stage is typically a dc-to-dc conversion, where the low voltage and high current of the solar panel output is converted to high voltage and low current. The reason for this is to raise the voltage to a level compatible to the peak voltage of the grid. The second stage typically converts the dc voltage and current to ac voltage and current, typically using an H-Bridge circuit. (See Isolation Technology Helps Integrate Solar Photovoltaic Systems onto the Smart Grid, Analog Dialogue article). AC CURRENT MEASURMENT USING SHUNT DC LINK = Current measurement in solar applications requires isolated measurement techniques. The AD7401A is just one of many Analog Devices products that offer such isolation applications in ac measurements. This type of isolation is based on iCoupler® Technology. ~ = Theory The AD7401A is a second-order, sigma-delta (Σ-Δ) modulator that converts an analog input signal into a high speed, 1-bit data stream with on-chip digital isolation based on Analog Devices, Inc., iCoupler® technology. The AD7401A operates from a 5 V power supply and accepts a differential input signal of ±250 mV (±320 mV full scale). The analog modulator, eliminating the need for external sample-and-hold circuitry, continuously samples the analog input. The input information is contained in the output stream as a density of ones with a data rate up to 20 MHz. The original information is reconstructed with an appropriate digital filter. The processor side (non-isolated) can use a 5 V or a 3 V supply (VDD2). DC-TO-AC CONVERSION = AD7401A ANALOG DEVICES DSP 2-WIRE CONNECTION SINGLE PHASE AC GRID An ADM8829 switched capacitor voltage inverter is driven by the isolated +5 V and generates a −5 V output which is regulated to −2.5 V using the ADP7182 negative linear regulator. ANALOG DEVICES DSP L N 10857-002 Power Supply Configuration SINGLE PHASE AC GRID Circuit Note Figure 2. Solar PV Inverter System Previous solar PV Inverters were simply modules that dumped power onto the utility grid. Solar inverters for new designs focus on safety, grid integration and cost reduction. To achieve this, Solar PV inverter designers are looking to new technology, some previously unused in existing solar inverter modules, to improve their performance and potentially reduce cost. In the circuit the DSP controls the dc-to-dc converter and the dc-to-ac converter. The connection to the utility grid is typically made with a relay. The ac current measurement is performed by the AD7401A which measures current output to the grid, typically 25A. Solar PV inverter systems may or may not have an isolation transformer at the output, mainly due to cost savings, but without a transformer, the solar PV inverter must measure the dc component of the output current. This current is known as dc injection, and its value is critical to the operation. Too much dc injection onto the grid may saturate any transformers in the path. The dc injection current must be limited to the low mA. Both of these tasks can be accomplished by the circuit in this application. Thus, cost savings can be accomplished because alternative methods like Hall effect current transducers may need two devices: one for the high current range and one for the lower current range. Offset Performance of AD7401A The offset of the AD7401A in the current measurement module was measured over temperature up to 125°C. The results are shown in Figure 3 and are in line with the data sheet specifications of the AD7401A. The maximum variation of offset measured over the temperature range with no current flowing in the shunt was approximately ±20 mA from −40°C to +125°C. Rev. 0 | Page 3 of 7 CN-0280 Circuit Note The following voltages were applied during the test. 0.5 0.4 • VDD_ISO = 5 V 0.3 • VDD_FPGA = 3.3 V • VIN = 6 V @ 62 mA (Current sense module input supply voltage). ERROR (%) 0.2 • MCLKIN = 16 MHz (EVAL-CED1Z with Altera FPGA , 256 Decimation Rate). 0.1 0 –0.1 –0.2 0.05 –0.3 0.04 –0.4 0 0.02 2 4 6 8 10 12 14 16 18 20 22 24 28 26 AMPS 0.01 Figure 4. AD7401A Linearity Performance 0 SINC3 Filter Performance –0.01 The AD7401A is specified for a decimation rate (DR) of 256. However, it is possible to use this device at higher decimation rates. For a DR = 256, the response of a sinc3 filter is shown in Figure 5 where the output data rate is 62.5 kHz, and the FFT noise floor is shown in Figure 6. –0.02 –0.03 10 20 30 40 50 70 80 90 105 TEMPERATURE (°C) 0 Figure 3. AD7401A Module Offset –10 –20 The linearity of the module with currents up to ±28A was analyzed. As seen in Figure 4, Linearity of <±0.2% can be achieved after calibration. The voltages specified in the previous section were applied during the analysis. Figure 4 shows both full scale error and absolute error analysis defined as follows: –30 AMPLITUDE (dB) Linearity Performance –40 –50 –60 –70 Full-Scale Error = (VSHUNT – VCALC) / VFULLSCALE –80 Absolute Error = ( VSHUNT – VCALC) / VSHUNT –90 where VSHUNT = Current in the precision shunt( measured with a DVM) VCALC = Calculated current from the output of the ADC (AD7401A) VFULLSCALE = Full-scale current range of the module (28 A). –100 0 20 80 40 60 FREQUENCY (kHz) 100 120 10857-005 0 10857-003 –0.04 –0.05 –40 –30 –20 –10 Figure 5. Sinc3 Filter Response for Decimation Rate = 256, Output Data Rate = 62.5 kHz 0 –20 –40 AMPLITUDE (dB) The advantage of using the absolute error method is to analyze the errors in the lower range of measurement where errors can be highlighted. This is important for solar applications where dc injection can be measured in the low current range. –60 –80 –100 –120 –140 0 5 10 20 15 FREQUENCY (kHz) 25 31.246 10857-006 AMPS IN SHUNT/PRIMARY –0.5 10857-004 AD7401 ABSOLUTE ERROR AD7401 FULL-SCALE ERROR 0.03 Figure 6. 16K-Point FFT Showing Noise Floor for Decimation Rate = 256, Output Data Rate = 62.5 kHz Rev. 0 | Page 4 of 7 Circuit Note CN-0280 For higher decimation rates, the sinc3 Filter response is greater improved. For a DR = 1024, the response of a sinc3 filter is shown in Figure 7 where the data rate is 15.6 kHz This improves the noise performance of the system as shown in Figure 8, however at a reduced data rate. 0 –20 AMPLITUDE (dB) –40 0 –10 –20 –80 –100 –30 –40 –120 –140 0 –60 –70 1 2 4 5 3 FREQUENCY (kHz) 7 6 7.811 10857-008 –50 Figure 8. 16K-Point FFT Showing Noise Floor for Decimation Rate = 1024, Output Data Rate = 15.6 kHz –80 Layout Considerations –90 –100 0 20 80 40 60 FREQUENCY (kHz) 100 120 Special care must be taken during printed circuit board (PCB) layout to meet emissions standards. See the AN-0971 Application Note for board layout recommendations. An example of such a layout is shown in Figure 9. The key to the layout is to ensure a good overlap between Layer 3 (Floating Plane) and Layer 2 (Ground) .This simple overlap keeps emissions to a minimum in the system. Figure 10 shows the top view of the PCB layout, and Figure 11 shows a photo of the actual board. 10857-009 Figure 7. Sinc3 Filter Response for Decimation Rate = 1024, Output Data Rate = 15.6 kHz 10857-007 Figure 9. 4-Layer Board Example 10857-010 AMPLITUDE (dB) –60 Figure 10. Proposed Layout for Current Measurementt Rev. 0 | Page 5 of 7 Circuit Note 10857-011 CN-0280 Figure 11. Photo of Current Measurement Board Analog Devices isolated ADCs and isoPower devices meet the needs of the Solar Industry, and provides new technology advances required for Power systems. This leads to improved performance in grid integration using this new technology from the conventional methods used in solar inverters today. Functional Block Diagram A functional block diagram of the test setup is shown in Figure 12. POWER SUPPLY +6V TO +20V AT 200mA COMMON VARIATIONS + I+ The AD8638 op amp is a single version of the AD8639. Other members of the AD7401A sigma-delta modulator family include the AD7400 that includes a 10 MHz on-chip clock. DC SOURCE 100V AT 28A 6.5 DIGIT DVM AND CALIBRATED SHUNT I− POWER SUPPLY +7V AT 2A COM TB2 TB1 EVAL-CN0280-EB1Z EVALUATION BOARD J7 DATA J4 GND EVAL-CED1Z CONVERTER EVALUATION AND DEVELOPMENT BOARD J12 J1 CLOCK GND 5V GND CIRCUIT EVALUATION AND TEST Equipment Needed PC • DC source capable of 28A output at 100V to simulate source. • 6.5 digit DVM and calibrated shunt to measure input current • EVAL-CN0280-EB1Z evaluation board • 6 V, 200 mA power supply • 7 V, 2 A power supply • EVAL-CED1Z converter evaluation and development board software. • Example code for implementing the sinc3 filter can be found in the AD7401A data sheet. Rev. 0 | Page 6 of 7 Figure 12. Test Setup Functional Diagram 10857-012 USB Circuit Note CN-0280 LEARN MORE Data Sheets and Evaluation Boards CN-0280 Design Support Package: http://www.analog.com/CN0280-DesignSupport AD7401A Data Sheet CN-0183 Circuit Note, A Novel Analog-to-Analog Isolator Using an Isolated Sigma-Delta Modulator, Isolated DC-to-DC Converter, and Active Filter, Analog Devices. ADuM6000 Data Sheet AD8639 Data Sheet Defining Smart Grids and Smart Opportunities, IMS Research, February 28, 2012. Cantrell, Mark. Application Note AN-0971, Recommendations for Control of Radiated Emissions with isoPower Devices. Analog Devices. Chen, Baoxing, John Wynne, and Ronn Kliger. High Speed Digital Isolators Using Microscale On-Chip Transformers, Analog Devices, 2003. ADM8829 Data Sheet ADP121 Data Sheet ADP7104 Data Sheet ADP7182 Data Sheet REVISION HISTORY 10/13—Revision 0: Initial Version Chen, Baoxing. iCoupler® Products with isoPower™ Technology: Signal and Power Transfer Across Isolation Barrier Using Microtransformers, Analog Devices, 2006. Chen, Baoxing. “Microtransformer Isolation Benefits Digital Control.” Power Electronics Technology. October 2008. Ghiorse, Rich. Application Note AN-825, Power Supply Considerations in iCoupler® Isolation Products, Analog Devices. Krakauer, David. “Digital Isolation Offers Compact, Low-Cost Solutions to Challenging Design Problems.” Analog Dialogue. Volume 40, December 2006. MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of “AGND” and “DGND”, Analog Devices. MT-101 Tutorial, Decoupling Techniques, Analog Devices. Murname, Martin, Isolation Technology Helps Integrate Solar Photovoltaic Systems onto the Smart Grid, Analog Dialogue, Analog Devices, 46-09, September 2012. (Continued from first page) Circuits from the Lab circuits are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may use the Circuits from the Lab circuits in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the Circuits from the Lab circuits. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab circuits are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices reserves the right to change any Circuits from the Lab circuits at any time without notice but is under no obligation to do so. ©2013 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN10857-0-10/13(0) Rev. 0 | Page 7 of 7