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MCP3918 Datasheet(PDF) 59 Page - Microchip Technology |
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MCP3918 Datasheet(HTML) 59 Page - Microchip Technology |
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59 / 88 page  2014 Microchip Technology Inc. DS20005287A-page 59 MCP3918 8.0 BASIC APPLICATION CONFIGURATION One of the main applications for the MCP3918 is energy/power measurement in systems where the ADC sampling the current needs to be isolated from the rest of the design. Figure 8-1 can be used as a starting point for MCP3918 applications. For power measurements, since MCP3918 is a 1-channel ADC, it is recommended to use it for current samples acquisition and to use the MCU ADC for voltage samples acquisition. The isolator used between MCU and ADC needs to be fast enough to support the high-speed clock between MCU and ADC and the data coming from ADC to MCU. This is typically the case in a polyphase shunt-based power/energy metering or monitoring application. In this case, each phase needs to be isolated from the rest of the design and, since the sensor is not providing this isolation, the isolation needs to be provided at the output of the analog front-end. The MCP3918 device is built to work seamlessly with a large variety of two-channel unidirectional digital isolators (opto-couplers, capacitive or inductive integrated digital isolators with or without embedded power supplies). FIGURE 8-1: MCP3918 Three-Phase Shunt Energy Meter – Typical Application Schematic for Each Phase. 8.1 Power Supply Design and Bypassing To power the isolated ADC, an isolated DC/DC converter that can be embedded with the isolated data communication channels (as in Figure 8-1) or other structures that provide isolated power supplies (e.g., fly-back converter) can be used. For single-phase designs where isolation between ADC and MCU is not required, the SPI connection is also available. This SPI interface could also be used with isolators but this would require four isolators instead of two (for the 2-wire mode) and, therefore, this configuration is not preferred. 8.2 Power Supply Design and Bypassing The MCP3918 device was designed to measure positive and negative voltages that might be generated by a current-sensing device. This current-sensing device, with a common-mode voltage close to 0V, is referred to as AGND, which is a shunt or current transformer (CT) with burden resistors attached to ground. The high performance and good flexibility that characterize this ADC enable it to be used in other applications, as long as the absolute voltage on each pin, referred to AGND, stays in the -1V to +1V range. 3910A_SDO A_GNDD A_GNDA A_GNDA 10 R76 0.1uF C52 10 R100 0.1uF C43 10 R86 A_GNDA A_GNDA A_GNDA 0.1uF C46 A_3.3D A_3.3A A_GNDD DR / GAIN1 14 DGND 11 AGND 8 CH0- 5 DVDD 2 OSC1/CLKI 15 SCK / MCLK 18 MDAT1 12 MDAT0 13 NC 6 NC 7 RFIN/OUT+ 9 RFIN- 10 RESET / OSR0 1 AVDD 3 CH0+ 4 CS / BOOST 17 OSC2 / MODE 16 SDO 19 SDI / OSR1 20 DR / GAIN1 DGND AGND CH0- DVDD OSC1/CLKI SCK / MCLK MDAT1 MDAT0 NC NC RFIN/OUT+ RFIN- RESET / OSR0 AVDD CH0+ CS / BOOST OSC2 / MODE SDO SDI / OSR1 U3 MCP3918A1T/ISS 1 2 3 J24 1 2 3 J25 A_3.3D 1k R80 A_GNDD 1 2 3 J26 1 2 3 J27 1 2 3 J28 HIGH LOW HIGH LOW LOW LOW HIGH HIGH GAIN1 GAIN0 BOOST OSR1 OSR0 HIGH LOW 1k R79 3910A_CLKIN 10 R87 3910A_SDO A_GNDD A_3.3D 3910A_CLKIN 3910_CLKIN_MCU_A PHASE A A_3.3D A_3.3D A_3.3A A_GNDA VBT1-S5-S5 -Vin 1 +Vin 2 +Vout 5 -Vout 4 -Vin +Vin +Vout -Vout DC DC U7 4.7uF C63 MCP1754-3.3V VIN 3 VOUT 2 VIN VOUT U18 A_GNDD 0.1uF C57 4.7uF C22 5V L2 GND 3.3D GND 0.1uF 0603 C68 3.3D GND 0.1uF 0603 C60 A_3.3D A_GNDD NT3 3910A_SDO_MCU/RC3 VDD1 1 VOA 2 VIB 3 GND1 4 VDD2 8 VIA 7 VOB 6 GND2 5 VDD1 VOA VIB GND1 VDD2V V VIA VOB GND2G G FOD8012 U21 A_GNDA A_GNDA A_GNDA A_GNDA A_GNDA 0.1uF C5 1k R5 1k R4 0.1uF C1 LINE_SHUNT1 LINE_SHUNT2 Via_1.6x1 CP1 Via_1.6x1 CP2 DNP R3 DNP R6 FB2 FB3 |
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