How to design 1-phase shunt electricity meters using standalone metrology ADCs
As processing requirements for electricity meters gradually increase, it becomes increasingly difficult to find one device that meets both the processing requirements for adding advanced metrology features to smart meters while also accurately sensing metrology parameters, such as RMS voltage, RMS current and active power. To deal with this design challenge, one option is to use a separate metrology microcontroller and a standalone ADC. This session will focus on how to design electricity meters using the ADS131M04 standalone delta-sigma metrology ADCs. The TIDA-010036 reference design with its onboard MSP432 ARM Cortex M4F metrology microcontroller will be used to illustrate how to use the ADS131M04 standalone ADC to implement a Class 0.5 single-phase meter with shunt current sensor.
Another design challenge for electricity meters is protecting meters against tampering. One form of tampering is magnetic tampering, which is one of the most common ways to tamper with a meter. In the TIDA-010036 design, this design challenge is addressed by using a shunt current sensor as the primary current sensor and a cap-drop supply as the design’s AC/DC power supply. Conventional cap-drop supplies have a limited maximum current; however, in this design, the switch-cap stage of the TPS7A78 enables us to get a higher maximum current output compared to conventional cap-drop supplies. The switch-cap stage does not use magnetic components, which enables the AC/DC power supply to be magnetically immune. Another method to tamper with the meter is to remove the neutral. The ADS131M04 in this design uses a special current detection mode for determining this neutral current disconnection in a low-power manner so that the backup power supply lifetime is maximized. The design also measures both the line and neutral current in case someone tries to tamper with the meter by bypassing the sensed current on one of these two channels.