The “smarts” in a smart grid come from the ability the grid has to measure, control and communicate. Once a measurement is done, communication between the power distribution company and the customer creates an automated and widely-distributed electricity network that can protect, monitor, and optimize the operation of the interconnected components. This new automated network allows for utility companies to respond more efficiently to customer demand, and enables consumers to take more control over their energy usage through increased access to information.
In today’s complex power grid, the number of channels of currents and voltages that need to be monitored or measured are increasing dramatically. One of the challenges that directly flow from this is how to handle the large number of analog inputs coming in and then even after digitizing the signal how to get it into the processor without overwhelming the processor interface.
A merging unit is the interface from the physical analog world to the digital world using communication networks. The analog signal is converted to a digital signal and transmitted through -Ethernet network based on IEC 61850-9-2 (sampled values) communication protocol.
The TI reference design for the analog front end (AFE) for merging units (TIDA-00307) shows how multiple channels (8 at a time) can be modularly added to increase signal handling by adding additional successive-approximation-register (SAR) analog to digital converters (ADCs).
Multiple ADCs can then be connected together by using the daisy chain feature available on the SPI on the ADC, thereby minimizing the connectivity overhead from multiple ADCs to the controller.
The ADS8688 SAR ADC used in the reference design operates at a throughput of 500 kSPS. The device features integrated analog front-end circuitry for each input channel with overvoltage protection up to ±20 V, an 8-channel multiplexer with automatic and manual scanning modes, and an on-chip 4.096-V reference with extremely low drift. Operating on a single analog supply of 5 V, each input channel on the device can support true bipolar input ranges of ±2.56 V, ±5.12 V and ±10.24 V.
The device also offers second-order antialiasing filter, ADC driver amplifier and an extended industrial-temperature range. Additionally, this solution enables simultaneous sampling. When two devices are connected in daisy chain and the same chip selected is asserted for sampling, sampling delay will be minimal between the CH0 of ADC1 and CH0 of ADC2.
This solution from TI shows how a merging unit can not only be designed in a modular fashion, which enables the easy increase in channels as needed, but also how to interface the ADC to the processor without overloading the processor’s interface. In addition, this solution is not only compact with a high level of integration but it also allows simultaneous sampling when using multiple ADCs.
More information can be found here in the TI Design library: TIDA-00307
