At the same time, measurement of these power characteristics requires tightly synchronized, high-resolution measurement of voltage and current across all three phases and neutral. Synchronized sampling allows extremely accurate measurement of phase angle between voltage and current on each line. Poor synchronization between voltage and current measurements can introduce artifacts, reduce overall accuracy, and significantly compromise more complex power calculations.
Simultaneous sampling
To reduce the complexity of high-accuracy multichannel measurement, designers can turn to highly integrated devices that combine high-resolution ADCs with analog front ends (AFEs) designed to optimize impedance matching, signal dynamic range, offset, and other factors that can erode performance and accuracy of data conversion. Manufacturers typically build these high-performance simultaneous sampling ADCs around successive-approximation register (SAR) converters. SAR ADCs offer high accuracy without the cycle latency associated with the oversampling used in delta-sigma ADCs to achieve high stability and conversion resolution.
For the most demanding applications, semiconductor manufacturers integrate a complex AFE and dedicated SAR ADC for each channel. For highly accurate power monitoring, eight-channel ADCs such as the Maxim Integrated MAX11046 and Texas Instruments ADS8568 allow simultaneous sampling of all three phases and neutral (Figure 2). With a dedicated AFE and ADC for each channel, these devices are able to achieve very high data rates—250 ksps per channel for the MAX11046 and 510 ksps for the ADS8568 (parallel-output interface).
Figure 2: For three-phase power monitoring, simultaneous sampling ADCs provide separate AFEs and ADCs for each of the eight channels required for highly accurate power calculations. (Courtesy of Maxim Integrated)
For some applications, designers can simply tie the unbuffered output of CTs and PTs directly into the inputs of ADCs including the Maxim Integrated MAX11046 and TI ADS8568 as suggested in Figure 2. Devices such as the MAX11046 provide high-input input impedance and self-protecting input clamps to support this type of simple configuration. To achieve maximum throughput, however, ADCs typically require op amp drivers between the transducers and ADC. In fact, evaluation boards from both Maxim Integrated and Texas Instruments include such drivers. On its MAXREFDES30 evaluation board for the MAX11046, Maxim Integrated includes its MAX44252 op amp drivers. On the TI ADS8688EVM-PDK evaluation board for the ADS8568, Texas Instruments includes its OPA2209 op amp drivers.
Less-stringent requirements
For power monitoring applications with less stringent accuracy requirements, designers can turn to devices with more modest performance attributes. The simpler requirements in a power monitor that only needs to detect faults in the power line can take advantage of lower resolution ADCs. The Texas Instruments LMP92064 current/voltage monitor IC uses this approach to offer a single-line power-monitoring IC that requires few additional components (Figure 3). The device integrates a precision current sense amplifier to measure a load current across a shunt resistor and a buffered voltage channel to measure the voltage supply of the load. Nevertheless, the ability to sample current and voltage channels simultaneously by the independent 125-kSps, 12-bit ADCs supports accurate power calculations.
