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ADE7758 Datasheet(PDF) 41 Page - Analog Devices |
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ADE7758 Datasheet(HTML) 41 Page - Analog Devices |
41 / 68 page ADE7758 Rev. A | Page 41 of 68 ENERGY REGISTERS SCALING The ADE7758 provides measurements of active, reactive, and apparent energies that use separate signal paths and filtering for calculation. The differences in the data paths can result in small differences in LSB weight between the active, reactive, and apparent energy registers. These measurements are internally compensated so that the scaling is nearly one to one. The relationship between the registers is shown in Table 11. Table 11. Energy Registers Scaling Frequency of 60 Hz Frequency of 50 Hz Integrator Off VAR = 1.004 × WATT VAR = 1.0054 × WATT VA = 1.00058 × WATT VA = 1.0085 × WATT Integrator On VAR = 1.0059 × WATT VAR = 1.0064 × WATT VA = 1.00058 × WATT VA = 1.00845 × WATT WAVEFORM SAMPLING MODE The waveform samples of the current and voltage waveform, as well as the active, reactive, and apparent power multiplier out- puts, can all be routed to the WAVEFORM register by setting the WAVSEL[2:0] bits (Bit 2 to Bit 4) in the WAVMODE register. The phase in which the samples are routed is set by setting the PHSEL[1:0] bits (Bit 0 and Bit 1) in the WAVMODE register. All energy calculation remains uninterrupted during waveform sampling. Four output sample rates may be chosen by using Bit 5 and Bit 6 of the WAVMODE register (DTRT[1:0]). The output sample rate may be 26.0 kSPS, 13.0 kSPS, 6.5 kSPS, or 3.3 kSPS (see Table 16). By setting the WSMP bit in the interrupt mask register to Logic 1, the interrupt request output IRQ goes active low when a sample is available. The 24-bit waveform samples are transferred from the ADE7758 one byte (8 bits) at a time, with the most significant byte shifted out first. The interrupt request output IRQ stays low until the interrupt routine reads the reset status register (see the ADE7758 Interrupts section). CALIBRATION A reference meter or an accurate source is required to calibrate the ADE7758 energy meter. When using a reference meter, the ADE7758 calibration output frequencies, APCF and VARCF, are adjusted to match the frequency output of the reference meter under the same load conditions. Each phase must be calibrated separately in this case. When using an accurate source for calibration, one can take advantage of the line cycle accu- mulation mode and calibrate the three phases simultaneously. There are two objectives in calibrating the meter: to establish the correct impulses/kW-hr constant on the pulse output and to obtain a constant that relates the LSBs in the energy and rms registers to Watt/VA/VAR hours, amps, or volts. Additionally, calibration compensates for part-to-part variation in the meter design as well as phase shifts and offsets due to the current sensor and/or input networks. Calibration Using Pulse Output The ADE7758 provides a pulsed output proportional to the active power accumulated by all three phases, called APCF. Additionally, the VARCF output is proportional to either the reactive energy or apparent energy accumulated by all three phases. The following section describes how to calibrate the gain, offset, and phase angle using the pulsed output information. The equations are based on the pulse output from the ADE7758 (APCF or VARCF) and the pulse output of the reference meter or CFEXPECTED. Figure 76 shows a flow chart of how to calibrate the ADE7758 using the pulse output. Since the pulse outputs are proportional to the total energy in all three phases, each phase must be cali- brated individually. Writing to the registers is fast in order to reconfigure the part for calibrating a different phase, therefore Figure 76 shows a method that calibrates all phases at a given test condition before changing the test condition. |
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