ADE7758ARWRL datasheet(23/68 Pages) AD

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ADE7758

Rev. A | Page 23 of 68

ADC

TO VOLTAGE RMS

CALCULATION AND

WAVEFORM SAMPLING

TO ACTIVE AND

REACTIVE ENERGY

CACLULATION

VAP

–

PGA

GAIN[6:5]

×1, ×2, ×4

LPF OUTPUT

WORD RANGE

0xD869

0x0

0x2797

50Hz

LPF OUTPUT

WORD RANGE

0xD8B8

0x0

0x2748

60Hz

0xD7AE

0x0

0x2852

PHASE

CALIBRATION

PHCAL[6:0]

ANALOG INPUT

RANGE

0.5V

GAIN

LPF1

Figure 49. ADC and Signal Processing in Voltage Channel

For active and reactive energy measurements, the output of the

ADC passes directly to the multipliers and is not filtered. This

solution avoids the much larger multibit multiplier and does not

affect the accuracy of the measurement. A HPF is not imple-

mented on the voltage channel to remove the dc offset because

the HPF on the current channel alone should be sufficient to

eliminate error due to ADC offsets in the power calculation.

However, ADC offset in the voltage channels produces large

errors in the voltage rms calculation and affects the accuracy

of the apparent energy calculation.

Voltage Channel Sampling

The waveform samples on the voltage channels can also be

routed to the WFORM register. However, before passing to the

WFORM register, the ADC outputs pass through a single-pole,

low-pass filter (LPF1) with a cutoff frequency at 260 Hz.

Figure 50 shows the magnitude and phase response of LPF1.

This filter attenuates the signal slightly. For example, if the line

frequency is 60 Hz, the signal at the output of LPF1 is attenuated

by 3.575%. The waveform samples are 16-bit, twos complement

data ranging between 0x2748 (+10,056d) and 0xD8B8 (−10,056d).

The data are sign extended to 24 bit in the WFORM register.

()

225

974

260

−

⎟

⎠

⎞

⎜

⎝

⎛

–20

–40

–60

–80

–10

–20

–30

–40

100

FREQUENCY (Hz)

(60Hz; –0.2dB)

(60Hz; –13

°)

Figure 50. Magnitude and Phase Response of LPF1

Note that LPF1 does not affect the active and reactive energy

calculation because it is used only in the waveform sampling

signal path. However, waveform samples are used for the

voltage rms calculation and the subsequent apparent energy

accumulation.

WAVSEL[2:0] bits in the WAVMODE register should be set to

001 (binary) to start voltage waveform sampling. PHSEL[1:0]

bits control the phase from which the samples are routed. When

in waveform sampling mode, one of four output sample rates

can be chosen by changing Bit 5 and Bit 6 of the WAVMODE

26.0 kSPS, 13.5 kSPS, 6.5 kSPS, or 3.3 kSPS. 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. Figure 40

shows the timing. 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 sign of the register is

extended in the upper 8 bits. The timing is the same as that

for the current channels (see Figure 40).

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