ADE7758_15 datasheet(22/72 Pages) AD

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ADE7758

Data Sheet

Rev. E | Page 22 of 72

ADC

TO VOLTAGE RMS

CALCULATION AND

WAVEFORM SAMPLING

TO ACTIVE AND

REACTIVE ENERGY

CALCULATION

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

f3dB = 260Hz

Figure 49. ADC and Signal Processing in Voltage Channel

VOLTAGE CHANNEL ADC

Figure 49 shows the ADC and signal processing chain for the

input VA in the voltage channel. The VB and VC channels have

similar processing chains.

For active and reactive energy measurements, the output of the

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

solution avoids the much larger multibit multiplier and does not

affect the accuracy of the measurement. An HPF is not

implemented 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 is sign extended to 24-bit in the

WFORM register.

()

225

974

260

−

⎟

⎠

⎞

⎜

⎝

⎛

(3)

–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 only used in the waveform sampling

signal path. However, waveform samples are used for the

voltage rms calculation and the subsequent apparent energy

accumulation.

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

to 001 (binary) to start the voltage waveform sampling. The

PHSEL[1:0] bits control the phase from which the samples are

routed. 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 WFSM

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

one byte

(8 bits) at a time, with the most significant byte shifted out first.

ADE7758

The sign of the register is extended in the upper 8 bits. The

timing is the same as for the current channels, as seen in Figure 40.

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