ADE7758 Analog Devices, ADE7758 Datasheet - Page 23

• Manufacturer Part Number: ADE7758
• Description: Poly Phase Multifunction Energy Metering IC with Per Phase Information
• Manufacturer: Analog Devices

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.
H
( )
f
=
1
+
⎛
⎜
⎜
⎝
260
60
1
Hz
H
z
VA
⎞
⎟
⎟
⎠
2
VAP
VN
0V
=
VA
. 0
ANALOG INPUT
RANGE
974
+
PGA
–
GAIN[6:5]
×
1,
=
×
−
2,
. 0
×
4
225
Figure 49. ADC and Signal Processing in Voltage Channel
GAIN
0.5V
ADC
dB
0xD7AE
0x2852
0x0
Rev. A | Page 23 of 68
CALIBRATION
PHCAL[6:0]
PHASE
LPF1
Φ
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
register (see Table 16). The available output sample rates are
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).
–20
–40
–60
–80
0
10
Figure 50. Magnitude and Phase Response of LPF1
50Hz
60Hz
0xD8B8
0xD869
0x2797
0x2748
TO ACTIVE AND
REACTIVE ENERGY
CACLULATION
TO VOLTAGE RMS
CALCULATION AND
WAVEFORM SAMPLING
0x0
0x0
LPF OUTPUT
WORD RANGE
LPF OUTPUT
WORD RANGE
FREQUENCY (Hz)
(60Hz; –13°)
100
(60Hz; –0.2dB)
ADE7758
1k
0
–10
–20
–30
–40