Datasheet

ADE7758 Data Sheet
Rev. E | Page 22 of 72
ADC
TO VOLTAGE RMS
CALCULATION AND
WAVEFORM SAMPLING
TO ACTIVE AND
REACTIVE ENERGY
CALCULATION
VAP
+
–
VN
PGA
VA
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]
Φ
ANALOG INPUT
RANGE
VA
0V
0.5V
GAIN
LPF1
f
3dB
= 260Hz
04443-049
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.
()
dB225.0974.0
Hz260
Hz60
1
1
2
−==
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
=fH
(3)
0
–20
–40
–60
–80
0
–10
–20
–30
–40
10 100 1k
FREQUENCY (Hz)
PHASE (Degrees)
GAIN (dB)
(60Hz; –0.2dB)
(60Hz; –13°)
04443-050
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
register (see Table 20). The available output sample rates are
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.