Datasheet

ADE7880 Data Sheet
Rev. A | Page 44 of 104
INSTANTANEOUS
PHASE A
ACTIVE POWER
HPFEN BIT
CONFIG3[0]
HPF
DIGITAL SIGNAL PROCESSOR
AIGAIN
AVGAIN
HPFEN BIT
CONFIG3[0]
INTEN BIT
CONFIG[0]
LPSEL BIT
CONFIG3[1]
HPF
VA
IA
LPF
APGAIN AWATTOS
APHCAL
2
4
AWATT
:
10193-045
Figure 70. Total Active Power Data Path
INSTANTANEOUS
POWER SIGNAL
INSTANTANEOUS
ACTIVE POWER
SIGNAL: V rms × I rms
p(t)= V rms × I rms – V rms × I rms × cos(2ωt)
0x339CBBC =
54,119,356
V rms × I rms
0x19CE5DE =
27,059,678
0x000 0000
i(t) = √2× I rms× sin(ωt)
v(t) = √2× V rms× sin(ωt)
10193-046
Figure 71. Active Power Calculation
Because LPF2 does not have an ideal brick wall frequency
response, the active power signal has some ripple due to the
instantaneous power signal. This ripple is sinusoidal and has a
frequency equal to twice the line frequency. Because the ripple
is sinusoidal in nature, it is removed when the active power
signal is integrated over time to calculate the energy. Bit 1
(LPFSEL) of CONFIG3 register selects the LPF2 strength. If
LPFSEL is 0 (default), the settling time is 650 ms and the ripple
attenuation is 65 dB. If LPFSEL is 1, the settling time is 1300 ms
and the ripple attenuation is 128 dB. Figure 72 shows the
frequency response of LPF2 when LPFSEL is 0 and Figure 73
shows the frequency response of LPF2 when LPFSEL is 1.
The ADE7880 stores the instantaneous total phase active
powers into the AWATT, BWATT, and CWATT registers. Their
equation is
∑
∞
=
××=
1k
FS
k
FS
k
I
I
U
U
xWATT
cos(φ
k
– γ
k
) × PMAX ×
4
2
1
(21)
where:
U
FS
, I
FS
are the rms values of the phase voltage and current when
the ADC inputs are at full scale.
PMAX = 27,059,678 it is the instantaneous power computed
when the ADC inputs are at full scale and in phase.
The xWATT[23:0] waveform registers can be accessed using
various serial ports. Refer to the Waveform Sampling Mode
section for more details.
0
–5
–10
–15
–20
–25
0.1 13
FREQUENCY (Hz)
MAGNITUDE (dB)
10
10193-172
Figure 72. Frequency Response of the LPF Used to Filter Instantaneous Power
in Each Phase When the LPFSEL Bit of CONFIG3 is 0 (Default)
0
–10
–20
–30
–40
0.1 11
FREQUENCY (Hz)
MAGNITUDE (dB)
0
10193-173
Figure 73. Frequency Response of the LPF Used to Filter Instantaneous Power
in Each Phase when the LPFSEL Bit of CONFIG3 is 1