Datasheet

Data Sheet ADE7880
Rev. A | Page 39 of 104
• If the ADE7880 registers located in the data memory RAM
have not been modified, write 0x0001 into the Run register to
start the DSP.
• If the ADE7880 registers located in the data memory RAM
have to be modified, first execute a software or a hardware
reset, initialize all ADE7880 registers at desired values,
enable the write protection and then write 0x0001 into the
Run register to start the DSP.
As mentioned in the Power Management section, when the
ADE7880 switch out of PSM0 power mode, it is recommended to
stop the DSP by writing 0x0000 into the Run register (see Table 10
and Table 11 for the recommended actions when changing
power modes).
ROOT MEAN SQUARE MEASUREMENT
Root mean square (rms) is a measurement of the magnitude of
an ac signal. Its definition can be both practical and mathematical.
Defined practically, the rms value assigned to an ac signal is the
amount of dc required to produce an equivalent amount of
power in the load. Mathematically, the rms value of a conti-
nuous signal f(t) is defined as
()
dttf
t
rmsF
∫
=
t
0
2
1
(10)
For time sampling signals, rms calculation involves squaring the
signal, taking the average, and obtaining the square root.
[]
∑
=
=
N
N
nf
N
rmsF
1
2
1
(11)
Equation 10 implies that for signals containing harmonics, the
rms calculation contains the contribution of all harmonics, not
only the fundamental. The ADE7880 uses two different methods
to calculate rms values. The first one is very accurate and is active
only in PSM0 mode. The second one is less accurate, uses the
estimation of the mean absolute value (mav) measurement, is
active in PSM0 and PSM1 modes.
The ADE7880 also computes the rms values of various
fundamental and harmonic components of phase currents,
phase voltages and neutral current as part of the harmonic
calculations block. Refer to Harmonics Calculations section for
details.
The first method is to low-pass filter the square of the input
signal (LPF) and take the square root of the result (see Figure 65).
If
(
k
k
k
γtωkFtf +=
∑
∞
=
sin2)(
1
)
)
(12)
then,
()
(
∑
∑∑
∞
≠
=
∞
=
∞
=
γ+ω×γ+ω××+
+γ+ω−=
mk
mk
m
k
m
k
k
kk
k
k
tmtkFF
tkFFtf
1,
1
2
1
22
sinsin22
)22cos()(
(13)
After the LPF and the execution of the square root, the rms
value of f(t) is obtained by
∑
∞
=
=
1
2
k
k
FF
(14)
The rms calculation based on this method is simultaneously
processed on all seven analog input channels. Each result is
available in the 24-bit registers: AIRMS, BIRMS, CIRMS,
AVRMS, BVRMS, CVRMS, and NIRMS.
The second method computes the absolute value of the input
signal and then filters it to extract its dc component. It computes
the absolute mean value of the input. If the input signal in
Equation 12 has a fundamental component only, its average
value is
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎣
⎡
××−××=
∫∫
T
T
1
T
1
DC
dttωFdttωF
T
F
2
2
0
)sin(2)sin(2
1
1
DC
F
π
F ××= 2
2
The calculation based on this method is simultaneously processed
only on the three phase currents. Each result is available in the
20-bit registers AIMAV, BMAV, and CMAV. Note that the
proportionality between mav and rms values is maintained for
the fundamental components only. If harmonics are present in the
current channel, the mean absolute value is no longer
proportional to rms.
Current RMS Calculation
This section presents the first approach to compute the rms
values of all phase and neutral currents. The ADE7880 also
computes the rms of the sum of the instantaneous values of the
phase currents if Bit 2 (INSEL) in the CONFIG3 register is set
to 1. Note that the instantaneous value of the sum is stored into
ISUM register presented in the Neutral Current Mismatch
section. In 3-phase four wired systems that only require sensing
the phase currents, this value provides a measure of the neutral
current.
Figure 65 shows the detail of the signal processing chain for the
rms calculation on one of the phases of the current channel.
The current channel rms value is processed from the samples
used in the current channel. The current rms values are signed
24-bit values and they are stored into the AIRMS, BIRMS, CIRMS,
NIRMS registers. The update rate of the current rms measurement
is 8 kHz. If Bit 2 (INSEL) of the CONFIG3 register is 0 (default),
the NIRMS register contains the rms value of the neutral
current. If the INSEL bit is 1, the NIRMS register contains the
rms value of the sum of the instantaneous values of the phase
currents.
With the specified full-scale analog input signal of 0.5 V, the
ADC produces an output code that is approximately ±5,326,737.
The equivalent rms value of a full-scale sinusoidal signal is
3,766,572 (0x39792C), independent of the line frequency. If