Datasheet

Data Sheet ADE7854/ADE7858/ADE7868/ADE7878
Rev. G | Page 43 of 100
an internal 8-bit register located at Address 0xE7FE, followed
by a write of 0x00 to an internal 8-bit register located at
Address 0xE7E3. Reinitialize the registers. Write the last
register in the queue three times. Enable the write protec-
tion by writing 0xAD to an internal 8-bit register located
at Address 0xE7FE, followed by a write of 0x80 to an internal
8-bit register located at Address 0xE7E3.
There is no obvious reason to stop the DSP if the ADE78xx is
maintained in PSM0 normal mode. All ADE78xx registers,
including ones located in the data memory RAM, can be
modified without stopping the DSP. However, to stop the DSP,
0x0000 has to be written into run register. To restart the DSP,
one of the following procedures must be followed:
• If the ADE7854/ADE7858/ADE7868/ADE7878 registers
located in the data memory RAM have not been modified,
write 0x0001 into the run register to start the DSP.
• If the ADE7854/ADE7858/ADE7868/ADE7878 registers
located in the data memory RAM have to be modified, first
execute a software or a hardware reset, and then follow the
recommended procedure to initialize the registers at power
up.
As mentioned in the Power Management section, when the
ADE7854/ADE7858/ADE7868/ADE7878 switch out of PSM0
power mode, it is recommended to stop the DSP by writing
0x0000 into the run register (see Figure 11 and Table 12 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
(9)
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
(10)
Equation 10 implies that for signals containing harmonics, the
rms calculation contains the contribution of all harmonics, not
only the fundamental. The ADE78xx 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, and is available for the
ADE7868 and ADE7878 only.
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 59).
( )
k
k
k
γt
ω
kF
tf
+
=
∑
∞
=
sin
2)
(
1
(11)
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()(
(12)
After the LPF and the execution of the square root, the rms
value of f(t) is obtained by
∑
∞
=
=
1
2
k
k
F
F
(13)
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 (NIRMS is available
on the ADE7868 and ADE7878 only).
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
dt
tω
F
dttω
F
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, which are available on the ADE7868 and
ADE7878 only: 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.
Figure 59 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,
and NIRMS (ADE7868/ADE7878 only) registers. The update
rate of the current rms measurement is 8 kHz.
With the specified full-scale analog input signal of 0.5 V, the
ADC produces an output code that is approximately ±5,928,256.
The equivalent rms value of a full-scale sinusoidal signal is
4,191,910 (0x3FF6A6), independent of the line frequency. If