ADE7854AACPZ View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'ADE7854AACPZ' PDF : 96 Pages View PDF
Data Sheet
ADE7854A/ADE7858A/ADE7868A/ADE7878A
the CF1, CF2, and CF3/HSCLK pins. Do this initialization
last, so that no spurious pulses are generated while the
ADE7854A/ADE7858A/ADE7868A/ADE7878A are
initialized.
There is no obvious reason to stop the DSP when maintaining
the device in PSM0 normal mode. All ADE7854A, ADE7858A,
ADE7868A, and ADE7878A registers, including ones located in
the data memory RAM, can be modified without stopping the
DSP. However, to stop the DSP, write 0x0000 into the run register.
To restart the DSP, select one of the following procedures:
If the ADE7854A/ADE7858A/ADE7868A/ADE7878A
registers located in the data memory RAM have not been
modified, write 0x0001 into the run register to start the DSP.
If the ADE7854A/ADE7858A/ADE7868A/ADE7878A
registers located in the data memory RAM must be
modified, first execute a software or hardware reset, and
then follow the recommended procedure to initialize the
registers at power-up.
As mentioned in the Power Management section, when the
ADE7854A/ADE7858A/ADE7868A/ADE7878A exit PSM0
power mode, it is recommended to stop the DSP by writing
0x0000 to the run register (see Table 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. is defined as
F rms
1
t
t
0
f
2
t
dt
(14)
where F rms is the mathematical rms value of a continuous
signal f(t).
For time sampling signals, rms calculation involves squaring the
signal, taking the average, and obtaining the square root.
F rms
1
N
N
f
N 1
2
n
(15)
Equation 15 implies that, for signals containing harmonics, the
rms calculation contains the contribution of all harmonics, not
only the fundamental. The device uses two different methods to
calculate rms values. The first method is very accurate and is active
only in PSM0 mode. The second method is less accurate and uses
the estimation of the mean absolute value (MAV) measurement;
this method is active in PSM0 and PSM1 modes and is available
for the ADE7868A and ADE7878A only.
The first method is to filter the square of the input signal using a
low-pass filter (LPF) and take the square root of the result (see
Figure 61).
f (t) Fk
2 sinkωt γk
(16)
k 1
The square of f(t) is
f 2 (t) Fk2 Fk2 cos(2kt 2k )
k 1
k 1
2
2
Fk
Fm
sinkt
k
sinmt
m
(17)
k,m1
km
After the LPF and the execution of the square root, the rms
value of f(t) is obtained by
F Fk2
(18)
k 1
All seven analog input channels simultaneously process the
rms calculation based on this method. Each result is available in
the following 24-bit registers: AIRMS, BIRMS, CIRMS, AVRMS,
BVRMS, CVRMS, and NIRMS (NIRMS is available on the
ADE7868A and ADE7878A only). An average of 1.024 sec of
these readings is also available (see the Low Ripple Current RMS
and Low Ripple Voltage sections for more information).
The second method computes the absolute value of the input
signal and then filters it to extract its dc component. This method
computes the absolute mean value of the input. When the input
signal in Equation 17 has a fundamental component only, its
average value is
T
Fdc
1
T
2
0
T
2 F1 sin(t)dt
T
2
2
F1
sin(t
)dt
Fdc
2
2 F1
(19)
The calculation based on this method is simultaneously processed
on the three phase currents only. Each result is available in the
following 20-bit registers: AIMAV, BIMAV, and CIMAV
(available on the ADE7868A and ADE7878A only). Note that
the proportionality between the 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 ADE7868A and
ADE7878A also compute the rms of the sum of the instantaneous
values of the phase currents when Bit 0 (INSEL) in the CONFIG_A
register is set to 1. The result is stored in the NIRMS register.
Note that the instantaneous value of the sum is stored into the
ISUM register (see the Neutral Current Mismatch—ADE7868A
and ADE7878A section). In 3-phase, 4-wire systems that require
sensing the phase currents only, these values provide a measure
of the neutral current.
Rev. C | Page 43 of 96
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