ANALOG DEVICES AN-578 Service Manual
9106
www.analog.com
Etienne Moulin
ADE7756 (i.e., AN-564 reference design) and, in addition
to the active energy measurement, provides the voltage
ADE7756 with a microcontroller. The communication
through a serial interface (SPI). The SPI port allows
the user to calibrate various components of the meter
to store various calibration parameters of the meter and
to store the meter’s data during a power-down.
tion of active energy. Circuitry is provided to null out
various system errors including gain, phase, and offset
All registers of the ADE7756 are available through the
to process rms calculation into the microcontroller. This
the meter.
14 of program memory, and a 4 kbytes
this document is to demonstrate that a cost-effective
voltage input.
further details on the accuracy achievements of this refer-
–2
–
THEORY OF CALCULATION
V
T
V t
dt
RMS
0
T
= ×
( )
∫
1
2
the square root
V
N
V i
RMS
i
N
= ×
∑
=
1
2
1
( )
factor to the mean absolute value
V
2
V
RMS MAV
=
×
×
π
2
tive Power and the Apparent Power consumed by a load.
PF
Active Power
Apparent Power
=
Active Power (t) Mean V(t) I(t)
Apparent Power V I
RMS RMS
= ×
( )
= ×
is the phase difference between the volt-
the reactive energy. When the active power and the real
Reactive Power Apparent Power Active Power
2 2
= −
Averaging
the more samples used for rms calculation, the more
the measurement depends on both measurement time
variation of these parameters in terms of dynamic range
the averaging time needed for an accurate rms measure-
Amplitude Dynamic Range:
Voltage 10:1
Period of Change of the Amplitude: 3 Seconds
Frequency of the Signal:
Windowing Effect
A commonly used method for canceling the windowing
–3
–
+ V
t) yields:
V
/2 + 2
V
V
cos (
t) + V
/2
cos (2
t)
DC COMPONENT (INCLUDING ERROR TERM) IS EXTRACTED
BY THE LPF FOR REAL POWER CALCULATION
FREQUENCY – rad/s
0 2
V
2
OS
V
2
2
V
2
2
2 V
OS
V
and 2
the rms calculation.
ADE7756 Sampling Mode
through a special mode called waveform sampling
ADE7756 data sheet). The sampling rate of the output
four ch o i c e s : CLKI N / 1 2 8 , CL K I N / 2 5 6 , CLKIN / 5 1 2 ,
IRQ
pin low for 16 µs
when a new sample is available in the waveform reg-
When the sampling period is over, another interrupt
0 0 0 01 HEX
SAMPLING RATE (CLKIN/128, CLKIN/256, CLKIN/512, OR CLKIN/1024)
IRQ
SCLK
DIN
DOUT
READ FROM WAVEFORM
CHANNEL 1 OR CHANNEL 2 DATA – 20 BITS
16s
SIGN
As discussed later, the rms computation implemented in
worst-case computation time of approximately 350 µs.
ADE7756 can be sped up to 5 MHz (see the ADE7756 data
ADE7756.
time needed for the microcontroller to process the data
through the required rms signal processing determines
the sampling frequency of the input.
With this frequency, the sampling periods available
through the ADE7756 are 35.7 µs, 71.5 µs, 143 µs, and
the signal processing, the lowest sampling rate is used
for the ADE7756 and the samples are read with a decima-
tion factor of 2. The actual sampling rate for the samples
which represents approximately 1.748 kHz.
SAMPLING RATE (CLKIN/1024)
IRQ
CS
READING RATE = CLKIN/2048
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