ade7566 Analog Devices, Inc., ade7566 Datasheet - Page 50

• Manufacturer Part Number: ade7566
• Description: Single-phase Energy Measurement Ic With 8052 Mcu, Rtc, And Lcd Driver
• Manufacturer: Analog Devices, Inc.

ADE7566/ADE7569
Voltage Channel RMS Calculation
Figure 57 shows details of the signal processing chain for the
rms calculation on the voltage channel. The voltage channel
rms value is processed from the samples used in the voltage
channel waveform sampling mode and is stored in the unsigned
24-bit V
The update rate of the voltage channel rms measurement is
MCLK/5. To minimize noise in the reading of the register, the
V
crossing of the voltage input. This configuration is done by
setting ZXRMS bit in the MODE2 register (0x0C).
With the specified full-scale ac analog input signal of 0.4 V, the
output from the LPF1 in Figure 57 swings between 0x28F5 and
0xD70B at 60 Hz (see the Voltage Channel ADC section). The
equivalent rms value of this full-scale ac signal is approximately
0d1,898,124 (0x1CF68C) in the V
measurement provided in the ADE7566/ADE7569 is accurate to
within ±0.5% for signal input between full scale and full scale/20.
The conversion from the register value to volts must be done
externally in the microprocessor using a V/LSB constant.
Voltage Channel RMS Offset Compensation
The ADE7566/ADE7569 incorporate a voltage channel rms
offset compensation register (VRMSOS). This is a 12-bit signed
register that can be used to remove offset in the voltage channel
rms calculation. An offset can exist in the rms calculation due
to input noises and dc offset in the input samples. One LSB of
the voltage channel rms offset is equivalent to 64 LSBs of the
rms register. Assuming that the maximum value from the
voltage channel rms calculation is 0d1,898,124 with full-scale ac
inputs, then 1 LSB of the voltage channel rms offset represents
3.37% of measurement error at −60 dB down of full scale.
where V
ACTIVE POWER CALCULATION
Active power is defined as the rate of energy flow from source
to load. It is the product of the voltage and current waveforms.
The resulting waveform is called the instantaneous power signal
and is equal to the rate of energy flow at every instant of time.
The unit of power is the watt or joules/sec. Equation 7 gives an
expression for the instantaneous power signal in an ac system.
rms
register can also be configured to update only with the zero
V
rms
rms0
rms
= V
register.
is the rms measurement without offset correction.
rms0
+ 64 × VRMSOS
VOLTAGE CHANNEL
rms
register. The voltage rms
Figure 57. Voltage Channel RMS Signal Processing
LPF1
VOLTAGE SIGNAL (V(t))
0xD70B
0x28F5
0x0
Rev. 0 | Page 50 of 136
(4)
LPF3
where:
v is the rms voltage.
i is the rms current.
The average power over an integral number of line cycles (n) is
given by the expression in Equation 8.
where:
T is the line cycle period.
P is referred to as the active or real power.
Note that the active power is equal to the dc component of the
instantaneous power signal p(t) in Equation 8, that is, VI. This
is the relationship used to calculate active power in the
ADE7566/ADE7569. The instantaneous power signal p(t) is
generated by multiplying the current and voltage signals. The dc
component of the instantaneous power signal is then extracted
by LPF2 (low-pass filter) to obtain the active power
information. This process is illustrated in Figure 58.
0xCCCCD
0x19999A
sgn
0x00000
2
v
t i
P
p
p
( )
16
( )
) (
VRMOS[11:0]
(
VI
t
t
=
t
2
)
15
=
=
nT
=
=
1
INSTANTANEOUS
POWER SIGNAL
VI
+
v
2
2
∫
(
2
CURRENT
i(t) = √2 × i × sin(ωt)
t
0
×
−
nT
8
+
×
)
VI
I
V
×
2
p
Figure 58. Active Power Calculation
7
sin(
) (
t i
0x28F5C2
VOLTAGE
v(t) = √2 × v × sin(ωt)
cos(
t
sin(
(
2
6
dt
)
ω
0x00
V
2
ω
t
=
rms
ω
)
t
VI
)
t
[23:0]
)
p(t) = v × i – v × i × cos(2ωt)
ACTIVE REAL POWER
SIGNAL = v × i
(5)
(6)
(7)
(8)