Datasheet

Data Sheet ADE5166/ADE5169/ADE5566/ADE5569
Rev. D | Page 59 of 156
LPF3
VOLTAGE CHANNEL
0x28F5C2
+
+
VRMSOS[11:0]
VOLTAGE SIGNAL (V(t))
0x28F5
0x0
0xD70B
2
16
sgn 2
15
2
7
2
6
2
8
07411-038
V
rms
(t)
0x00
V
rms
[23:0]
|
X
|
HPF
LPF1
Figure 63. Voltage Channel RMS Signal Processing
Voltage Channel RMS Calculation
The voltage channel rms estimation is performed in the
ADE5166/ADE5169/ADE5566/ADE5569 using a mean absolute
value calculation. This calculation accurately estimates the rms of
the voltage channel fundamental signal. Figure 63 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 VRMS SFRs
(VRMSL, Address 0xD1; VRMSM, Address 0xD2; and
VRMSH, Address 0xD3).
The update rate of the voltage channel rms measurement is
MCLK/5. To minimize noise in the reading of the register, the
VRMS SFRs can also be configured to update only with the zero
crossing of the voltage input. This configuration is done by setting
the ZXRMS bit (Bit 2) in the MODE2 register (Address 0x0C).
With the specified full-scale ac analog input signal of 0.5 V, t h e
output from the LPF1 in Figure 63 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 VRMS SFRs. The voltage rms
measurement provided in the ADE5166/ADE5169/ADE5566/
ADE5569 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 ADE5166/ADE5169/ADE5566/ADE5569 incorporate the
voltage channel rms offset compensation register (VRMSOS,
Address 0x23). This 12-bit, signed register 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 voltage channel 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 from full scale.
V
rms
= V
rms0
+ 64 × VRMSOS (5)
where V
rms0
is the rms measurement without offset correction.
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/second. Equation 8 gives an
expression for the instantaneous power signal in an ac system.
( )
)sin(2 tVtV ω×=
(6)
( )
)sin(2 tItI ω×=
(7)
where:
V is the rms voltage.
I is the rms current.
P(t) = V(t) × I(t)
)2cos()( tVIVItP ω−=
(8)
The average power over an integral number of line cycles (n) is
given by the expression in Equation 9.
∫
==
nT
VIdttP
nT
P
0
)(
1
(9)
where:
T is the line cycle period.
P is referred to as the active or real power.