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ADE7754
are particularly noticeable at low power factors. The ADE7754
provides a means of digitally calibrating these small phase
For time sampling signals, rms calculation involves squaring the
signal, taking the average, and obtaining the square root:
∑ f
errors. The ADE7754 allows a small time delay or time advance
to be introduced into the signal processing chain to compensate
for small phase errors. Because the compensation is in time, this
technique should be used only for small phase errors in the
F rms =
1
N
×
N
i = 1
2
( i )
(2)
V ( t ) × V ( t ) = V rms ? V rms × cos( 2 ω t )
range of 0.1 ° to 0.5 ° . Correcting large phase errors using a
time shift technique can introduce significant phase errors at
higher harmonics.
The phase calibration registers (APHCAL, BPHCAL, and
CPHCAL) are twos complement, 5-bit signed registers that
can vary the time delay in the voltage channel signal path from
–19.2 μ s to +19.2 μ s (CLKIN = 10 MHz). One LSB is equiva-
lent to 1.2 μ s. With a line frequency of 50 Hz, this gives a
phase resolution of 0.022 ° at the fundamental (i.e., 360 °
1.2 μ s 50 Hz).
Figure 19 illustrates how the phase compensation is used to
remove a 0.091 ° phase lead in IA of the current channel caused
by an external transducer. In order to cancel the lead (0.091 ° )
in IA of the current channel, a phase lead must also be intro-
duced into VA of the voltage channel. The resolution of the
phase adjustment allows the introduction of a phase lead of
0.086 ° . The phase lead is achieved by introducing a time advance
into VA. A time advance of 4.8 μ s is made by writing –4 (1Ch)
to the time delay block (APHCAL[4:0]), thus reducing the
amount of time delay by 4.8 μ s. See the Calibration of a 3-Phase
Meter Based on the ADE7754 Application Note AN-624.
The method used to calculate the rms value in the ADE7754 is
to low-pass filter the square of the input signal (LPF3) and take
the square root of the result.
With
V ( t ) = V rms × 2 × sin( ω t )
then
2 2
The rms calculation is simultaneously processed on the six analog
input channels. Each result is available on separate registers.
Current RMS Calculation
Figure 20 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 waveform sampling mode. Note
that the APGAIN adjustment affects the result of the rms calcu-
lation. See the Current RMS Gain Adjust section. The current
rms values are stored in unsigned 24-bit registers (AIRMS,
BIRMS, and CIRMS). One LSB of the current rms register is
equivalent to 1 LSB of a current waveform sample. The update
I AP
24
rate of the current rms measurement is CLKIN/12. With the
specified full-scale analog input signal of 0.5 V, the ADC produces
IA
I AN
PGA1
ADC
HPF
24
an output code which is approximately ± 2,684,354d. See the
Current Channel ADC section. The equivalent rms values of a
VA
V AP
PGA2
ADC
1
0.69 AT 50Hz, 0.022
0.83 AT 60Hz, 0.024
LPF2
full-scale ac signal is 1,898,124d. With offset calibration, the
current rms measurement provided in the ADE7754 is accurate
within ± 2% for signal input between full scale and full scale/100.
V N
7
0
I rms (t)
0
0
0
1
1
1
0
0
–100% to +100% FS
APHCAL[4:0]
IRMSOS[11:0]
1CF68Ch
–19.2 s TO +19.2 s
SGN 2 11
2 10
2 9
2 2
2 1
2 0
00h
+
24
24 IRMS
V2
V1
0.1
VA
IA
VA DELAYED BY 4.8 s
(–0.0868 AT 50Hz) 1CH
IA
AAPGAIN
HPF
LPF3
CURRENT
SIGNAL – i(t)
CURRENT
CHANNEL (rms)
50Hz
50Hz
FS
Figure 19. Phase Calibration
400000h
2378EDh
+ 122.5% FS
28F5C2h
+ FS
1CF68Ch
+ 100% FS
ROOT MEAN SQUARE MEASUREMENT
Root Mean Square (rms) is a fundamental measurement of the
00000h
147AE0h
0000h
+ 70.7% FS
AAPGAIN[11:0]
magnitude of an ac signal. Its definition can be practical or
D70A3Eh
– FS
EB852Fh
– 70.7% FS
mathematical. Defined practically, the rms value assigned to an
ac signal is the amount of dc required to produce an equivalent
amount of heat in the same load. Mathematically the rms value
C00000h
ADC OUTPUT
WORD RANGE
E30974h
DC8713h
000h
7FFh
800h
– 100% FS
– 122.5% FS
of a continuous signal f(t) is defined as
Figure 20. Current RMS Signal Processing
F rms =
1
T
T
× ∫ f 2 ( t ) dt
0
(1)
Note that a crosstalk between phases can appear in the ADE7754
current rms measurements. This crosstalk follows a specific
REV. 0
–15 –
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