Datasheet

REV. 0
ADE7754
–15–
are particularly noticeable at low power factors. The ADE7754
provides a means of digitally calibrating these small phase
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
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 µs to +19.2 µs (CLKIN = 10 MHz). One LSB is equiva-
lent to 1.2 µs. With a line frequency of 50 Hz, this gives a
phase resolution of 0.022° at the fundamental (i.e., 360° 
1.2 µs  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 µs is made by writing –4 (1Ch)
to the time delay block (APHCAL[4:0]), thus reducing the
amount of time delay by 4.8 µs. See the Calibration of a 3-Phase
Meter Based on the ADE7754 Application Note AN-624.
V1
V2
0.1
50Hz
IA
VA
I
AP
I
AN
ADC
IA
PGA1
HPF
V
AP
V
N
ADC
VA
PGA2
0.69 AT 50Hz, 0.022
0.83 AT 60Hz, 0.024
LPF2
24
1
VA DELAYED BY 4.8s
(–0.0868 AT 50Hz) 1CH
50Hz
APHCAL[4:0]
–19.2s TO +19.2s
0
0
1
0
1
1
0
0
0
7
24
Figure 19. Phase Calibration
ROOT MEAN SQUARE MEASUREMENT
Root Mean Square (rms) is a fundamental measurement of the
magnitude of an ac signal. Its definition can be practical or
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
of a continuous signal f(t) is defined as
F
T
ftdt
rms
T
=
× ∫
1
0
2
()
(1)
For time sampling signals, rms calculation involves squaring the
signal, taking the average, and obtaining the square root:
F
N
fi
rms
i
N
=
=
×
∑
1
2
1
()
(2)
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
Vt V t
rms
() sin( )=××2 ω
then
Vt Vt V V t
rms rms
() () cos( )×= − ×
22
2ω
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
rate of the current rms measurement is CLKIN/12. With the
specified full-scale analog input signal of 0.5 V, the ADC produces
an output code which is approximately ±2,684,354d. See the
Current Channel ADC section. The equivalent rms values of a
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.
CURRENT
SIGNAL – i(t)
CURRENT
CHANNEL (rms)
0000h
1CF68Ch
+ 100% FS
FS
– 100% FS
E30974h
+ 122.5% FS
+ 70.7% FS
– 70.7% FS
– 122.5% FS
2378EDh
147AE0h
EB852Fh
DC8713h
AAPGAIN[11:0]
000h
7FFh 800h
00000h
400000h
C00000h
28F5C2h
D70A3Eh
+ FS
– FS
ADC OUTPUT
WORD RANGE
IA
AAPGAIN
HPF
LPF3
2
11
SGN
2
9
2
10
2
2
2
0
2
1
I
rms
(t)
–100% to +100% FS
1CF68Ch
00h
IRMS
IRMSOS[11:0]
2424
+
Figure 20. Current RMS Signal Processing
Note that a crosstalk between phases can appear in the ADE7754
current rms measurements. This crosstalk follows a specific