Datasheet

ADE7763 Data Sheet
Rev. C | Page 42 of 56
Calibrating Phase with an Accurate Source Example
With an accurate source, line cycle accumulation is a good
method of calibrating phase error. The value of LAENERGY
must be obtained at two power factors, PF = 1 and PF = 0.5
inductive.
SET HALF LINE CYCLES FOR ACCUMULATION
IN LINECYC REGISTER ADDR. 0x1C
SET I
TEST
= I
b
, V
TEST
= V
NOM
, PF = 0.5
CALCULATE PHCAL. SEE EQUATION 55.
SET MODE FOR LINE CYCLE
ACCUMULATION ADDR. 0x09 = 0x0080
ENABLE LINE CYCLE ACCUMULATION
INTERRUPT ADDR. 0x0A = 0x04
READ LINE ACCUMULATION ENERGY
ADDR. 0x04
RESET THE INTERRUPT STATUS
READ REGISTER ADDR. 0x0C
INTERRUPT?
NO
NO
YES
YES
RESET THE INTERRUPT STATUS
READ REGISTER ADDR. 0x0C
INTERRUPT?
WRITE PHCAL VALUE TO THE PHCAL
REGISTER: ADDR. 0x10
04481-A-090
Figure 76. Calibrating Phase with an Accurate Source
For this example:
Meter Constant: MeterConstant(imp/Wh) = 3.2
Line Voltage: V
nominal
= 220 V
Line Frequency: f
L
= 50 Hz
CF Numerator: CFNUM = 0
CF Denominator: CFDEN = 489
Base Current: I
b
= 10 A
Half Line Cycles Used at Base Current:
LINECYC
IB
= 2000
PERIOD Register: PERIOD = 8959
Expected Line Accumulation at Unity Power Factor (from Watt
Gain section): LAENERGY
IB(expected)
= 19186
Active Energy Reading at PF = 0.5 inductive:
LAENERGY
IB, PF = 0.5
= 9613
The error using Equation 52 is
Error =
0021
.
0
2
19186
2
19186
9613
=
−
Phase Error (°) = −Arcsin
°
−=








07
.
0
3
0021.
0
Using Equation 55, PHCAL is 11.
PHCAL = INT
111320x0D
360
8959
07
.0
=+−=+






°
×
°
−
Note that PHCAL is a signed, twos complement register.
The phase lead is corrected by 0.08 degrees when the PHCAL
register is set to 11:
Phase Correction (°) = −
PERIOD
PHCAL
°
×−
360
)0x0D(
Phase Correction (°) = −
°=
°
×− 08.0
8960
360
)0x0D11
(
VRMS and IRMS Calibration
VRMS and IRMS are calculated by squaring the input in a
digital multiplier.
)2cos()sin(
V2)
sin(V2)( tVVt
ttv
222
ω×−=ω×ω=
(59)
The square of the rms value is extracted from v
2
(t) by a low-pass
filter. The square root of the output of this low-pass filter gives
the rms value. An offset correction is provided to cancel noise
and offset contributions from the input.
There is ripple noise from the 2ω term because the low-pass
filter does not completely attenuate the signal. This noise can be
minimized by synchronizing the rms register readings with the
zero crossing of the voltage signal. The
IRQ
output can be
configured to indicate the zero crossing of the voltage signal.
This flowchart demonstrates how VRMS and IRMS readings
are synchronized to the zero crossings of the voltage input.
SET INTERRUPT ENABLE FOR ZERO
CROSSING ADDR. 0x0A = 0x0010
RESET THE INTERRUPT STATUS
READ REGISTER ADDR. 0x0C
INTERRUPT?
NO
YES
04481-A-091
READ VRMS OR IRMS
ADDR. 0x17; 0x16
RESET THE INTERRUPT STATUS
READ REGISTER ADDR. 0x0C
Figure 77. Synchronizing VRMS and IRMS Readings with Zero Crossings