Datasheet

ADE7758 Data Sheet
Rev. E | Page 46 of 72
PHASE CALIBRATION USING PULSE OUTPUT
The ADE7758 includes a phase calibration register on each phase
to compensate for small phase errors. Large phase errors should
be compensated by adjusting the antialiasing filters. The ADE7758
phase calibration is a time delay with different weights in the
positive and negative direction (see the Phase Compensation
section). Because a current transformer is a source of phase error,
a fixed nominal value can be decided on to load into the xPHCAL
registers at power-up. During calibration, this value can be adjusted
for CT-to-CT error. Figure 79 shows the steps involved in
calibrating the phase using the pulse output.
START
ALL
PHASES
PHASE ERROR
CALIBRATED?
END
YES NO
STEP 1
SET UP PULSE
OUTPUT FOR
PHASE A, B, OR C
AND ENABLE CF
OUTPUTS
STEP 2
SET UP SYSTEM
FOR I
TEST
,V
NOM
,
PF = 0.5, INDUCTIVE
STEP 3
MEASURE %
ERROR IN APCF
STEP 4
CALCULATE PHASE
ERROR (DEGREES)
STEP 5
PERIOD OF
SYSTEM
KNOWN?
MEASURE
PERIOD USING
FREQ[11:0]
REGISTER
NO YES
CALCULATE AND
WRITE TO
xPHCAL
04443-078
SELECT PHASE
FOR LINE PERIOD
MEASUREMENT
CONFIGURE
FREQ[11:0] FOR A
LINE PERIOD
MEASUREMENT
Figure 79. Phase Calibration Using Pulse Output
Step 1: Step 1 and Step 3 from the gain calibration should be
repeated to configure the ADE7758 pulse output. Ensure the
xPHCAL registers are zero.
Step 2: Set the test system for I
TEST
, V
NOM
, and 0.5 power factor
inductive.
Step 3: Measure the percent error in the pulse output, APCF,
from the reference meter using Equation 49.
Step 4: Calculate the Phase Error in degrees by









3%100
–
%Error
ArcsinErrorPhase
(53)
Step 5: Calculate xPHCAL.



360
1
)(
1
__
1
sPeriodLineWeightLSBPHCAL
ErrorPhase
xPHCAL
(54)
where
PHCAL_LSB_Weight is 1.2 μs if the %Error is negative or
2.4 μs if the %Error is positive (see the Phase Compensation
section).
If it is not known, the line period is available in the ADE7758
frequency register, FREQ (0x10). To configure line period
measurement, select the phase for period measurement in the
MMODE[1:0] and set LCYCMODE[7]. Equation 55 shows how
to determine the value that needs to be written to
xPHCAL
using the period register measurement.





360
]0:11[
__
6.9
FREQ
WeightLSBPHCAL
s
ErrorPhase
xPHCAL
(55)
Example: Phase Calibration of Phase A Using Pulse Output
For this example, I
TEST
= 10 A, V
NOM
= 220 V, V
FULLSCALE
= 500 V,
I
FULLSCALE
= 130 A, MC = 3200 impulses/kWh, power factor = 0.5
inductive, and frequency = 50 Hz.
With Phase A contributing to CF, at I
TEST
, V
NOM
, and 0.5
inductive power factor, the example ADE7758 meter shows
0.9668 Hz on the pulse output. This is equivalent to −1.122%
error from the reference meter value using Equation 49.
The
Phase Error in degrees using Equation 53 is 0.3713°.









 3713.0
3%100
1.122%–
– ArcsinErrorPhase
If at 50 Hz the FREQ register = 2083d, the value that should be
written to APHCAL is 17d, or 0x11 using Equation 55. Note
that a PHCAL_LSB_Weight of 1.2 μs is used because the
%Error is negative.
1101719.17
360
2083
μs2.1
μs6.9
3713.0 xAPHCAL 


Power Offset Calibration Using Pulse Output
Power offset calibration should be used for outstanding
performance over a wide dynamic range (1000:1). Calibration
of the power offset is done at or close to the minimum current
where the desired accuracy is required.
The ADE7758 has power offset registers for watts and VAR
(xWATTOS and xVAROS). Offsets in the VA measurement are
compensated by adjusting the rms offset registers (see the
Calibration of IRMS and VRMS Offset section). Figure 80
shows the steps to calibrate the power offsets using the pulse
outputs.