ADE7754 Analog Devices, ADE7754 Datasheet - Page 16

• Manufacturer Part Number: ADE7754
• Description: Poly-phase Multi-Function Energy Metering IC with Serial Port
• Manufacturer: Analog Devices

ADE7754
PHASE COMPENSATION
When the HPFs are disabled the phase error between the
current channel (IA, IB and IC) and the voltage channel (VA,
VB and VC) is zero from DC to 3.3kHz. When the HPFs are
enabled, the current channels
trated in Figure 16a & 16b. Also shown in Figure 16c is the
magnitude response of the filter. As can be seen from the
plots, the phase response is almost zero from 45Hz to 1kHz,
This is all that is required in typical energy measurement
applications.
Figure 16c – Gain response of HPF & Phase Compensation
(Degree)
(Degree)
(Degree)
Phase
Phase
Phase
Figure 16a – Phase response of the HPF & Phase
Figure 16b - Phase response of the HPF & Phase
-0.002
-0.004
-0.01
-0.002
-0.004
0.008
0.006
0.004
0.002
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.008
0.006
0.004
0.002
0.01
0.01
(deviation of Gain as % of Gain at 54Hz)
0
0
0
0
40
44
Compensation (10Hz to 1kHz)
compensation (40Hz to 70Hz)
100
45
46
200
300
50
48
Frequency (Hz)
400
Frequency (Hz)
Frequency (Hz)
PRELIMINARY TECHNICAL DATA
have a phase response illus-
500
55
50
600
60
52
700
800
65
54
900 1000
56
70
–16–
However despite being internally phase compensated, the
ADE7754 must work with transducers which may have
inherent phase errors. For example a phase error of 0.1° to
0.3° is not uncommon for a CT (Current Transformer).
These phase errors can vary from part to part and they must
be corrected in order to perform accurate power calculations.
The errors associated with phase mismatch 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 in order to
compensate for small phase errors. Because the compensa-
tion is in time, this technique should only be used 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 2’s complement 5-bit signed registers which
can vary the time delay in the voltage channel signal path from
–19.2µs to +19.2µs (CLKIN = 10MHz). One LSB is
equivalent to 1.2µs. With a line frequency of 50Hz this gives
a phase resolution of 0.022° at the fundamental (i.e., 360° x
1.2µs x 50Hz).
Figure 17 illustrates how the phase compensation is used to
remove a 0.091° phase lead in IA of the current channel due
to some external transducer. In order to cancel the lead
(0.091°) in IA of the current channel, a phase lead must also
be introduced 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 4.8µs is made by
writing -4 (1Ch) to the time delay block (APHCAL[4:0]),
thus reducing the amount of time delay by 4.8µs - see
Calibration of a 3-phase meter based on the ADE7754.
IA
VA
IAP
IAN
VAP
VN
V2
PGA1
PGA2
V1
50Hz
Figure 17 – Phase Calibration
0.1
ADC
ADC
7
0 0 0 1 1
1
±0.69 @ 50Hz, 0.022
±0.83 @ 60Hz, 0.024
-19.2µs to +19.2µs
APHCAL[4:0]
CALIBRATION
PHASE
HPF
1
0
0
24
0
REV. PrG 01/03
24
VA
IA
50Hz
VA delayed by 4.8µs
LPF2
(-0.086 @ 50Hz)
1Ch