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PRELIMINARY TECHNICAL DATA
EVAL-ADE7758EB
Interrupts
This window gives access to the Interrupt Mask and STA-
TUS registers of the ADE7758 - see Figure 14 . For details
on how to use the Interrupt and Mask registers, refer to the
ADE7758 datasheet.
the phase difference at the input to the Current channel is now
59.89° lag instead of 60° lag. Determining whether the error
is a lead or lag can also be figured intuitively from the
frequency output. Figure 14 shows how the output frequency
varies with phase (cos{ φ }). Since the output frequency B
(1.83817Hz) at the PF=0.5 lag setting in the example is
actually greater than A/2 (1.833105Hz), this means the phase
error between the current channel and the voltage channel
was actually less than 60°. This means there was additional
lead in the current channel due to the CT.
CF (Hz)
Frequency B > A/2
PF=1
Phase difference < 60 lag
PF>0.5
PF=0.5
PF<0.5
PF=0
360
60
Phase lag
Figure 14—CF Frequency Vs Phase(PF)
Phase Error ( ° ) = Tan ? 1 ?
? ? A ? 3 ? ?
Phase Error ( ) = Tan ? 1 ?
? ?
? ?
3 . 66621 ? 3
Figure 14 - Interrupts Window
Measuring CT Phase Errors using the ADE7758
The ADE7758 itself can be used to measure a CT (and
external components) phase error during calibration. The
assumption is that the ADE7758 has no internal phase error
(APHCAL = BPHCAL = CPHCAL = 00 hex) and the error
due to external components is small (<0.5°). The procedure
is based on a two point measurement, at PF=1 and PF = 0.5
(lag). The PF is set up using the test bench source and this
source must be very accurate. The ADE7758 should be
configured for Active Energy measurement mode.
An Active Energy measurement is first made with PF=1
(measurement A). A second Active Energy measurement
should be made at PF=0.5 (measurement B). The frequency
output CF can be used for this measurement. Using the
formula shown below the phase error is easily calculated:
? B ? A ?
2 ?
2
For example, using the frequency output CF to measure
power, a frequency of 3.66621Hz is recorded for a PF=1.
The PF is then set to 0.5 lag and a measurement of
1.83817Hz is obtained. Using the formula above the phase
error on Channel 1 is calculated as:
? 1 . 83817 ? 3 . 66621 ?
° 2 ? = + 0 . 0 9 1 °
2
The formula will also give the correct sign for the phase
error. In this example the phase error is calculated as +0.091°
at the input to the Current channel of ADE7758. This means
that the CT has introduced a phase lead of 0.091°. Therefore
REV. PrB 08/03 –9–
Using the Phase Calibration to correct small (<0.5°)
external phase errors
From the previous example it is seen that the CT introduced
a phase lead in the current channel of 0.091° . Therefore
instead of 60° phase difference between the current channel
and the voltage channel, it is actually 59.89°. In order to
bring the phase difference back to 60°, the phase compensa-
tion circuit in the voltage channel is used to introduce an
extra lead of 0.091°. This is achieved by reducing the amount
of time delay in the voltage channel.
The maximum time delay adjustment in the voltage channel
is ±75.6μs with a CLKIN of 10MHz. The PHCAL registers
are signed 2's complement 7 bit registers. Each LSB is
equivalent to 1.2μs. In this example the line frequency is
50Hz. This means each LSB is equivalent to (360° x 1.2μs
x 50) = 0.022°. To introduce a lead of 0.091° the delay in the
voltage channel must be reduced. This is achieved by writing
-4 (1Ch) or +0.088° to the PHCAL register.
Correcting large external phase errors
In this example the phase correction range at 50Hz is only
approximately ±1.3°. However it is best to only use the
PHCAL register for small phase corrections, i.e., <0.5°. If
larger corrections are required the larger part of the correc-
tion can be made using external passive component. For
example the resistors in the anti-alias filter can be modified
to shift the corner frequency of the filter so as to introduce
more or less lag. The lag through the anti-alias filters with
1k ? and 33nF is 0.56° at 50Hz. Fine adjustments can be
made with the PHCAL register. Note that typically CT phase
shift will not vary significantly from part to part. If a CT
phase shift is 1°, then the part to part variation should only be
about ±0.1°. Therefore the bulk of the phase shift (1°) can be
canceled with fixed component values at design. The remain-
ing small adjustments can be made in production using the
PHCAL registers.
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