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ADE7759
For 255 half cycles this would give a total integration time of 2.125
seconds. This would mean that the energy register was
updated 2.125/1.1175 μ s (4/CLKIN) times. The average output
value of LPF2 is given as:
Contents of LENERGY [ 39 : 0 ] at the end
Number of times LENERGY [ 39 : 0 ] was updated
Or, equivalently, in terms of contents of various ADE7759
registers and CLKIN and line frequencies ( f l ):
frequency is accurately known during calibration. Using line
cycle energy accumulation mode, the calibration time can be
reduced by synchronizing energy accumulation to the zero
crossing of the voltage channel—see the Line Cycle Energy
Accumulation Mode section. However, this requires the line
frequency to be precisely known. As shown in Equation 16, the
average value of LPF2 is directly proportional to the line fre-
quency. Any deviation from the nominal frequency will directly
affect the calibration result. The line frequency could be mea-
sured using the ZX output of the ADE7759. Alternatively, the
average value of LPF2 can be calculated from the output frequency
Average Word ( LPF 2 ) =
LENERGY [ 39 : 0 ] × 8 × f l
LINECYC [ 13 : 0 ] × CLKIN
(16)
from CF—see the Energy to Frequency Conversion section.
Note that besides CFNUM and CFDEN registers, changing
APGAIN[11:0] register will also affect the output frequency
where f l is the line frequency.
Calibrating the Frequency at CF
Once the average active power signal is calculated, it can be
used to determine the frequency at CF before calibration. When
the frequency before calibration is known, the pair of CF fre-
quency divider registers (CFNUM and CFDEN) can be adjusted
so as to produce the required frequency on CF. In this example,
a meter constant of 3200 imp/kWh is chosen as an appropriate
constant. This means that under a steady load of 1 kW, the
output frequency on CF would be:
from CF. The APGAIN register has a resolution of 0.0244%/LSB.
Energy Meter Display
Besides the pulse output, which is used to verify calibration, a
solid state energy meter will very often require some form of
display. The display should show the amount of energy con-
sumed in kWh (kilowatthours). One convenient and simple way
to interface the ADE7759 to a display or energy register (e.g.,
MCU with nonvolatile memory) is to use CF. For example, the
CF frequency could be calibrated to 1,000 imp/kWhr. The
MCU would count pulses from CF. Every pulse would be
equivalent to 1 watt-hour. If more resolution is required, the CF
Frequency ( CF ) =
3200 imp / kWh
60 min · 60 sec
3200
= = 0 . 8888 Hz
3600
frequency could be set to, say, 10,000 imp/kWh.
If more flexibility is required when monitoring energy usage, the
Frequency ( CF ) =
= 349 . 566 Hz
Assuming the meter is set up with a test current (basic current)
of 20 A and a line voltage of 220 V for calibration, the load is
calculated as 220 V × 20 A = 4.4 kW. Therefore, the expected
output frequency on CF under this steady load condition would
be 4.4 × 0.8888 Hz = 3.9111 Hz. Under these load conditions,
the transducers on Channel 1 and Channel 2 should be selected
such that the signal on the voltage channel should see approxi-
mately half scale and the signal on the current channel about 1/8
of full scale (assuming a maximum current of 80 A). The aver-
age value from LPF2 is calculated as 3,276.81 decimal using the
calibration mode as described above. Then using Equation 8
(energy-to-frequency conversion), the frequency under this
load is calculated as:
3276.81 × 3.579545 MHz
2 25
This is the frequency with the contents of the CFNUM and
CFDEN registers equal to 000h. The desired frequency out is
3.9111 Hz. Therefore, the CF frequency must be divided by
349.566/3.9111 Hz or 89.3779 decimal. This is achieved by
loading the pair of CF divider registers with the closest rational
number. In this case, the closest rational number is found to be
25/2234 (or 19h/8BAh). Therefore, 18h and 8B9h should be
written to the CFNUM and CFDEN registers, respectively.
active energy register (AENERGY) can be used to calculate
energy. A full description of this register can be found in the
Energy Calculation section. The AENERGY register gives the
user both sign and magnitude information regarding energy
consumption. On completion of the CF frequency output cali-
bration, i.e., after the active power gain (APGAIN) register has
been adjusted, a second calibration sequence can be initiated.
The purpose of this second calibration routine is to determine a
kWh/LSB coefficient for the AENERGY register. Once the
coefficient has been calculated, the MCU can determine the
energy consumption at any time by reading the AENERGY
contents and multiplying by the coefficient to calculate kWh.
CLKIN FREQUENCY
In this data sheet, the characteristics of the ADE7759 are
shown with the CLKIN frequency equal to 3.579545 MHz.
However, the ADE7759 is designed to have the same accu-
racy at any CLKIN frequency within the specified range. If
the CLKIN frequency is not 3.579545 MHz, various timing
and filter characteristics will need to be redefined with the
new CLKIN frequency. For example, the cutoff frequencies
of all digital filters (LPF1, LPF2, HPF1, etc.) will shift in
proportion to the change in CLKIN frequency according to
the following equation:
Note that the CF frequency is divided by the contents of
(CFNUM + 1)/(CFDEN + 1). With the CF divide registers
New Frequency = Original Frequency ×
CLKIN Frequency
3 . 579545 MHz
(17)
contents equal to 18h/8B9h, the output frequency is given as
349.566 Hz/89.36 = 3.91188 Hz. Note that this setting has an
error of +0.02%.
Calibrating CF is made easy by using the line cycle energy
accumulation mode on the ADE7759, provided that the line
REV. A
The change of CLKIN frequency does not affect the timing
characteristics of the serial interface because the data transfer is
synchronized with serial clock signal (SCLK). But one needs to
observe the read/write timing of the serial data transfer—see
Timing Characteristics. Table III lists various timing changes
that are affected by CLKIN frequency.
–25 –
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