ADE7759 Analog Devices, ADE7759 Datasheet - Page 25

• Manufacturer Part Number: ADE7759
• Description: Active Energy Metering IC with di/dt Sensor Interface
• Manufacturer: Analog Devices

For 255 half cycles this would give a total integration time of 2.125
seconds. This would mean the energy register was updated
2.125/1.1175 µs (4/CLKIN) times. The average output value of
LPF2 is given as:
Or, equivalently, in terms of contents of various ADE7759
registers and CLKIN and line frequencies (f
where f
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 Frequency
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:
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 cal-
culated 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 approximately
half scale and the signal on the current channel about 1/8 of full
scale (assuming a maximum current of 80 A). The average value
from LPF2 is calculated as 3,276.81 decimal using the calibration
mode as described above. Then using Equation 8 (Energy to Fre-
quency Conversion), the frequency under this load is calculated as:
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. Note that the CF frequency is
divided by the contents of (CFNUM + 1)/(CFDEN + 1). With
the CF Divide registers 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
frequency is accurately known during calibration. Using Line
Frequency CF
Average Word LPF
Frequency CF
l
Number of times LENERGY
is the line frequency.
Contents of LENERGY
(
(
)
(
=
)
=
3276 81 3 579545
3200
60
2
)
min
.
=
LINECYC
imp kWh
LENERGY
×
×
60
2
/
.
25
sec
[
39 0
[
39 0
=
: ]
[ : ]
13 0
: ]
[
3200
3600
MHz
39 0
at the end
l
was updated
):
: ]
×
=
CLKIN
× ×
=
0 8888
8
349 566
.
f
.
l
Hz
Hz
(16)
Cycle Energy Accumulation mode, the calibration time can be
reduced by synchronizing energy accumulation to the zero cross-
ing of the voltage channel. See 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 frequency. Any deviation
from the nominal frequency will directly affect the calibration
result. The line frequency could be measured using the ZX output
of the ADE7759. Alternatively, the average value of LPF2 can
be calculated from the output frequency from CF—see Energy
to Frequency Conversion section.
Note that besides CFNUM and CFDEN registers, changing
APGAIN[11:0] register will also affect the output frequency 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 consumed
in kWh (Killowatt Hours). 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
could be set to, say, 10,000 imp/kWh.
If more flexibility is required when monitoring energy usage, the
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 accuracy at any
CLKIN frequency within the specified range. If the CLKIN
frequency is not 3.579545 MHz, various timing and filter charac-
teristics 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:
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.
New Frequency Original Frequency
=
×
CLKIN Frequency
3 579545
.
ADE7759
MHz
(17)