ADE7757 Analog Devices, ADE7757 Datasheet - Page 13

• Manufacturer Part Number: ADE7757
• Description: Energy Metering IC with Integrated Oscillator
• Manufacturer: Analog Devices

1
0
1
0
1
0
1
0
As shown in Table I, the user can select one of four fre-
quencies. This frequency selection determines the maxi-
mum frequency on F1 and F2. These outputs are intended
for driving an energy register (electromechanical or oth-
ers). Since only four different output frequencies can be
selected, the available frequency selection has been opti-
mized for a meter constant of 100 imp/kWhr with a maxi-
mum current of between 10 A and 120 A. Table IV shows
the output frequency for several maximum currents (I
with a line voltage of 220 V. In all cases the meter con-
stant is 100 imp/kWhr.
The F
output frequencies (F1, F2). When designing an energy meter
the nominal design voltage on Channel V2 (voltage) should be
set to half-scale to allow for calibration of the meter constant.
The current channel should also be no more than half-scale
when the meter sees maximum load. This will allow over cur-
rent signals and signals with high crest factors to be accommo-
dated. Table V shows the output frequency on F1 and F2 when
both analog inputs are half-scale. The frequencies listed in
Table V align very well with those listed in Table IV for maxi-
mum load.
0
0
1
1
When selecting a suitable F
sign, the frequency output at I
meter constant of 100 imp/kWhr should be compared with
REV. PrC.
1–4
0
1
0
1
0
0
0
0
1
1
1
1
frequencies allow complete coverage of this range of
0
0
1
1
0
0
1
1
0.85
1.7
3.4
6.8
120.0 A
25.0 A
40.0 A
60.0 A
80.0 A
12.5 A
128 x F1, F2 = 22.4
64 x F1, F2 = 11.2
64 x F1, F2 = 22.4
32 x F1, F2 = 11.2
32 x F1, F2 = 22.4
16 x F1, F2 = 11.2
16 x F1, F2 = 22.4
2048 x F1, F2 = 2.867 kHz
PRELIMINARY TECHNICAL DATA
1–4
MAX
frequency for a meter de-
0.076
0.733
0.153
0.244
0.367
0.489
(maximum load) with a
0.0438 Hz
0.0875 Hz
0.175 Hz
0.35 Hz
MAX
)
–13–
Column 4 of Table V. The closest frequency in Table V
will determine the best choice of frequency (F
example, if a meter with a maximum current of 25 A is
being designed, the output frequency on F1 and F2 with
a meter constant of 100 imp/kWhr is 0.153 Hz at 25 A and
220 V (from Table IV). Looking at Table V, the closest
frequency to 0.153 Hz in column four is 0.175 Hz. There-
fore F
Figure 1 shows a timing diagram for the various frequency
outputs. The outputs F1 and F2 are the low frequency outputs
that can be used to directly drive a stepper motor or elec-
tromechanical impulse counter. The F1 and F2 outputs
provide two alternating low frequency pulses. The
pulsewidth (t
1100 ms (0.909 Hz) the pulsewidth of F1 and F2 is set to
half of their period. The maximum output frequencies for
F1 and F2 are shown in Table II.
The high frequency CF output is intended to be used for
communications and calibration purposes. CF produces a
180 ms-wide active high pulse (t
tional to active power. The CF output frequencies are
given in Table III. As in the case of F1 and F2, if the
period of CF (t
set to half the period. For example, if the CF frequency is
20 Hz, the CF pulsewidth is 25 ms.
NOTE: When the high frequency mode is selected, (i.e.,
SCF = 0, S1 = S0 = 1) the CF pulsewidth is fixed at
36 µs. Therefore t
output frequency on CF.
The ADE7757 also includes a “no load threshold” and “start-
up current” feature that will eliminate any creep effects in
the meter. The ADE7757 is designed to issue a minimum
output frequency. Any load generating a frequency lower than
this minimum frequency will not cause a pulse to be issued on
F1, F2 or CF. The minimum output frequency is given as
0.0014% of the full-scale output frequency for each of the F
frequency selections—see Table I. For example, an energy
meter with a meter constant of 100 imp/kWhr on F1, F2
using F
or F2 would be 0.0014% of 3.4 Hz or 4.76 x 10
This would be 3.05 x 10
SCF = S0 = 1, S1 = 0. In this example the no load
threshold would be equivalent to 1.7 W of load or a start-
up current of 8 mA at 220 V. Comparing this value to
the IEC1036 specification which states that the meter
must start up with a load equal to or less than 0.4% Ib.
For a 5A (Ib) meter 0.4% of Ib is equivalent to 20 mA.
3
3
(3.4 Hz—see Table I) is selected for this design.
(3.4 Hz), the minimum output frequency at F1
1
) is set such that if F1 and F2 falls below
5
) falls below 360 ms, the CF pulsewidth is
4
will always be 36 µs, regardless of
–3
Hz at CF (64 x F1 Hz) when
4
) at a frequency propor-
1–4
). For
–5
Hz.
1–4