ADE7757 View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'ADE7757' PDF : 16 Pages View PDF
Table I. F1–4 Frequency Selection
S1
S0
OSC Relation1
F1–4 at Nominal
OSC (Hz)2
0
0
OSC/219
0.86
0
1
OSC/218
1.72
1
0
OSC/217
3.44
1
1
OSC/216
6.86
NOTES
1F1–4 is a binary fraction of the internal oscillator frequency (OSC).
2Values are generated using the nominal frequency of 450 kHz.
Example
In this example, with ac voltages of ± 30 mV peak applied to V1
and ± 165 mV peak applied to V2, the expected output frequency
is calculated as follows:
F1–4
V1rms
V2rms
VREF
= OSC/219 Hz, S0 = S1 = 0
= 0.03/√2 V
= 0.165/√2 V
= 2.5 V (nominal reference value)
NOTE: If the on-chip reference is used, actual output frequencies may vary
from device to device due to reference tolerance of ± 8%.
Freq
=
515.85 × 0.03 × 0.165 ×
2 × 2 × 2.52
F1
=
0.204 ×
F1
=
0.175
Table II. Maximum Output Frequency on F1 and F2
Max Frequency*
S1
S0
OSC Relation
for AC Inputs (Hz)
0
0
0
1
1
0
1
1
0.204 × F1
0.204 × F2
0.204 × F3
0.204 × F4
0.175
0.35
0.70
1.40
*Values are generated using the nominal frequency of 450 kHz
Frequency Output CF
The pulse output CF (calibration frequency) is intended for
calibration purposes. The output pulse rate on CF can be up to
2048 times the pulse rate on F1 and F2. The lower the F1–4
frequency selected, the higher the CF scaling (except for the
high frequency mode SCF = 0, S1 = S0 = 1). Table III shows
how the two frequencies are related, depending on the states of
the logic inputs S0, S1, and SCF. Due to its relatively high
pulse rate, the frequency at CF logic output is proportional to
the instantaneous real power. As with F1 and F2, CF is derived
from the output of the low-pass filter after multiplication. How-
ever, because the output frequency is high, this real power
information is accumulated over a much shorter time. There-
fore, less averaging is carried out in the digital-to-frequency
conversion. With much less averaging of the real power signal,
the CF output is much more responsive to power fluctuations
(see the Signal Processing Block in Figure 3).
ADE7757
Table III. Maximum Output Frequency on CF
SCF S1
S0
CF Max for AC Signals (Hz)*
1
0
0
128 × F1, F2 = 22.4
0
0
0
64 × F1, F2 = 11.2
1
0
1
64 × F1, F2 = 22.4
0
0
1
32 × F1, F2 = 11.2
1
1
0
32 × F1, F2 = 22.4
0
1
0
16 × F1, F2 = 11.2
1
1
1
16 × F1, F2 = 22.4
0
1
1
2048 × F1, F2 = 2.867 kHz
*Values are generated using the nominal frequency of 450 kHz.
SELECTING A FREQUENCY FOR AN ENERGY METER
APPLICATION
As shown in Table I, the user can select one of four frequencies.
This frequency selection determines the maximum frequency on
F1 and F2. These outputs are intended for driving an energy
register (electromechanical or others). Since only four different
output frequencies can be selected, the available frequency
selection has been optimized for a meter constant of 100 imp/kWh
with a maximum current of between 10 A and 120 A. Table IV
shows the output frequency for several maximum currents (IMAX)
with a line voltage of 220 V. In all cases, the meter constant is
100 imp/kWh.
Table IV. F1 and F2 Frequency at 100 imp/kWh
IMAX (A)
12.5
25.0
40.0
60.0
80.0
120.0
F1 and F2 (Hz)
0.076
0.153
0.244
0.367
0.489
0.733
The F1–4 frequencies allow complete coverage of this range of
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 overcurrent
signals and signals with high crest factors to be accommodated.
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 maximum load.
REV. A
–13–
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