ADE7769_15 Datasheet, PDF (15/20 Pages) Analog Devices – Energy Metering IC with Integrated Oscillator and No-Load Indication

Table 5. 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
3.43
1
1
OSC/216
6.86
1 _{F1–4 is a binary fraction of the internal oscillator frequency.}
2 _{Values 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
F1–4 = OSC/2¹⁹ Hz, S0 = S1 = 0
V1rms = 0.03/√2 V
V2rms = 0.165/√2 V
VREF = 2.45 V (nominal reference value)
If the on-chip reference is used, actual output frequencies
may vary from device to device due to the reference tolerance
of ±200 mV.
Freq
= ^{494.75 × 0.03 × 0.165 × F1}
2 × 2 × 2.45²
= 0.204 × F₁
= 0.175
(11)
Table 6. Maximum Output Frequency on F1 and F2
S1 S0 OSC Relation Max Frequency¹ or AC Inputs (Hz)
0 0 0.204 × F1
0.175
0 1 0.204 × F2
0.35
1 0 0.204 × F3
0.70
1 1 0.204 × F4
1.40
1 _{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 7 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 the 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.
However, because the output frequency is high, this real
power information is accumulated over a much shorter time.
Therefore, 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 diagram shown in
Figure 15).
ADE7769
Table 7. Maximum Output Frequency on CF
SCF S1 S0 CF Max for AC Signals (Hz)¹
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
1 _{Values are generated using the nominal frequency of 450 kHz.}
SELECTING A FREQUENCY FOR AN ENERGY
METER APPLICATION
As shown in Table 5, 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 other). Because 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 8 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 8. F1 and F2 Frequency at 100 imp/kWh
IMAX (A)
F1 and F2 (Hz)
12.5
0.076
25.0
0.153
40.0
0.244
60.0
0.367
80.0
0.489
120.0
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 allows overcurrent
signals and signals with high crest factors to be accommodated.
Table 9 shows the output frequency on F1 and F2 when both
analog inputs are half scale. The frequencies in Table 9 align
very well with those in Table 8 for maximum load.
Table 9. F1 and F2 Frequency with Half-Scale AC Inputs
Frequency on F1 and F2—
S1 S0 F1–4 (Hz) CH1 and CH2 Half-Scale AC Input¹
0 0 0.86
0.051 × F1
0.044 Hz
0 1 1.72
0.051 × F2
0.088 Hz
1 0 3.43
0.051 × F3
0.176 Hz
1 1 6.86
0.051 × F4
0.352 Hz
1 _{Values are generated using the nominal frequency of 450 kHz.}
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