AD71056 Ver la hoja de datos (PDF) - Analog Devices
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AD71056 Datasheet PDF : 20 Pages
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AD71056
Table 6. Maximum Output Frequency on the F1 and F2 Pins
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 the F1 and F2 pins. The lower the
f1…4 frequency selected, the higher the CF scaling (except for the
high frequency mode where 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 in Figure 15).
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 listed in Table 5, the user can select one of four frequencies.
This frequency selection determines the maximum frequency
on the F1 and F2 pins. 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 is optimized for a meter constant of
100 imp/kWh with a maximum current between 10 A and
120 A. Table 8 shows the output frequency for several maxi-
mum 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 lists the output frequency on the F1 and F2 pins when
both analog inputs are half-scale. The frequencies listed in Table 9
align very well with those listed 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.
When selecting a suitable f1…4 frequency for a meter design,
compare the frequency output at IMAX (maximum load) based
on a meter constant of 100 imp/kWh against the last column of
Table 9. The closest frequency in Table 9 determines the best
choice of frequency (f1…4). For example, if a meter with a
maximum current of 25 A is being designed, the output
frequency on the F1 and F2 pins with a meter constant of
100 imp/kWh is 0.153 Hz at 25 A and 220 V (from Table 8). In
the last column of Table 9, the closest frequency to 0.153 Hz is
0.176 Hz. Therefore, f3 (3.43 Hz) is selected for this design (see
Table 5).
Frequency Outputs
Figure 2 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 electro-
mechanical impulse counter. The F1 and F2 outputs provide
two alternating low frequency pulses. The F1 and F2 pulse
widths (t1) are set such that if they fall below 240 ms (0.24 Hz),
they are set to half of their period. The maximum output
frequencies for F1 and F2 are shown in Table 6.
The high frequency CF output is intended to be used for
communications and calibration purposes. CF produces a
90-ms-wide active high pulse (t4) at a frequency proportional
Rev. A | Page 16 of 20
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