AD7755 Ver la hoja de datos (PDF) - Analog Devices
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AD7755 Datasheet PDF : 16 Pages
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AD7755
Example 1
Thus if full-scale differential dc voltages of +470 mV and –660 mV
are applied to V1 and V2 respectively (470 mV is the maximum
differential voltage that can be connected to Channel 1 and
660 mV is the maximum differential voltage that can be con-
nected to Channel 2), the expected output frequency is calcu-
lated as follows:
Gain
F1–4
V1
V2
VREF
= 1, G0 = G1 = 0
= 1.7 Hz, S0 = S1 = 0
= +470 mV dc = 0.47 V (rms of dc = dc)
= –660 mV dc = 0.66 V (rms of dc = |dc|)
= 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 = 8.06 × 0.47 × 0.66 × 1 × 1.7 = 0.68
2.52
Example 2
In this example, with ac voltages of ± 470 mV peak applied to
V1 and ± 660 mV peak applied to V2, the expected output fre-
quency is calculated as follows:
Gain
F1–4
V1
V2
VREF
= 1, G0 = G1 = 0
= 1.7 Hz, S0 = S1 = 0
= rms of 470 mV peak ac = 0.47/√2 volts
= rms of 660 mV peak ac = 0.66/√2 volts
= 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 = 8.06 × 0.47 × 0.66 × 1 × 1.7 = 0.34
2 × 2 × 2.52
As can be seen from these two example calculations, the maxi-
mum output frequency for ac inputs is always half of that for dc
input signals. Table III shows a complete listing of all maximum
output frequencies.
Table III. Maximum Output Frequency on F1 and F2
Max Frequency
Max Frequency
S1
S0 for DC Inputs (Hz) for AC Inputs (Hz)
0
0
0.68
0.34
0
1
1.36
0.68
1
0
2.72
1.36
1
1
5.44
2.72
Frequency Output CF
The pulse output CF (Calibration Frequency) is intended for
use during calibration. 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 IV shows
how the two frequencies are related, depending on the states of
the logic inputs S0, S1 and SCF. Because of its relatively high
pulse rate, the frequency at this logic output is proportional to
the instantaneous real power. As is the case with F1 and F2, the
frequency 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. Hence 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 Signal Processing Block in Figure 20.
Table IV. Maximum Output Frequency on CF
SCF S1
10
00
10
00
11
01
11
01
S0 F1–4 (Hz)
0 1.7
0 1.7
1 3.4
1 3.4
0 6.8
0 6.8
1 13.6
1 13.6
CF Max for AC Signals (Hz)
128 × F1, F2 = 43.52
64 × F1, F2 = 21.76
64 × F1, F2 = 43.52
32 × F1, F2 = 21.76
32 × F1, F2 = 43.52
16 × F1, F2 = 21.76
16 × F1, F2 = 43.52
2048 × F1, F2 = 5.57 kHz
SELECTING A FREQUENCY FOR AN ENERGY METER
APPLICATION
As shown in Table II, the user can select one of four frequen-
cies. This frequency selection determines the maximum fre-
quency on F1 and F2. These outputs are intended to be used to
drive the energy register (electromechanical or other). Since only
four different output frequencies can be selected, the available
frequency selection has been optimized for a meter constant of
100 imp/kWhr with a maximum current of between 10 A and
120 A. Table V 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/kWhr.
Table V. F1 and F2 Frequency at 100 imp/kWhr
IMAX
12.5 A
25 A
40 A
60 A
80 A
120 A
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 on F1 and F2. When designing an energy
meter the nominal design voltage on Channel 2 (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 current
signals and signals with high crest factors to be accommodated.
Table VI shows the output frequency on F1 and F2 when both
analog inputs are half-scale. The frequencies listed in Table
VI align very well with those listed in Table V for maximum load.
REV. B
–15–
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