AD7755
Table VI. F1 and F2 Frequency with Half-Scale AC Inputs
The high frequency CF output is intended to be used for com-
munications and calibration purposes. CF produces a 90 ms-
wide active high pulse (t4) at a frequency proportional to active
power. The CF output frequencies are given in Table IV. As in
the case of F1 and F2, if the period of CF (t5) falls below 180 ms,
the CF pulsewidth is set to half the period. For example, if the CF
frequency is 20 Hz, the CF pulsewidth is 25 ms.
Frequency on F1 and F2–
CH1 and CH2 Half-Scale AC Inputs
S1
S0
F1–4
0
0
1
1
0
1
0
1
1.7
3.4
6.8
13.6
0.085 Hz
0.17 Hz
0.34 Hz
0.68 Hz
NOTE: When the high frequency mode is selected, (i.e., SCF =
0, S1 = S0 = 1) the CF pulsewidth is fixed at 18 µs. Therefore t4
will always be 18 µs, regardless of output frequency on CF.
When selecting a suitable F1–4 frequency for a meter design, the
frequency output at IMAX (maximum load) with a meter con-
stant of 100 imp/kWhr should be compared with Column 4 of
Table VI. The frequency that is closest in Table VI will deter-
mine the best choice of frequency (F1–4). For example, if a meter
with a maximum current of 25 A is being designed, the out-
put 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 V). Looking
at Table VI, the closest frequency to 0.153 Hz in column four is
0.17 Hz. Therefore F2 (3.4 Hz—see Table II) is selected for this
design.
NO LOAD THRESHOLD
The AD7755 also includes a “no load threshold” and “start-up
current” feature that will eliminate any creep effects in the
meter. The AD7755 is designed to issue a minimum output
frequency on all modes except when SCF = 0 and S1 = S0 = 1.
The no-load detection threshold is disabled on this output mode
to accommodate specialized application of the AD7755. Any
load generating a frequency lower than this minimum frequency
will not cause a pulse to be issued on F1, F2 or CF. The mini-
mum output frequency is given as 0.0014% of the full-scale
output frequency for each of the F1–4 frequency selections—see
Table II. For example, an energy meter with a meter constant
of 100 imp/kWhr on F1, F2 using F2 (3.4 Hz), the maximum
output frequency at F1 or F2 would be 0.0014% of 3.4 Hz or
4.76 × 10–5 Hz. This would be 3.05 × 10–3 Hz at CF (64 × F1 Hz).
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. Compar-
ing 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.
Frequency Outputs
Figure 1 shows a timing diagram for the various frequency out-
puts. The outputs F1 and F2 are the low frequency outputs that
can be used to directly drive a stepper motor or electromechani-
cal impulse counter. The F1 and F2 outputs provide two alter-
nating low going pulses. The pulsewidth (t1) is set at 275 ms and
the time between the falling edges of F1 and F2 (t3) is approxi-
mately half the period of F1 (t2). If however the period of F1
and F2 falls below 550 ms (1.81 Hz) the pulsewidth of F1 and
F2 is set to half of their period. The maximum output frequen-
cies for F1 and F2 are shown in Table III.
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
24-Lead Plastic DIP
(N-24)
24-Lead Shrink Small Outline Package
(RS-24)
0.328 (8.33)
0.318 (8.08)
1.275 (32.30)
1.125 (28.60)
24
13
12
0.280 (7.11)
0.240 (6.10)
24
13
12
1
0.325 (8.25)
0.300 (7.62)
PIN 1
0.311 (7.9)
0.301 (7.64)
0.212 (5.38)
0.205 (5.207)
0.060 (1.52)
0.015 (0.38)
0.210
(5.33)
MAX
0.195 (4.95)
0.115 (2.93)
0.150
(3.81)
MIN
1
0.200 (5.05)
0.125 (3.18)
0.015 (0.381)
0.008 (0.204)
0.100
(2.54)
BSC
0.022 (0.558)
0.014 (0.356)
0.070 (1.77) SEATING
PLANE
PIN 1
0.07 (1.78)
0.066 (1.67)
0.045 (1.15)
0.078 (1.98)
0.068 (1.73)
8°
0°
0.0256
(0.65)
BSC
0.015 (0.38)
0.010 (0.25)
0.008 (0.203)
0.002 (0.050)
SEATING 0.009 (0.229)
0.037 (0.94)
0.022 (0.559)
PLANE
0.005 (0.127)
REV. B
–16–
ADI [ ADI ]
