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## ADE7755 Datasheet(HTML) 18 Page - Analog Devices

 18 / 20 page ADE7755Rev. A | Page 18 of 20Table 8. Maximum Output Frequency on CFSCFS1S0f1, f2, f3, and f4 (Hz)CF Maximum for AC Signals100f1 = 1.7128 × F1, F2 = 43.52 Hz000f1 = 1.764 × F1, F2 = 21.76 Hz101f2 = 3.464 × F1, F2 = 43.52 Hz001f2 = 3.432 × F1, F2 = 21.76 Hz110f3 = 6.832 × F1, F2 = 43.52 Hz010f3 = 6.816 × F1, F2 = 21.76 Hz111f4 = 13.616 × F1, F2 = 43.52 Hz011f4 = 13.62048 × F1, F2 = 5.57 kHzSELECTING A FREQUENCY FOR AN ENERGYMETER APPLICATIONAs shown in Table 6, the user can select one of four frequencies.This frequency selection determines the maximum frequencyon F1 and F2. These outputs are intended to be used to drivethe energy register (electromechanical or other). Because onlyfour different output frequencies can be selected, the availablefrequency selection has been optimized for a meter constant of100 imp/kWh with a maximum current between 10 A and 120 A.Table 9 shows the output frequency for several maximum currents(IMAX) with a line voltage of 220 V. In all cases, the meterconstant is 100 imp/kWh.Table 9. F1 and F2 Frequency at 100 imp/kWhIMAX (A)F1 and F2 (Hz)12.50.076250.153400.244600.367800.4891200.733The fi frequencies (i = 1, 2, 3, or 4) allow complete coverage ofthis range of output frequencies on F1 and F2. When designingan energy meter, the nominal design voltage on Channel 2 (voltage)should be set to half scale to allow for calibration of the meterconstant. The current channel should also be no more than halfscale when the meter sees maximum load. This allows overcurrentsignals and signals with high crest factors to be accommodated.Table 10 shows the output frequency on F1 and F2 when bothanalog inputs are half scale. The frequencies listed in Table 10align well with those listed in Table 9 for maximum load.Table 10. F1 and F2 Frequency with Half-Scale AC InputsS1S0f1, f2, f3, and f4 (Hz)F1 and F2 Frequency on CH1 andCH2 Half-Scale AC Inputs (Hz)00f1 = 1.70.08501f2 = 3.40.1710f3 = 6.80.3411f4 = 13.60.68When selecting a suitable fi frequency (i = 1, 2, 3, or 4) for ameter design, the frequency output at IMAX (maximum load)with a meter constant of 100 imp/kWh should be compared withColumn 4 of Table 10. The frequency that is closest in Table 10determines the best choice of fi frequency (i = 1, 2, 3, or 4). Forexample, if a meter with a maximum current of 25 A is beingdesigned, the output frequency on F1 and F2 with a meter constantof 100 imp/kWh is 0.153 Hz at 25 A and 220 V (from Table 9).Table 10, the closest frequency to 0.153 Hz in Column 4, is 0.17 Hz.Therefore, f2 (3.4 Hz, see Table 6) is selected for this design.FREQUENCY OUTPUTSFigure 2 shows a timing diagram for the various frequencyoutputs. The F1 and F2 outputs are the low frequency outputsthat can be used to directly drive a stepper motor or electro-mechanical impulse counter. The F1 and F2 outputs providetwo alternating low going pulses. The pulse width (t1) is set at275 ms, and the time between the falling edges of F1 and F2 (t3)is approximately half the period of F1 (t2). If, however, the period ofF1 and F2 falls below 550 ms (1.81 Hz), the pulse width of F1 andF2 is set to half of their period. The maximum output frequenciesfor F1 and F2 are shown in Table 7.The high frequency CF output is intended to be used forcommunications and calibration purposes. CF produces a90 ms wide active high pulse (t4) at a frequency proportionalto active power. The CF output frequencies are listed in Table 8.As in the case of F1 and F2, if the period of CF (t5) falls below180 ms, the CF pulse width is set to half the period. For example,if the CF frequency is 20 Hz, the CF pulse width is 25 ms.When the high frequency mode is selected (that is, SCF = 0,S1 = S0 = 1), the CF pulse width is fixed at 18 μs. Therefore, t4is always 18 μs, regardless of the output frequency on CF.