ADE7760ARS datasheet(16/24 Pages) AD

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ADE7760

Rev. 0 | Page 16 of 24

The output frequency on CF can be up to 2048 times higher

than the frequency on F1 and F2. This higher output frequency

is generated by accumulating the instantaneous active power

signal over a much shorter time while converting it to a

frequency. This shorter accumulation period means less

averaging of the cos(2ωt) component. As a consequence, some

of this instantaneous power signal passes through the digital-to-

frequency conversion. This is not a problem in the application.

Where CF is used for calibration purposes, the frequency

should be averaged by the frequency counter, which removes

any ripple. If CF is being used to measure energy, such as in a

microprocessor-based application, the CF output should also be

averaged to calculate power. Because the outputs F1 and F2

operate at a much lower frequency, a lot more averaging of the

instantaneous active power signal is carried out. The result is a

greatly attenuated sinusoidal content and a virtually ripple-free

frequency output.

TRANSFER FUNCTION

Frequency Outputs F1 and F2

The ADE7760 calculates the product of two voltage signals (on

Channel 1 and Channel 2) and then low-pass filters this product

to extract active power information. This active power

information is then converted to a frequency. The frequency

information is output on F1 and F2 in the form of active high

pulses. The pulse rate at these outputs is relatively low, for

example, 0.34 Hz maximum for ac signals with S0 = S1 = 0

(see Table 7). This means that the frequency at these outputs is

generated from active power information accumulated over a

relatively long period of time. The result is an output frequency

that is proportional to the average active power. The averaging

of the active power signal is implicit to the digital-to-frequency

conversion. The output frequency or pulse rate is related to the

input voltage signals by the following equation:

REF

rms

Frequency

−

(7)

where:

F1 − F2 Frequency is the output frequency on F1 and F2 (Hz).

V1rms is the differential rms voltage signal on Channel 1 (V).

V2rms is the differential rms voltage signal on Channel 2 (V).

VREF is the reference voltage (2.5 V ± 8%) (V).

F1–4 is one of four possible frequencies selected by using the

logic inputs S0 and S1 (see Table 5).

Table 5. F1–4 Frequency Solution

F1–4 (Hz)1

OSC/CLKIN2

1.72

OSC/218

3.44

OSC/217

6.86

OSC/216

13.7

OSC/215

1 Values are generated using the nominal frequency of 450 kHz.

2 F1–4 are a binary fraction of the master clock and, therefore, varies, if the

internal oscillator frequency (OSC).

Frequency Output CF

The pulse output calibration frequency (CF) 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. Table 6 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 active power. As with F1 and F2, the fre-

quency is derived from the output of the low-pass filter after

multiplication. However, because the output frequency is high,

this active 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 active power signal, the CF output is much more responsive

to power fluctuations (see Figure 17).

Table 6. Relationship between CF and F1, F2 Frequency

Outputs

SCF

F1–4 (Hz)

CF Frequency output

1.72

128 × F1, F2

1.72

64 × F1, F2

3.44

64 × F1, F2

3.44

32 × F1, F2

6.86

32 × F1, F2

6.86

16 × F1, F2

13.7

16 × F1, F2

13.7

2048 × F1, F2

Example

In this example, if ac voltages of ±660 mV peak are applied to

V1 and V2, then the expected output frequency on CF, F1, and

F2 is calculated as follows:

F1–4 = 1.7 Hz, SCF = S1 = S0 = 0

V1rms = rms of 660 mV peak ac = 0.66/√2 V

V2rms = rms of 660 mV peak ac = 0.66/√2 V

VREF = 2.5 V (nominal reference value)

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