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ADE7769
CF
AVERAGE
FREQUENCY
ADE7769
CF
FREQUENCY
RIPPLE
TIME
MCU
COUNTER
TIMER
± 10%
INTERNAL OSCILLATOR (OSC)
The nominal internal oscillator frequency is 450 kHz when
used with RCLKIN, with a nominal value of 6.2 kΩ. The
frequency outputs are directly proportional to the oscillator
frequency, thus RCLKIN must have low tolerance and low
temperature drift to ensure stability and linearity of the chip.
The oscillator frequency is inversely proportional to the
RCLKIN, as shown in Figure 27. Although the internal
oscillator operates when used with RCLKIN values between
5.5 kΩ and 20 kΩ, choosing a value within the range of the
nominal value, as shown in Figure 27, is recommended.
490
480
Figure 26. Interfacing the ADE7769 to an MCU
As shown in Figure 26, the frequency output, CF, is connected
to an MCU counter or port. This counts the number of pulses
in a given integration time, which is determined by an MCU
internal timer. The average power proportional to the average
frequency is given by
470
460
450
440
430
Average Frequency = Average Power =
Counter
Time
(8)
420
410
The energy consumed during an integration period is given by
400
5.8
5.9
6.0
6.1
6.2
6.3
6.4
6.5
6.6
6.7
Energy = Average Power × Time =
Counter
Time
× Time = Counter (9)
RESISTANCE (k Ω)
Figure 27. Effect of RCLKIN on Internal Oscillator Frequency (OSC)
For the purpose of calibration, this integration time could be
10 seconds to 20 seconds to accumulate enough pulses to
ensure correct averaging of the frequency. In normal operation,
the integration time could be reduced to 1 or 2 seconds,
depending, for example, on the required update rate of a
display. With shorter integration times on the MCU, the
amount of energy in each update may still have some small
amount of ripple, even under steady load conditions. However,
over a minute or more the measured energy has no ripple.
Power Measurement Considerations
Calculating and displaying power information always has some
associated ripple, which depends on the integration period used
in the MCU to determine average power and also on the load.
For example, at light loads, the output frequency may be 10 Hz.
With an integration period of 2 seconds, only about 20 pulses
TRANSFER FUNCTION
Frequency Outputs F1 and F2
The ADE7769 calculates the product of two voltage signals
(on Channel V1 and Channel V2) and then low-pass filters this
product to extract real power information. This real power
information is then converted to a frequency. The frequency
information is output on F1 and F2 in the form of active low
pulses. The pulse rate at these outputs is relatively low, for
example, 0.175 Hz maximum for ac signals with S0 = S1 = 0
(see Table 6). This means that the frequency at these outputs is
generated from real power information accumulated over a
relatively long period of time. The result is an output frequency
that is proportional to the average real power. The averaging of
the real 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:
are counted. The possibility of missing one pulse always exists,
because the ADE7769 output frequency is running asynchro-
nously to the MCU timer. This results in a 1-in-20, or 5%, error
in the power measurement.
Freq =
where:
494 . 75 × V1 rms × V2 rms × F 1 ? 4
V REF 2
(10)
Freq is the output frequency on F1 and F2 (Hz).
V1 rms is the differential rms voltage signal on Channel V1 (V).
V2 rms is the differential rms voltage signal on Channel V2 (V).
V REF = is the reference voltage (2.45 V ± 200 mV) (V).
F 1–4 = are one of four possible frequencies selected by using
the S0 and S1logic inputs (see Table 5).
Rev. A | Page 14 of 20
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