ADE7757 View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'ADE7757' PDF : 16 Pages View PDF
ADE7757
Interfacing the ADE7757 to a Microcontroller for Energy
Measurement
The easiest way to interface the ADE7757 to a microcontroller
is to use the CF high frequency output with the output frequency
scaling set to 2048 × F1, F2. This is done by setting SCF = 0
and S0 = S1 = 1 (see Table III). With full-scale ac signals on
the analog inputs, the output frequency on CF will be approxi-
mately 2.867 kHz. Figure 14 illustrates one scheme that could
be used to digitize the output frequency and carry out the neces-
sary averaging mentioned in the previous section.
CF
AVERAGE
FREQUENCY
FREQUENCY
RIPPLE
؎10%
ADE7757
CF
TIME
MCU
COUNTER
TIMER
Figure 14. Interfacing the ADE7757 to an MCU
As shown, the frequency output CF is connected to an MCU
counter or port. This will count 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
Average Frequency = Average Power = Counter
Time
The energy consumed during an integration period is given by
Energy = Average Power × Time = Counter × Time = Counter
Time
For the purpose of calibration, this integration time could be
10 seconds to 20 seconds in order to accumulate enough pulses to
ensure correct averaging of the frequency. In normal opera-
tion, the integration time could be reduced to one or two seconds,
depending, for example, on the required update rate of a dis-
play. 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 will have no ripple.
Power Measurement Considerations
Calculating and displaying power information will always have
some associated ripple that will depend 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 two seconds, only about
20 pulses will be counted. The possibility of missing one pulse
always exists as the ADE7757 output frequency is running
asynchronously to the MCU timer. This would result in a one-
in-twenty or 5% error in the power measurement.
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 15. 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 15,
is recommended.
490
480
470
460
450
440
430
420
410
400
5.8 5.9 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7
RESISTANCE – k⍀
Figure 15. Effect of RCLKIN on Internal Oscillator
Frequency (OSC)
TRANSFER FUNCTION
Frequency Outputs F1 and F2
The ADE7757 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, e.g.,
0.175 Hz maximum for ac signals with S0 = S1 = 0 (see Table II).
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 propor-
tional 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
Freq = 515.84 ×V 1rms ×V 2rms × F1–4
VREF 2
where
Freq = Output frequency on F1 and F2 (Hz).
V1rms = Differential rms voltage signal on Channel V1 (V).
V2rms = Differential rms voltage signal on Channel V2 (V).
VREF = The reference voltage (2.5 V ± 8%) (V).
F1-4 = One of four possible frequencies selected by using the
logic inputs S0 and S1—see Table I.
–12–
REV. A
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