Datasheet

AD71056
Rev. A | Page 14 of 20
Digital-to-Frequency Conversion
As previously described, the digital output of the low-pass
filter after multiplication contains the real power information.
However, because this LPF is not an ideal brick wall filter
implementation, the output signal also contains attenuated
components at the line frequency and its harmonics—that is,
cos(hωt), where h = 1, 2, 3, . . . and so on.
The magnitude response of the filter is given by
()
2
2
45.4
1
1
f
fH
+
=
(7)
For a line frequency of 50 Hz, this gives an attenuation of
the 2ω (100 Hz) component of approximately 22 dB. The
dominating harmonic is twice the line frequency (2ω) due
to the instantaneous power calculation.
Figure 25 shows the instantaneous real power signal at the
output of the LPF that still contains a significant amount of
instantaneous power information, that is, cos(2ωt). This signal
is then passed to the digital-to-frequency converter where it is
integrated (accumulated) over time to produce an output
frequency. The accumulation of the signal suppresses or
averages out any non-dc components in the instantaneous real
power signal. The average value of a sinusoidal signal is zero.
Thus, the frequency generated by the AD71056 is proportional
to the average real power.
Figure 25 shows the digital-to-
frequency conversion for steady load conditions, that is,
constant voltage and current.
F1
F2
DIGITAL-TO-
FREQUENCY
CF
DIGITAL-TO-
FREQUENCY
MULTIPLIER
F1
TIME
CF
TIME
FREQUENCY FREQUENCY
V
I
0
FREQUENCY (RAD/s)
ω
2ω
COS (2ω)
ATTENUATED BY LPF
V×I
2
LPF TO EXTRACT
REAL POWER
(DC TERM)
INSTANTANEOUS REAL POWER SIGNAL
(FREQUENCY DOMAIN)
LPF
05636-025
Figure 25. Real Power-to-Frequency Conversion
Figure 25 shows that the frequency output CF varies over time,
even under steady load conditions. This frequency variation is
primarily due to the cos(2ωt) component in the instantaneous
real power signal. 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 real 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. Consequently,
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 to
remove any ripple. If CF is being used to measure energy, for
example in a microprocessor-based application, the CF output
should also be averaged to calculate power.
Because the F1 and F2 outputs operate at a much lower
frequency, a lot more averaging of the instantaneous real power
signal is carried out. The result is a greatly attenuated sinusoidal
content and a virtually ripple free frequency output.
Connecting to a Microcontroller for Energy
Measurement
The easiest way to interface the AD71056 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 7). With full-scale ac signals on the analog
inputs, the output frequency on CF is approximately 2.867 kHz.
Figure 26 illustrates one scheme to digitize the output frequency
and carry out the necessary averaging mentioned in the
Digital-
to-Frequency Conversion
section.
CF
TIME
±10%
FREQUENCY
RIPPLE
AVERAGE
FREQUENCY
AD71056
COUNTER
TIMER
MCU
CF
05636-026
Figure 26. Interfacing the AD71056 to an MCU
As shown, the frequency output CF is connected to an MCU
counter or port. This counts the number of pulses in a given
integration time that is determined by an MCU internal timer.
The average power proportional to the average frequency is
given by
Time
Counter
PowerAverageFrequencyAverage ==
(8)