Datasheet

AD71056
Rev. A | Page 15 of 20
The energy consumed during an integration period is given by
CounterTime
Time
Counter
TimePowerAverageEnergy =×=×=
(9)
For the purpose of calibration, this integration time can be as
long as 10 seconds to 20 seconds to accumulate enough pulses
to ensure correct averaging of the frequency. In normal operation,
the integration time can be reduced to one or two 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 can 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 that depends on the load as well as the integration
period used in the MCU to determine average power. For
example, at light loads, the output frequency may be 10 Hz.
With an integration period of two seconds, only about 20 pulses
are counted. The possibility of missing one pulse always exists,
because the output frequency of the AD71056 is running
asynchronously to the MCU timer. This results in a 1-in-20, 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 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.
RESISTANCE (kΩ)
5.8 5.9 6.1 6.3 6.7
FREQUEN
C
Y (kHz)
420
430
440
450
460
480
470
490
6.0 6.2 6.4 6.5 6.6
410
400
05636-027
Figure 27. Effect of RCLKIN on Internal Oscillator Frequency (OSC)
TRANSFER FUNCTION
Frequency Outputs F1 and F2
The AD71056 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:
2
...
75.494
REF
41
rmsrms
V
fV2V1
Freq
×
×
×
=
(10)
where:
Freq = output frequency on F1 and F2 (Hz).
V1
rms
= differential rms voltage signal on Channel V1 (V).
V2
rms
= differential rms voltage signal on Channel V2 (V).
V
REF
= the reference voltage (2.45 V ±200 mV) (V).
f
1…4
= one of four possible frequencies selected by using
Logic Input S0 and Logic Input S1 (see Table 5).
Table 5. f
1…4
Frequency Selection
S1 S0 OSC Relation
1
f
1
…
4
at Nominal OSC (Hz)
2
0 0 OSC/2
19
0.86
0 1 OSC/2
18
1.72
1 0 OSC/2
17
3.43
1 1 OSC/2
16
6.86
1
f
1…4
is a binary fraction of the internal oscillator frequency (OSC).
2
Values are generated using the nominal frequency of 450 kHz.
Example
In this example, with ac voltages of ±30 mV peak applied to
V1 and ±165 mV peak applied to V2, the expected output
frequency is calculated as
f
1…4
= OSC/2
19
Hz, S0 = S1 = 0
V1
rms
= 0.03/√2 V
V2
rms
= 0.165/√2 V
V
REF
= 2.45 V (nominal reference value)
Note that if the on-chip reference is used, actual output
frequencies can vary from device to device due to the reference
tolerance of ±200 mV.
175.0204.0
45.222
165.003.075.494
2
=×=
××
×
×
×
=
1
1
f
f
Freq
(11)