AD7755AAN Analog Devices Inc, AD7755AAN Datasheet - Page 10

• Manufacturer Part Number: AD7755AAN
• Manufacturer: Analog Devices Inc

Specifications of AD7755AAN

• Operating Temperature (max): 85C
• Operating Temperature (min): -40C
• Pin Count: 24
• Mounting: Through Hole
• Package Type: PDIP
• Screening Level: Industrial
• Lead Free Status / Rohs Status: Not Compliant

AD7755
THEORY OF OPERATION
The two ADCs digitize the voltage signals from the current and
voltage transducers. These ADCs are 16-bit second order
sigma-delta with an oversampling rate of 900 kHz. This analog
input structure greatly simplifies transducer interfacing by
providing a wide dynamic range for direct connection to the
transducer and also simplifying the antialiasing filter design. A
programmable gain stage in the current channel further facili-
tates easy transducer interfacing. A high pass filter in the current
channel removes any dc component from the current signal.
This eliminates any inaccuracies in the real power calculation
due to offsets in the voltage or current signals—see HPF and
Offset Effects section.
The real power calculation is derived from the instantaneous
power signal. The instantaneous power signal is generated by a
direct multiplication of the current and voltage signals. In order
to extract the real power component (i.e., the dc component),
the instantaneous power signal is low-pass filtered. Figure 20
illustrates the instantaneous real power signal and shows how the
real power information can be extracted by low-pass filtering the
instantaneous power signal. This scheme correctly calculates real
power for nonsinusoidal current and voltage waveforms at all
power factors. All signal processing is carried out in the digital
domain for superior stability over temperature and time.
The low frequency output of the AD7755 is generated by accu-
mulating this real power information. This low frequency inher-
ently means a long accumulation time between output pulses.
The output frequency is therefore proportional to the average
real power. This average real power information can, in turn, be
accumulated (e.g., by a counter) to generate real energy infor-
mation. Because of its high output frequency and hence shorter
integration time, the CF output is proportional to the instanta-
neous real power. This is useful for system calibration purposes
that would take place under steady load conditions.
Power Factor Considerations
The method used to extract the real power information from the
instantaneous power signal (i.e., by low-pass filtering) is still
valid even when the voltage and current signals are not in phase.
Figure 21 displays the unity power factor condition and a DPF
(Displacement Power Factor) = 0.5, i.e., current signal lagging
CH1
CH2
V I
V I
2
TIME
PGA
POWER SIGNAL – p(t)
INSTANTANEOUS
p(t) = i(t) v(t)
WHERE:
ADC
ADC
v(t) = V cos( t)
i(t) = I cos( t)
p(t) =
MULTIPLIER
V I
2
HPF
{
1+cos (2 t)}
LPF
INSTANTANEOUS REAL
V I
2
POWER SIGNAL
FREQUENCY
FREQUENCY
DIGITAL-TO-
DIGITAL-TO-
CF
F1
F2
the voltage by 60 . If we assume the voltage and current wave-
forms are sinusoidal, the real power component of the instanta-
neous power signal (i.e., the dc term) is given by
This is the correct real power calculation.
Nonsinusoidal Voltage and Current
The real power calculation method also holds true for nonsinu-
soidal current and voltage waveforms. All voltage and current
waveforms in practical applications will have some harmonic
content. Using the Fourier Transform, instantaneous voltage
and current waveforms can be expressed in terms of their har-
monic content.
where:
where:
V I
2
v(t)
V
Vh
and
i(t)
I
Ih
and
v t
i t
O
( )
h
h
( )
O
cos(60 )
V I
is the instantaneous current
is the dc component
is the rms value of current harmonic h
is the phase angle of the current harmonic.
is the instantaneous voltage
is the average value
is the rms value of voltage harmonic h
is the phase angle of the voltage harmonic.
2
I
0V
0V
V
O
O
CURRENT
VOLTAGE
VOLTAGE
2
2
INSTANTANEOUS
POWER SIGNAL
h
INSTANTANEOUS
POWER SIGNAL
V
h
0
2
0
Ih
Vh
I
sin(
60
sin(
cos (60 ).
h t
h t
INSTANTANEOUS
REAL POWER SIGNAL
CURRENT
INSTANTANEOUS
REAL POWER SIGNAL
h
)
h
)
(1)
(2)