ade7753 Analog Devices, Inc., ade7753 Datasheet - Page 19

• Manufacturer Part Number: ade7753
• Description: Active And Apparent Energy Metering Ic With Di/dt Sensor Interface
• Manufacturer: Analog Devices, Inc.

sec. Equation 3 gives an expression for the instantaneous
power signal in an ac system.
v
where
p
p
The average power over an integral number of line cycles (n)
is given by the expression in Equation 4.
where T is the line cycle period.
P is referred to as the Active or Real Power. Note that the
active power is equal to the dc component of the instanta-
neous power signal p(t) in Equation 3 , i.e., VI. This is the
relationship used to calculate active power in the ADE7753.
The instantaneous power signal p(t) is generated by multiply-
ing the current and voltage signals. The dc component of the
instantaneous power signal is then extracted by LPF2 (Low
Pass Filter) to obtain the active power information. This
process is illustrated graphically in Figure 31.
REV. PrF 10/02
t i
P
t
t
t
nT
1
v
VI
2
2
V = rms voltage,
I = rms current.
19999Ah
CCCCDh
I
V
00000h
t
Figure 31 – Active Power Calculation
0
sin
nT
V. I.
sin
V
I
p t dt VI
VI
i
Current Signal - i(t)
t
Instantaneous
Power Signal
Current
i(t) = 2 I sin( t)
t
t
cos
Voltage Signal - v(t)
Voltage
v(t) = 2 V sin( t)
t
HPF
p(t) = V I-V I cos(2 t)
PRELIMINARY TECHNICAL DATA
MULTIPLIER
Active Real Power
Signal = V x I
Figure 33 – Active Power Signal Processing
19999Ah
Instantaneous
Power Signal - p(t)
000000h
24
LPF2
(1)
(2)
(3)
(4)
sgn
–19–
2
6
+
2
APOS[15:0]
5
Since LPF2 does not have an ideal “brick wall” frequency
response—see Figure 32, the Active Power signal will have
some ripple due to the instantaneous power signal. This
ripple is sinusoidal and has a frequency equal to twice the line
frequency. Since the ripple is sinusoidal in nature it will be
removed when the Active Power signal is integrated to
calculate Energy – see Energy Calculation.
Figure 33 shows the signal processing chain for the
ActivePower calculation in the ADE7753. As explained, the
Active Power is calculated by low pass filtering the instanta-
neous power signal. Note that for when reading the waveform
samples from the output of LPF2,
The gain of the Active Energy can be adjusted by using the
multiplier and Watt Gain register (WGAIN[11:0]). The
gain is adjusted by writing a 2’s complement 12-bit word to
the Watt Gain register. Below is the expression that shows
how the gain adjustment is related to the contents of the Watt
Gain register.
Output
For example when 7FFh is written to the Watt Gain register
the Power output is scaled up by 50%. 7FFh = 2047d,
2047/2
Complement) and power output is scaled by –50%.
Shown in Figure 34 is the maximum code (in hex) output
range for the Active Power signal (LPF2). Note that the
output range changes depending on the contents of the Watt
Gain register. The minimum output range is given when the
Watt Gain register contents are equal to 800h, and the
+
Σ
2
-6
WGAIN[11:0]
WGAIN
12
2
-7
Figure 32 —Frequency Response of LPF2
-12
-16
-20
-24
-4
-8
= 0.5. Similarly, 800h = -2048 Dec (signed 2’s
2
0
1.0Hz
-8
Active
3.0Hz
Power
24
32
Frequency
10Hz
For Waveform
Sampling
CCCCDh
For Energy
Accumulation
19999h
1
WGAIN
Active Power
Signal - P
2
12
30Hz
ADE7753
100Hz