ADE7759 Analog Devices, ADE7759 Datasheet - Page 21

• Manufacturer Part Number: ADE7759
• Description: Active Energy Metering IC with di/dt Sensor Interface
• Manufacturer: Analog Devices

Figure 34 shows the signal processing chain for the Active Power
calculation in the ADE7759. As explained, the Active Power is
calculated by low pass filtering the instantaneous power signal.
Shown in Figure 35 is the maximum code (hexadecimal) output
range for the Active Power signal (LPF2) when the digital inte-
grator is disabled. Note that when the integrator is enabled, the
output range changes depending on the input signal frequency.
Furthermore, the output range can also be changed by the Active
Power Gain register—see Channel 1 ADC section. The minimum
output range is given when the Active Power Gain register con-
tents are equal to 800h, and the maximum range is given by writing
7FFh to the Active Power Gain register. This can be used to calibrate
the Active Power (or Energy) calculation in the ADE7759.
1999Ah
CCCDh
00000h
V
I
VOLTAGE SIGNAL – v(t)
CURRENT SIGNAL – i(t)
VI
–12
–16
–20
–24
–4
–8
0
1
INSTANTANEOUS
POWER SIGNAL
CURRENT
i(t) = 2
MULTIPLIER
HPF
1
VOLTAGE
v(t) = 2
I
3
sin( t)
p(t) = V
V
FREQUENCY – Hz
1999Ah
1999Ah
INSTANTANEOUS POWER SIGNAL – p(t)
–40% TO +40% FS
sin( t)
20
00h
I – V
10
LPF2
I
cos(2 t)
CCCDh
ACTIVE POWER
ACTIVE REAL POWER
SIGNAL = V
SIGNAL – P
30
I
100
ENERGY CALCULATION
As stated earlier, power is defined as the rate of energy flow.
This relationship can be expressed mathematically as:
Where P = Power and E = Energy
Conversely, Energy is given as the integral of Power:
The AD7759 achieves the integration of the Active Power signal
by continuously accumulating the Active Power signal in the 40-
bit Active Energy register (ASENERGY[39:0]). This discrete
time accumulation or summation is equivalent to integration in
continuous time. Equation 7 expresses this relationship.
Where n is the discrete time sample number and T is the sample period.
The discrete time sample period (T) for the accumulation regis-
ter in the ADE7759 is 1.1 µs (4/CLKIN). As well as calculating
the Energy, this integration removes any sinusodial components
which may be in the Active Power signal.
Figure 36 shows a graphical representation of this discrete time
integration or accumulation. The Active Power signal in the
Waveform register is continuously added to the Active Energy
register. This addition is a signed addition; therefore negative
energy will be subtracted from the Active Energy contents.
As shown in Figure 36, the Active Power signal is accumulated
in a 40-bit signed register (AENERGY[39:0]). The Active
Power signal can be read from the Waveform register by setting
MODE[14:13] = 0, 0 and setting the WSMP bit (Bit 3) in the
Interrupt Enable register to 1. Like Channel 1 and Channel 2
waveform sampling modes, the waveform data is available at
sample rates of 27.9 kSPS, 14 kSPS, 7 kSPS, or 3.5 kSPS—see
Figure 24. Figure 37 shows this energy accumulation for full-
scale signals (sinusodial) on analog inputs. The three curves
displayed illustrate the minimum period of time it takes the
energy register to roll over when the Active Power Gain register
contents are 7FFh, 000h, and 800h. The Active Power Gain
register is used to carry out power calibration in the ADE7759.
P
E
E
=
= ∫
= ∫
dE
dt
Pdt
P t dt
ECCCDh
F999Ah
CCCDh
F3333h
13333h
00000h
( )
6666h
=
CHANNEL 1 (ACTIVE POWER)
CALIBRATION RANGE
Lim
T
000h
→
0
{APGAIN [11:0]}



n
∑
∞
=
0
7FFh
p nT
(
)
800h
×
T



+30% FS
–20% FS
+20% FS
+10% FS
–10% FS
–30% FS
ADE7759
POSITIVE
POWER
NEGATIVE
POWER
(5)
(6)
(7)