ADE7758 Analog Devices, ADE7758 Datasheet - Page 32

• Manufacturer Part Number: ADE7758
• Description: Poly Phase Multifunction Energy Metering IC with Per Phase Information
• Manufacturer: Analog Devices

ADE7758
The ADE7758 achieves the integration of the active power
signal by continuously accumulating the active power signal
in the internal 41-bit energy registers. The watt-hr registers
(AWATTHR, BWATTHR, and CWATTHR) represent the upper
16 bits of these internal registers. This discrete time accumulation
or summation is equivalent to integration in continuous time.
Equation 9 expresses the relationship
where n is the discrete time sample number and T is the sample
period.
Figure 66 shows a signal path of this energy accumulation. The
average active power signal is continuously added to the internal
active energy register. This addition is a signed operation.
Negative energy is subtracted from the active energy register.
Note the values shown in Figure 65 are the nominal full-scale
values, i.e., the voltage and current inputs at the corresponding
phase are at their full-scale input level. The average active power
is divided by the content of the watt divider register before it is
added to the corresponding watt-hr accumulation registers.
When the value in the WDIV[7:0] register is 0 or 1, active
power is accumulated without division. WDIV is an 8-bit
unsigned register that is useful to lengthen the time it takes
before the watt-hr accumulation registers overflow.
Figure 67 shows the energy accumulation for full-scale signals
(sinusoidal) on the analog inputs. The three displayed curves
show the minimum time it takes for the watt-hr accumulation
register to overflow when the watt gain register of the cor-
responding phase equals to 0x7FF, 0x000, and 0x800. The watt
gain registers are used to carry out a power calibration in the
ADE7758. As shown, the fastest integration time occurs when
the watt gain registers are set to maximum full scale, i.e., 0x7FF.
This is the time it takes before overflow can be scaled by writing
Energy
=
V
0xD7AE14
∫
0x2851EC
I
p
0xD7AE
0x2852
( )
CURRENT SIGNAL–i(t)
VOLTAGE SIGNAL–v(t)
0x00
000x
t
dt
HPF
=
Lim
T
→
0
INTEGRATOR
⎧
⎨
⎩
n
∑
∞
DIGITAL
=
0
p
( )
0xCCCCD
nT
0x00000
×
MULTIPLIER
T
⎫
⎬
⎭
Figure 66. ADE7758 Active Energy Accumulation
SIGN 2
AVERAGE POWER
SIGNAL–P
LPF2
T
6
WATTOS[11:0]
Rev. A | Page 32 of 68
+
(9)
2
0
TIME (nT)
+
2
–1
AWG[11:0]
2
–2
2
to the WDIV register and therefore can be increased by a
maximum factor of 255.
Note that the active energy register content can roll over to full-
scale negative (0x8000) and continue increasing in value when
the active power is positive (see Figure 66). Conversely, if the
active power is negative, the energy register would under flow
to full-scale positive (0x7FFF) and continue decreasing in value.
By setting the AEHF bit (Bit 0) of the interrupt mask register,
the ADE7758 can be configured to issue an interrupt ( IRQ )
when Bit 14 of any one of the three watt-hr accumulation
registers has changed, indicating that the accumulation register
is half full (positive or negative).
Setting the RSTREAD bit (Bit 6) of the LCYMODE register
enables a read-with-reset for the watt-hr accumulation registers,
i.e., the registers are reset to 0 after a read operation.
WDIV[7:0]
–3
ACCUMULATION REGISTER
%
2
CONTENTS OF WATTHR
–4
0x7FFF
0x3FFF
0xC000
0x0000
0x8000
Figure 67. Energy Register Roll-Over Time for Full-Scale Power
+
+
40
15
(Minimum and Maximum Power Gain)
WATTHR[15:0]
0.13
TOTAL ACTIVE POWER IS
ACCUMULATED (INTEGRATED) IN
THE ACTIVE ENERGY REGISTER
0.52
0.79
TIME (Sec)
0
1.05
1.31
WATT GAIN = 0x7FF
WATT GAIN = 0000
WATT GAIN = 0800
1.58
0