EVAL-ADE7816EBZ AD [Analog Devices], EVAL-ADE7816EBZ Datasheet - Page 20

• Manufacturer Part Number: EVAL-ADE7816EBZ
• Description: Six Current Channels, One Voltage Channel
• Manufacturer: AD [Analog Devices]

ADE7816
ENERGY MEASUREMENTS
This section describes the energy measurements available in
the ADE7816. For information about the theory behind these
measurements, refer to the AN-1137 Application Note.
STARTING AND STOPPING THE DSP
To obtain energy measurements, the internal processor must first
be started by setting the run register (Address 0xE228) to 0x0001.
It is recommended that all registers be initialized before starting
the DSP and that the last register in the queue be written three
times to flush the pipeline. When this procedure is complete, the
DSP should be started. There is no reason to stop the DSP, once
started, because all of the registers can be modified while the DSP
is running. The DSP can be stopped, however, by writing 0x0000
to the run register.
Within the DSP core, there is a two-stage pipeline. This means
that when a single register must be initialized, two or more writes
are required to ensure that the value has been written. If two or
more registers must be initialized, the last register must be written
two more times to ensure that the value is written into the RAM.
It is recommended that the last register be written three times to
ensure successful communication. See the Register Protection
section for details on protecting these registers.
ACTIVE ENERGY MEASUREMENT
Definition of Active Power and Active Energy
Active power is the product of voltage and current and is the
power dissipated in a purely resistive load. Active energy is the
accumulation of active power over time and is measured in watts.
The average power over an integral number of line cycles (n) is
given by the following expression:
where:
V is the rms voltage.
I is the rms current.
P is the active or real power.
T is the line cycle period.
Active Energy Registers
The
energy is accumulated for each of the six channels separately:
AWATTHR (Address 0xE400), BWATTHR (Address 0xE401),
ADE7816
P =
VA
IA
nT
1
PCF_A_COEFF
nT
∫
0
P
has six active energy registers, where the active
) (
t
dt
= VI
IAGAIN
VGAIN
HPF
HPF
INTEGRATOR
DIGITAL
Figure 28. Active Energy Signal Path
LPF
Rev. 0 | Page 20 of 48
AWATTOS
(1)
AWGAIN
CWATTHR (Address 0xE402), DWATTHR (Address 0xE403),
EWATTHR (Address 0xE404) and FWATTHR (Address 0xE405).
All active energy registers are in 32-bit, signed format. The
ADE7816
power indicates that the angle between the voltage and current is
greater than 90°, and power is being injected back into the grid.
The
positive power is added and negative power is subtracted.
Figure 28 shows the configurations of the active energy signal path.
Active Energy Threshold
The
The first step occurs internally, using the two threshold registers,
WTHR1 (Address 0x43AB) and WTHR0 (Address 0x43AC).
These registers make up the most significant and least significant
24 bits, respectively, of an internal threshold register that is used
to control the frequency at which the external xWATTHR registers
are updated. The WTHR1 and WTHR0 registers affect all six active
energy measurements. For standard operation, the WTHR1 regi-
ster should be set to 0x2 and the WTHR0 register set to 0x0.
Thus, the update rate of the xWATTHR registers is set to slightly
below the maximum of 8 kHz with full-scale inputs. If the rate at
which energy is accumulated in the xWATTHR registers must be
reduced, the WTHR1and WTHR0 registers can be modified.
Note that the maximum output with full scale inputs is 8 kHz.
Do not adjust the threshold to try to produce more than 8 kHz.
Such an adjustment may result in saturation of the output
frequency and, therefore, a loss of accuracy.
The second stage of the accumulation occurs in the external
registers, xWATTHR. With the recommended values provided
in Equation 2, the energy updates at a rate of 8 kHz with full-
scale inputs (see Figure 28).
Energy Accumulation and Register Roll-Over
As shown in Equation 2, the active energy accumulates at a maxi-
mum rate of 8 kHz with full-scale inputs. The maximum positive
value that the 32-bit, signed xWATTHR registers can store before
they overflow is 0x7FFFFFFF. Assuming steady accumulation
with full-scale inputs, the accumulation time is
ADE7816
ADE7816
Threshold = 0x2000000 ×
Time = 0x7FFFFFFF × 125 μs = 74 hr, 33 min, 55 sec
accumulates both positive and negative power. Negative
ACCUMULATOR
accumulates energy in two steps (see Figure 28).
provides a signed accumulation of the power;
WTHR[47:0]
Required
AWATTHR[31:0]
Update
8
REGISTER
kHz
32-BIT
Data Sheet
Rate
(kHz)
(2)