ADE7816 datasheet(20/48 Pages) AD

20 / 48 page

ADE7816

Data Sheet

Rev. 0 | Page 20 of 48

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:

P =

∫

)

(

= VI

(1)

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 ADE7816 has six active energy registers, where the active

energy is accumulated for each of the six channels separately:

AWATTHR (Address 0xE400), BWATTHR (Address 0xE401),

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 accumulates both positive and negative power. Negative

power indicates that the angle between the voltage and current is

greater than 90°, and power is being injected back into the grid.

The ADE7816 provides a signed accumulation of the power;

positive power is added and negative power is subtracted.

Figure 28 shows the configurations of the active energy signal path.

Active Energy Threshold

The ADE7816 accumulates energy in two steps (see Figure 28).

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.

Threshold = 0x2000000 ×

(kHz)

kHz

Rate

Update

Required

(2)

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

Time = 0x7FFFFFFF × 125 μs = 74 hr, 33 min, 55 sec

VGAIN

HPF

IAGAIN

DIGITAL

INTEGRATOR

HPF

AWGAIN

AWATTOS

PCF_A_COEFF

ACCUMULATOR

WTHR[47:0]

AWATTHR[31:0]

32-BIT

LPF

Figure 28. Active Energy Signal Path

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