ADE7754 datasheet(22/44 Pages) AD

22 / 44 page

REV. PrG 01/03

PRELIMINARY TECHNICAL DATA

ADE7754

– 22 –

ENERGY CALCULATION

As stated earlier, power is defined as the rate of energy flow.

This relationship can be expressed mathematically as

Equation 7.

(7)

Where P = Power and E = Energy.

Conversely Energy is given as the integral of Power.

E= Pdt

∫

(8)

The ADE7754 achieves the integration of the Active Power

signal by continuously accumulating the Active Power signal

in an internal non-readable 54-bit Energy register. The

Active Energy register (AENERGY[23:0]) represents the

upper 24 bits of this internal register. This discrete time

accumulation or summation is equivalent to integration in

continuous time. Equation 9 below expresses the relationship

Lim

p(t)

∫

∑

=×













→

∞

pnT

()

(9)

Where n is the discrete time sample number and T is the

sample period.

The discrete time sample period (T) for the accumulation

calculating the Energy, this integration removes any sinusoi-

dal component which may be in the Active Power signal.

Figure 26 shows a graphical representation of this discrete

time integration or accumulation. The Active Power signal

is continuously added to the internal Energy register. This

addition is a signed addition, therefore negative energy will

be subtracted from the Active Energy contents.

TOTAL ACTIVE POWER

00000h

26667h

time (nT)

TOTAL ACTIVE POWER ARE

ACCUMULATED (INTEGRATED) IN

THE ACTIVE ENERGY REGISTER

Active Power

Signal - P

AENERGY[23:0]

WDIV

Figure 25 –ADE7754 Active Energy calculation

The 54-bit of the internal Energy register are divided by

WDIV. If the value in the WDIV register is equal to 0 then

the internal Active Energy register is divided by 1. WDIV is

an 8-bit unsigned register. The upper 24-bit of the result of

the division are then available in the 24-bit Active Energy

the same internal Active energy register. They differ by the

the state in which they are leaving the internal Active energy

the RAENERGY register: Read and reset to zero the internal

Active Energy register. Only one operation is held when

reading the AENERGY register: read the internal Active

Energy register.

Figure 26 shows the energy accumulation for full scale

signals (sinusoidal) on the analog inputs. The three displayed

curves, illustrate the minimum time it takes the energy

contents are all equal to 3FFh, 000h and 800h. The Watt

Gain registers are used to carry out a power calibration in the

ADE7754. As shown, the fastest integration time occurs

when the Watt Gain registers are set to maximum full scale,

i.e., 3FFh.

00,0000h

7F,FFFFh

80,0000h

3F,FFFFh

40,0000h

AENERGY[23:0]

Time

(seconds)

AWG = BWG = CWG = 3FFh

176

264

132

220

AWG = BWG = CWG = 000h

AWG = BWG = CWG = 800h

Figure 26 –Energy register roll-over time for full-scale

power (Minimum & Maximum Power Gain)

Note that the Active Energy register contents roll over to full-

scale negative (80,0000h) and continue increasing in value

when the power or energy flow is positive -See Figure 26.

Conversely if the power is negative the energy register would

under flow to full scale positive (7F,FFFFh) and continue

decreasing in value.

By using the Interrupt Enable register, the ADE7754 can be

configured to issue an interrupt (IRQ) when the Active

Energy register is half full (positive or negative).

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