ADE7754_15 datasheet(20/44 Pages) AD

20 / 44 page

REV. 0

–20–

ADE7754

The discrete time sample period (T) for the accumulation

µs (4/10 MHz). In addition to

calculating the energy, this integration removes any sinusoidal

component that may be in the active power signal. Figure 27

shows a graphical representation of this discrete time integration

or accumulation. The active power signal is continuously added

to the internal energy register. Because this addition is a signed

addition, negative energy will be subtracted from the active

energy contents.

TOTAL ACTIVE POWER

00000h

26667h

TIME (nT)

TOTAL ACTIVE POWER IS

ACCUMULATED (INTEGRATED) IN

THE ACTIVE ENERGY REGISTER

ACTIVE POWER

SIGNAL (P)

AENERGY[23:0]

WDIV

Figure 27. Active Energy Calculation

The 54-bit value of the internal energy register is divided by

WDIV. If the value in the WDIV register is 0, then the internal

active energy register is divided by 1. WDIV is an 8-bit unsigned

available in the 24-bit active energy register. The AENERGY

and RAENERGY registers read the same internal active energy

internal active energy register after a read. Two operations are

held when reading the RAENERGY register: read and reset to 0

the internal active energy register. Only one operation is held

when reading the AENERGY register: read the internal active

energy register.

Figure 28 shows the energy accumulation for full-scale (sinusoidal)

signals on the analog inputs. The three displayed curves illustrate

the minimum time it takes the energy register to roll over when

the individual watt gain registers 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 (sec)

AWG = BWG = CWG = 3FFh

176

264

132

AWG = BWG = CWG = 000h

AWG = BWG = CWG = 800h

220

Figure 28. Energy Register Roll-Over Time for Full-

Scale Power (Minimum and 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 28.

Conversely, if the power is negative, the energy register would

underflow 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

Integration Times Under Steady Load

As mentioned in the last section, the discrete time sample

period (T) for the accumulation register is 0.4

µs (4/CLKIN).

With full-scale sinusoidal signals on the analog inputs and the

watt gain registers set to 000h, the average word value from

each LPF2 is D1B717h. See Figures 22 and 24. The maximum

value that can be stored in the active energy register before it

overflows is 2

23 – 1 or 7F,FFFFh. As the average word value is

added to the internal register, which can store 2

53 – 1 or

1F,FFFF,FFFF,FFFFh before it overflows, the integration time

under these conditions with WDIV = 0 is calculated as follows:

Time

F FFFF FFFF FFFFh

×=

31 717

,,,

When WDIV is set to a value different from 0, the integration

time varies as shown in Equation 10.

Time

WDIV

=×

(10)

The WDIV register can be used to increase the time before the

active energy register overflows, thereby reducing the communi-

cation needs with the ADE7754.

Energy to Frequency Conversion

The ADE7754 also provides energy-to-frequency conversion

for calibration purposes. After initial calibration at manufac-

ture, the manufacturer or the customer will often verify the

energy meter calibration. One convenient way to verify the

meter calibration is for the manufacturer to provide an output

frequency proportional to the energy or active power under

steady load conditions. This output frequency can provide a

simple single-wire, optically isolated interface to external cali-

bration equipment. Figure 29 illustrates the energy to frequency

conversion in the ADE7754.

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