ADE7756ARS datasheet(22/32 Pages) AD

22 / 32 page

REV. 0

ADE7756

–22–

ENERGY CALCULATION

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

This relationship can be expressed mathematically as Equation 5.

(5)

where

P = Power, and

E = Energy.

Conversely, Energy is given as the integral of Power.

Pdt

=∫

(6)

The ADE7756 achieves the integration of the Active Power signal

by continuously accumulating the Active Power signal in the 40-bit

Active Energy register (AENERGY[39:0]). This discrete time

accumulation or summation is equivalent to integration in con-

tinuous time. Equation 7 expresses the relationship

p t dt

Lim

p nT

= ∫

=×

∑













→

∞

()

(

)

(7)

where

n is the discrete time sample number

and

T is the sample period.

The discrete time sample period (T) for the accumulation regis-

ter in the ADE7756 is 1.1

µs (4/CLKIN). As well as calculating

the Energy, this integration removes any sinusoidal components

that may be in the Active Power signal.

Figure 30 shows a graphical representation of this discrete time

integration or accumulation. The Active Power signal in the

Waveform register is continuously added to the Active Energy

energy will be subtracted from the Active Energy contents.

APOS [11:0]

CLKIN

TIME – nT

WAVEFOR

VALUES

AENERGY[39:0]

WAVEFORM REGISTER VALUES ARE

ACCUMULATED (INTEGRATED) IN

THE ACTIVE ENERGY REGISTER

ACTIVE POWER

SIGNAL – P

LPF2

CURRENT

CHANNEL

VOLTAGE

CHANNEL

WAVEFORM [23:0]

Figure 30. Energy Calculation

As shown in Figure 30, the Active Power signal is accumulated

in a 40-bit signed register (AENERGY[39:0]).

The Active Power signal can be read from the Waveform regis-

ter by setting MODE[14:13] = 0,0 and setting the WSMP bit

(Bit 3) in the Interrupt Enable register to 1. Like the Channel 1

and Channel 2 waveform sampling modes, the waveform date

is available at sample rates of 27.9 kSPS, 14 kSPS, 7 kSPS or

3.5 kSPS—see Figure 19.

Figure 31 shows this energy accumulation for full-scale signals

(sinusoidal) on the analog inputs. The three curves displayed,

illustrate the minimum period of time it takes the energy register

to roll-over when the Active Power Gain register contents are

3FFh, 000h and 800h. The Active Power Gain register is used

to carry out power calibration in the ADE7756. As shown, the

fastest integration time will occur when the Active Power Gain

APGAIN = 800h

APGAIN = 000h

APGAIN = 3FFh

00,0000,0000h

7F,FFFF,FFFFh

80,0000,0000h

3F,FFFF,FFFFh

40,0000,0000h

AENERGY[39:0]

TIME

(SECONDS)

11.5s

23s

5.8s

Figure 31. Energy Register Roll-Over Time for Full-Scale

Power (Minimum and Maximum Power Gain)

Note that the energy register contents will roll over to full-scale

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

the power or energy flow is positive—see Figure 31. Conversely,

if the power is negative the energy register would underflow to

full-scale positive (7F, FFFF, FFFFh) and continue decreasing

in value.

By using the Interrupt Enable register, the ADE7756 can be

configured to issue an interrupt (

IRQ) when the Active Energy

flow occurs.

Integration Times under Steady Load

As mentioned in the last section, the discrete time sample

period (T) for the accumulation register is 1.1

µs (4/CLKIN).

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

Active Power Gain register set to 000h, the average word value

from LPF2 is CCCDh—see Figure 34. The maximum value

that can be stored in the Active Energy register before it over-

flows is 2

39 or 7F, FFFF, FFFFh. Therefore the integration

time under these conditions is calculated as follows:

Time

F FFFF FFFFh

CCCDh

=×

1 1

11 53

seconds

ADE7756ARS 数据表(PDF) 22 Page - Analog Devices

ADE7756ARS 数据表(HTML) 22 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

数据表下载