ADE7756ARS datasheet(24/32 Pages) AD

24 / 32 page

REV. 0

ADE7756

–24–

reduce the ripple. Also, averaging the output frequency by using

a longer gate time for the counter will achieve the same results.

E(t)

VIt

fl(1 + 2

fl/8.9Hz

SIN(4

Figure 33. Output Frequency Ripple

ENERGY CALIBRATION

By using the on-chip zero-crossing detection on Channel 2 the

energy calibration can be greatly simplified and the time required

to calibrate the meter can be significantly reduced. To use the

zero-cross detection the ADE7756 is placed in calibration mode

by setting Bit 7 (CMODE) in the Mode register. In Calibration

Mode the ADE7756 accumulates the Active Power signal in the

Active Energy register for an integral number of half cycles, as

shown in Figure 34. The number of half-line cycles is specified

in the SAGCYC register. The ADE7756 can accumulate Active

Power for up to 255 half-cycles. Because the Active Power is

integrated on an integral number of line cycles, the sinusoidal

component is reduced to zero. This eliminates any ripple in the

energy calculation. Energy is calculated more accurately and in

a shorter time because integration period can be shortened. At

the end of an energy calibration cycle the SAG flag in the Inter-

rupt Status register is set; this will cause the

SAG output to go

active low. If the SAG enable bit in the Interrupt Enable register

is enabled, the

IRQ output will also go active low. Thus the IRQ

line can be used to signal the end of a calibration also. Another

calibration cycle will start as long as the CMODE bit in the

Mode register is set. Note that the result of the first calibration

is invalid and must be ignored. The result of all subsequent

calibration cycles is correct.

From Equations 5 and 11,

E t

VIdt

tdt

()

–

cos (

)

8 9

∫









∫

(13)

where n is an integer and T is the line cycle period.

Since the sinusoidal component is integrated over an integer

number of line cycles, its value is always zero.

Therefore:

E t

VIdt

()

= ∫

(14)

E t

VInt

()

(15)

APOS [11:0]

AENERGY[39:0}

ACTIVE POWER

SIGNAL – P

LPF2

WAVEFORM [23:0]

SAGCYC[7:0]

CCCDh

CALIBRATION

CONTROL

ZERO CROSS

DETECT

LPF1

00h

CHANNEL 2

ADC

FROM

MULTIPLIER

Figure 34. Energy Calculation in Calibration Mode

CALIBRATING THE ENERGY METER

Calculating the Average Active Power

When calibrating the ADE7756, the first step is to calibrate

the frequency on CF to some required meter constant, e.g.,

3200 imp/kWh.

In order to determine the output frequency on CF, the average

value of the Active Power signal (output of LPF2) must first be

determined. One convenient way to do this is to use the calibra-

tion mode. When the CMODE (Bit 7) bit in the Mode register

is set to a Logic 1, energy is accumulated over an integer num-

ber of half-line cycles as described in the last section.

Since the line frequency is fixed at, say, 60 Hz, and the number

of half-cycles of integration is specified, the total integration

time is given as:

260

number of half cycles

For 255 half-cycles this would give a total integration time of

2.125 seconds. This would mean the energy register was updated

2.125/1.1175

µs (4/CLKIN) times. The average output value of

LPF2 is given as:

Contents of AENERGY

at the end

Number of times AENERGY

was updated

[: ]

39 0

Or equivalently, in terms of contents of various ADE7756 regis-

ters and CLKIN and line frequencies (fl):

Average word LPF

AENERGY

SAGCYC

CLKIN

()

[: ]

39 0

××

(16)

where fl is the line frequency.

Calibrating the Frequency at CF

Once the average Active Power signal is calculated it can be used

to determine the frequency at CF before calibration. When the

frequency before calibration is known, the Calibration Frequency

Divider register (CFDIV) and the Active Power Gain register

(APGAIN) can be adjusted to produce the required frequency

on CF. In this example, a meter constant of 3200 imp/kWh is

chosen as an appropriate constant. This means that under a

steady load of 1 kW, the output frequency on CF would be,

Frequency CF

imp kWh

()

3200

3600

0 8888

min

sec

ADE7756ARS 数据表(PDF) 24 Page - Analog Devices

ADE7756ARS 数据表(HTML) 24 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