ADE7753ARSZ datasheet(31/60 Pages) AD

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ADE7753

Rev. C | Page 31 of 60

0xFFFF,FFFF,FFFF before it overflows. The integration time

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

Time =

xCCCCD

FFFF

FFFF,

xFFFF,

× 1.12 μs = 375.8 s = 6.26 min(15)

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

time varies, as shown in Equation 16.

WDIV

Time

WDIV

(16)

POWER OFFSET CALIBRATION

The ADE7753 also incorporates an active power offset register

(APOS[15:0]). This is a signed twos complement 16-bit register

that can be used to remove offsets in the active power calculation—

see Figure 65. An offset could exist in the power calculation due

to crosstalk between channels on the PCB or in the IC itself.

The offset calibration allows the contents of the active power

The 256 LSBs (APOS = 0x0100) written to the active power

offset register are equivalent to 1 LSB in the waveform sample

0xCCCCD (838,861d) when inputs on Channels 1 and 2 are

both at full scale. At −60 dB down on Channel 1 (1/1000 of the

Channel 1 full-scale input), the average word value output from

LPF2 is 838.861 (838,861/1,000). One LSB in the LPF2 output

has a measurement error of 1/838.861 × 100% = 0.119% of the

average value. The active power offset register has a resolution

equal to 1/256 LSB of the waveform register, therefore the power

offset correction resolution is 0.00047%/LSB (0.119%/256) at –60 dB.

ENERGY-TO-FREQUENCY CONVERSION

ADE7753 also provides energy-to-frequency conversion for

calibration purposes. After initial calibration at manufacturing,

the manufacturer or end customer often verify the energy meter

calibration. One convenient way to verify the meter calibration

is for the manufacturer to provide an output frequency, which is

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 calibration

equipment. Figure 67 illustrates the energy-to-frequency

conversion in the ADE7753.

CFNUM[11:0]

CFDEN[11:0]

AENERGY[48:0]

02875-0-066

DFC

Figure 67. ADE7753 Energy-to-Frequency Conversion

A digital-to-frequency converter (DFC) is used to generate the

CF pulsed output. The DFC generates a pulse each time 1 LSB

in the active energy register is accumulated. An output pulse is

generated when (CFDEN + 1)/(CFNUM + 1) number of pulses

are generated at the DFC output. Under steady load conditions,

the output frequency is proportional to the active power.

The maximum output frequency, with ac input signals at full scale

and CFNUM = 0x00 and CFDEN = 0x00, is approximately 23 kHz.

The ADE7753 incorporates two registers, CFNUM[11:0] and

CFDEN[11:0], to set the CF frequency. These are unsigned

12-bit registers, which can be used to adjust the CF frequency to

a wide range of values. These frequency-scaling registers are

12-bit registers, which can scale the output frequency by 1/212 to

1 with a step of 1/212.

If the value 0 is written to any of these registers, the value 1

would be applied to the register. The ratio (CFNUM + 1)/

(CFDEN + 1) should be smaller than 1 to ensure proper

operation. If the ratio of the registers (CFNUM + 1)/(CFDEN + 1)

is greater than 1, the register values would be adjusted to a ratio

(CFNUM + 1)/(CFDEN + 1) of 1. For example, if the output

frequency is 1.562 kHz while the contents of CFDEN are 0

(0x000), then the output frequency can be set to 6.1 Hz by

writing 0xFF to the CFDEN register.

When CFNUM and CFDEN are both set to one, the CF pulse

width is fixed at 16 CLKIN/4 clock cycles, approximately 18 μs

with a CLKIN of 3.579545 MHz. If the CF pulse output is

longer than 180 ms for an active energy frequency of less than

5.56 Hz, the pulse width is fixed at 90 ms. Otherwise, the pulse

width is 50% of the duty cycle.

The output frequency has a slight ripple at a frequency equal to

twice the line frequency. This is due to imperfect filtering of the

instantaneous power signal to generate the active power signal—

see the Active Power Calculation section. Equation 9 from the

Active Power Calculation section gives an expression for the

instantaneous power signal. This is filtered by LPF2, which has

a magnitude response given by Equation 17.

)

(

(17)

The active power signal (output of LPF2) can be rewritten as

p(t) = VI −

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

× cos(4πfLt)

(18)

where fL is the line frequency, for example, 60 Hz.

From Equation 13,

E(t) = VIt −

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

× sin(4πfLt)

(19)

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