ADE7756ARS datasheet(23/32 Pages) AD

23 / 32 page

REV. 0

ADE7756

–23–

POWER OFFSET CALIBRATION

The ADE7756 also incorporates an Active Power Offset regis-

ter (APOS[11:0]). This is a signed, two’s complement, 12-bit

calculation—see Figure 30. An offset may exist in the power

calculation due to crosstalk between channels on the PCB or

in the IC itself. The offset calibration will allow the contents

of the Active Power register to be maintained at zero when no

power is being consumed.

Sixteen LSBs (APOS = 010h) written to the Active Power Off-

set register are equivalent to 1 LSB in the Waveform Sample

in the Waveform Register is CCCDh (52,429 in Decimal) when

inputs on Channels 1 and 2 are both at full scale. At –60 dB

down on Channel 1 (1/1000 of the full-scale input), the average

word value outputs from LPF2 is 52.429 (52,429/1,000). 1 LSB

in the Waveform register has a measurement error of 1/52.429

100% = 1.9% of the average value. The Active Power Offset

(1.9%/16) at –60 dB.

ENERGY-TO-FREQUENCY CONVERSION

ADE7756 also provides energy-to-frequency conversion for

calibration purposes. After initial calibration at manufacture, the

manufacturer or end 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 that 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 equip-

ment. Figure 32 illustrates the Energy-to-Frequency conversion

in the ADE7756.

ACTIVE POWER

OFFSET

CALIBRATION

MSB TRANSITION

APOS [11:0]

ENERGY-TO-FREQUENCY

ACTIVE POWER

SIGNAL – P

LPF2

WAVEFORM [23:0]

CFDIV[11:0]

Figure 32. Energy-to-Frequency Conversion

The energy-to-frequency conversion is accomplished by accumu-

lating the Active power signal in a 24-bit register. An output

pulse is generated when there is a zero to one transition on the

MSB (most significant bit) of the register. Under steady load con-

ditions the output frequency is proportional to the Active Power.

The output frequency at CF, with full-scale ac signals on Chan-

nel 1 and Channel 2 and CFDIV = 000h and APGAIN = 000h,

is approximately 5.593 kHz. This can be calculated as follows:

with the Active Power Gain register set to 000h, the average

value of the instantaneous power signal (output of LPF2) is

CCCDh or 52,429 decimal. An output frequency is generated

on CF when the MSB in the Digital-to-Frequency register (24

bits) toggles, i.e., when the register accumulates 2

23. This means

the register is updated 2

23/CCCDh times (or 159.999 times). Since

the update rate is 4/CLKIN or 1.1175

µs, the time between

MSB toggles (CF pulses) is given as:

159.999

× 1.1175 µs = 1.78799 × 10–4s = 5592.86 Hz.

Equation 8 gives an expression for the output frequency at CF

with the CFDIV register = 0.

CF Hz

Average LPF Output

CLKIN

()

(8)

This output frequency is easily scaled by the Calibration Fre-

quency Division register (CFDIV[11:0]). This frequency scaling

to 2

12. The output frequency is given in Equation 9.

Frequency

Frequency CFDIV

CFDIV

()

(9)

For example, if the output frequency is 5.59286 kHz while the

content of CFDIV is zero (000h), the output frequency can be

set to 5.4618 Hz by writing 3FFh Hex (1023 Decimal) to the

CFDIV register. The power-up default value in CFDIV is 3Fh.

The output frequency will have 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 Active Power Calculation section. Equation 3 gives

an expression for the instantaneous power signal. This is

filtered by LPF2, which has a magnitude response given by

Equation 10.

|( )|

fHz

18 9

(10)

The Active Power signal (output of LPF2) can be rewritten as

()

–

cos (

)









×× ×

8 9

(11)

where fl is the line frequency (e.g., 60 Hz)

From Equation 6

VIt

()

–

sin (

)

××

()













××

8 9

(12)

From Equation 12 it can be seen that there is a small ripple in

the energy calculation due to a sin(2

ωt) component. This is

shown graphically in Figure 33. The Active Energy calculation

is shown by the dashed straight line and is equal to V

× I × t.

The sinusoidal ripple in the Active Energy calculation is also

shown. Since the average value of a sinusoid is zero, this ripple

will contribute nothing to the energy calculation over time. How-

ever, the ripple can be observed in the frequency output, especially

at higher output frequencies. The ripple will get larger as a

percentage of the frequency at larger loads and higher output

frequencies. The reason is simply that at higher output frequen-

cies the integration or averaging time in the energy-to-frequency

conversion process is shorter. As a consequence, some of the

sinusoidal ripple is observable in the frequency output. Choosing

a lower output frequency at CF for calibration can significantly

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