ADE7759_15 datasheet(22/36 Pages) AD

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REV. A

ADE7759

–22–

7F,FFFF,FFFFh

3F,FFFF,FFFFh

00,0000,0000h

40,0000,0000

80,0000,0000h

AENERGY [39:0]

5.8s

11.5s

23s

TIME – sec

APGAIN = 7FFh

APGAIN = 000h

APGAIN = 800h

Figure 37. Energy Register Rollover 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 37. 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 ADE7759

can be configured to issue an interrupt (

IRQ) when the active

energy register is half-full (positive or negative) or when an

over/underflow occurs.

Integration Time 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, digital

integrator turned off, and the active power gain register set to

000h, the average word value from LPF2 is CCCD—see

Figures 34 and 35. The maximum value that can be stored in

the active energy register before it overflows is 2

39 or

7F,FFFF,FFFFh. Therefore, the integration time under these

conditions is calculated as follows:

Time

F FFFF FFFFh

CCCDh

onds

=×

11 53

sec

POWER OFFSET CALIBRATION

The ADE7759 also incorporates an active power offset regis-

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

calculation—see Figure 36. 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.

The 256 LSBs (APOS = 0100h) written to the active power

offset register are equivalent to 1 LSB in the waveform sample

store in the waveform register is CCCDh (52,429 in decimal)

when inputs on Channels 1 and 2 are both at full scale and

the digital integrator is turned off. At –60 dB down on Chan-

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

word value output from LPF2 is 52.429 (52,429/1,000). One

LSB in the waveform register has a measurement error of

1/52.429

× 100% = 1.9% of the average value. The active

power offset register has a resolution equal to 1/256 LSB of

the waveform register, thus the power offset correction reso-

lution is 0.007%/LSB (1.9%/256) at –60 dB. When the digital

integrator is turned on, the resolution of the LSB varies slightly

with the line frequency.

ENERGY-TO-FREQUENCY CONVERSION

ADE7759 also provides energy-to-frequency conversion for

calibration purposes. After initial calibration at manufacturing,

the manufacturer or end customer will often verify the energy

meter calibration. One convenient way to verify the meter cali-

bration is for the manufacturer to provide an output frequency

that is proportional to the energy or active power under steady

SIGN 26

2–1 2–2 2–3 2–4 2–5 2–6 2–7 2–8

APOS [15:0]

LPF2

CURRENT CHANNEL

VOLTAGE CHANNEL

TIME – nT

CLKIN

WAVEFORM

VALUES

ACTIVE POWER

SIGNAL = P

WAVEFORM [24:0]

AENERGY [39:0]

WAVEFORM REGISTER VALUES ARE

ACCUMULATED (INTEGRATED) IN

THE ACTIVE ENERGY REGISTER

Figure 36. Energy Calculation

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