ADE7759ARS datasheet(22/32 Pages) AD

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REV. 0

ADE7759

–22–

As shown, the fastest integration time will occur when the Active

Power Gain register is set to maximum full scale, i.e., 7FFh.

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

39 or 7F,FFFF,FFFFh. Therefore the

integration time under these conditions is calculated as follows:

Time

F FFFF FFFFh

CCCDh

onds

=×

1 1

11 53

sec

POWER OFFSET CALIBRATION

The ADE7759 also incorporates an Active Power Offset register

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

that can be used to remove offsets in the active power 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

Two hundred fifty-six LSBs (APOS = 0100h) written to the Active

Power Offset register are equivalent to 1 LSB in the Waveform

Sample register. Assuming the average value outputs from LPF2

to 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 Channel 1 (1/1000

of the Channel 1 full-scale input), the average word value outputs

from LPF2 is 52.429 (52,429/1,000). One LSB in the Waveform

× 100% = 1.9% of

the average value. The Active Power Offset register has a resolution

equal to 1/256 LSB of the Waveform register, hence the power

offset correction resolution 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 cali-

bration 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 calibration

is for the manufacturer to provide an output frequency that is pro-

portional to the energy or active power under steady load conditions.

This output frequency can provide a simple, single-wire, optically

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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