ADE7758 datasheet(22/68 Pages) AD

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ADE7758

Rev. A | Page 22 of 68

Note that the integrator has a −20 dB/dec attenuation and

approximately −90° phase shift. When combined with a di/dt

sensor, the resulting magnitude and phase response should be a

flat gain over the frequency band of interest. However, the di/dt

sensor has a 20 dB/dec gain associated with it and generates

significant high frequency noise. A more effective antialiasing filter

is needed to avoid noise due to aliasing (see the Theory of

Operation section).

When the digital integrator is switched off, the ADE7758 can be

used directly with a conventional current sensor, such as a

current transformer (CT) or a low resistance current shunt.

PEAK CURRENT DETECTION

The ADE7758 can be programmed to record the peak of the

current waveform and produce an interrupt if the current

exceeds a preset limit.

Peak Current Detection Using the PEAK Register

The peak absolute value of the current waveform within a

fixed number of half-line cycles is stored in the IPEAK

peak current detection.

CONTENT OF

IPEAK[7:0]

NO. OF HALF

LINE CYCLES

SPECIFIED BY

LINECYC[15:0]

CURRENT WAVEFORM

(PHASE SELECTED BY

PEAKSEL[2:0] IN

MMODE REGISTER)

Figure 47. Peak Current Detection Using the IPEAK Register

Note that the content of the IPEAK register is equivalent to Bit 14

to Bit 21 of the current waveform sample. At full-scale analog

input, the current waveform sample is 0x2851EC . The IPEAK

at full-scale input is therefore expected to be 0xA1.

In addition, multiple phases can be activated for the peak

detection simultaneously by setting multiple bits to logic high

among the PEAKSEL[2:4] bits in the MMODE register. These

bits select the phase for both voltage and current peak measure-

ments. Note that if more than one bit is set, the VPEAK and

IPEAK registers can hold values from two different phases, that

is, the voltage and current peak are independently processed

(see the Peak Current Detection section).

Note that the number of half-line cycles is based on counting

the zero crossing of the voltage channel. The ZXSEL[2:0] bits in

the LCYCMODE register determine which voltage channels are

used for the zero-crossing detection. The same signal is also

used for line cycle energy accumulation mode if activated (see

the Line Cycle Accumulation Mode Register (0x17) section).

OVERCURRENT DETECTION INTERRUPT

Figure 48 illustrates the behavior of the overcurrent detection.

IPINTLVL[7:0]

READ RSTATUS

PKI INTERRUPT FLAG

(BIT 15 OF STATUS

PKI RESET LOW

WHEN RSTATUS

CURRENT PEAK WAVEFORM BEING MONITORED

(SELECTED BY PKIRQSEL[2:0] IN MMODE REGISTER)

Figure 48. ADE7758 Overcurrent Detection

Note that the content of the IPINTLVL[7:0] register is equivalent

to Bit 14 to Bit 21 of the current waveform sample. Therefore,

setting this register to A1 (hex) represents putting peak detection

at full-scale analog input. Figure 48 shows a current exceeding a

threshold. The overcurrent event is recorded by setting the PKI

flag (Bit 15) in the interrupt status register. If the PKI enable bit

is set to Logic 1 in the interrupt mask register, the IRQ logic

output goes active low (see the ADE7758 Interrupts section).

Similar to peak level detection, multiple phases can be activated

for peak detection. If any of the active phase produces waveform

samples above the threshold, the PKI flag in the interrupt status

set by the PKIRQSEL[2:0] bits in the MMODE register (see

Table 15).

VOLTAGE CHANNEL ADC

Figure 49 shows the ADC and signal processing chain for the

input VA in the voltage channel (same for VB and VC).

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