ADE75 datasheet(49/148 Pages) AD

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Preliminary Technical Data

ADE75xx/ADE71xx

Rev. PrE | Page 49 of 148

sensor, the resulting magnitude and phase response should be a

flat gain over the frequency band of interest. The di/dt sensor

has a 20 dB/dec gain associated with it. It also generates signifi-

cant high frequency noise, therefore a more effective anti-

aliasing filter is needed to avoid noise due to aliasing—see the

Anti-aliasing Filter section.

When the digital integrator is switched off, the

ADE75xx/ADE71xx can be used directly with a conventional

current sensor such as a current transformer (CT) or with a low

resistance current shunt.

POWER QUALITY MEASUREMENTS

Zero-Crossing Detection

The ADE75xx/ADE71xx has a zero-crossing detection circuit

on the voltage channel. This zero crossing is used to produce an

external zero-crossing signal (ZX), and it is also used in the

calibration mode.

The zero-crossing is generated, by default, from the output of

LPF1. As explained in the following paragraph, this filter has a

low cut-off frequency and is intended for use for 50 and 60Hz

system. If needed this filter can be disabled to allow a higher

frequency signal to be detected or to limit the group delay of the

detection. If the voltage input fundamental frequency is below

60Hz and a time delay in ZX detection is acceptable, it is

recommended to enable LPF1. Enabling LPF1 will limit the

variability in the ZX detection by eliminating the high

frequency components.

Figure 29 shows how the zero-crossing signal is generated.

x1, x2, x4,

x8, x16

ADC 2

REFERENCE

LPF1

f–3dB = 63.7Hz

PGA2

{GAIN [7:5]}

ZERO

CROSS

HPF

MODE1[6]

43.24° @ 60Hz

1.0

0.73

LPF1

Figure 29. Zero-Crossing Detection on Voltage channel

The zero-crossing signal ZX is generated from the output of

LPF1 (bypassed or not). LPF1 has a single pole at 63.7 Hz (at

MCLK = 4.096 MHz). As a result, there is a phase lag between

the analog input signal V2 and the output of LPF1. The phase

lag response of LPF1 results in a time delay of approximately

2 ms (@ 60 Hz) between the zero crossing on the analog inputs

of the voltage channel and ZX detection.

The zero-crossing detection also drives the ZX flag in the

Interrupt Status Register 3 SFR (MIRQSTH, 0xDE). If the ZX

bit in the Interrupt Enable Register 3 SFR (MIRQENH, 0xDB) is

set, the 8052 core has a pending ADE interrupt.

The ADE interrupt stays active until the ZX status bit is

cleared—see Energy measurement interrupts section.

Zero-Crossing Timeout

The zero-crossing detection also has an associated timeout

(1 LSB) every 160/MCLK seconds. The register is reset to its

user programmed full-scale value every time a zero crossing is

detected on the voltage channel. The default power on value in

this register is 0xFFF. If the internal register decrements to 0

before a zero crossing is detected and the ZXTOUT flag in the

Interrupt Status Register 3 SFR (MIRQSTH, 0xDE) is set. If the

ZXTO bit in the Interrupt Enable Register 3 SFR (MIRQENH,

0xDB) is set, the 8052 core has a pending ADE interrupt.

The ADE interrupt stays active until the ZXTO status bit is

cleared—see Energy measurement interrupts section.

The ZXOUT register (Address 0x11) can be written or read by

the user—see Energy Measurement Register List. The resolution

of the register is 160/MCLK seconds per LSB. Thus the maxi-

mum delay for an interrupt is 0.16 second (128/MCLK × 212)

when MCLK = 4.096MHz.

Figure 30 shows the mechanism of the zero-crossing timeout

detection when the line voltage stays at a fixed dc level for more

than CLKIN/160 × ZXTOUT seconds.

12-BIT INTERNAL

ZXTOUT

VOLTAGE

CHANNEL

ZXTO

FLAG

BIT

Figure 30. Zero-Crossing Timeout Detection

Period or Frequency Measurements

The ADE75XX/ADE71XX provides the period or frequency

measurement of the line. The period or frequency measurement

is selected by clearing or setting FREQSEL bit in the MODE2

16-bit register and is updated every period. If LPF1 is enabled, a

settling time of 1.8 seconds is associated with this filter before

the measurement is stable.

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