ADE5166 datasheet(48/156 Pages) AD

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ADE5166/ADE5169/ADE5566/ADE5569

Rev. B | Page 48 of 156

Antialiasing Filter

Figure 44 also shows an analog LPF (RC) on the input to the

modulator. This filter is present to prevent aliasing, an artifact

of all sampled systems. Aliasing means that frequency components

in the input signal to the ADC that are higher than half the

sampling rate of the ADC appear in the sampled signal at a fre-

quency below half the sampling rate. Figure 45 illustrates the effect.

Frequency components (the black arrows) above half the sampling

frequency (also known as the Nyquist frequency, that is, 409.6 kHz)

are imaged or folded back down below 409.6 kHz. This happens

with all ADCs, regardless of the architecture. In Figure 45, only

frequencies near the sampling frequency (819.2 kHz) move into

the band of interest for metering (40 Hz to 2 kHz). This allows

the use of a very simple LPF to attenuate high frequency (at

approximately 819.2 kHz) noise and prevents distortion in the

band of interest.

409.6

819.2

FREQUENCY (kHz)

SAMPLING

FREQUENCY

ALIASING EFFECTS

IMAGE

FREQUENCIES

Figure 45. ADC and Signal Processing in Current Channel Outline Dimensions

For conventional current sensors, a simple RC filter (single-pole

LPF) with a corner frequency of 10 kHz produces an attenuation

of approximately 40 dB at 819.2 kHz (see Figure 45). The 20 dB

per decade attenuation is usually sufficient to eliminate the effects

of aliasing for conventional current sensors. However, for a di/dt

sensor such as a Rogowski coil, the sensor has a 20 dB per decade

gain. This neutralizes the −20 dB per decade attenuation produced

by one simple LPF. Therefore, when using a di/dt sensor, care

should be taken to offset the 20 dB per decade gain. One simple

approach is to cascade two RC filters to produce the −40 dB per

decade attenuation needed.

ADC Transfer Function

Both ADCs in the ADE5166/ADE5169/ADE5566/ADE5569 are

designed to produce the same output code for the same input

signal level. With a full-scale signal on the input of 0.5 V and an

internal reference of 1.2 V, the ADC output code is nominally

2,147,483 or 0x20C49B. The maximum code from the ADC is

±4,194,304; this is equivalent to an input signal level of ±0.794 V.

However, for specified performance, it is recommended that the

full-scale input signal level of 0.5 V not be exceeded.

Current Channel ADC

Figure 46 and Figure 47 show the ADC and signal processing

chain for the current channel. In waveform sampling mode, the

ADC outputs a signed, twos complement, 24-bit data-word at a

maximum of 25.6 kSPS (4.096 MHz/160).

ADE5166 数据表(PDF) 48 Page - Analog Devices

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