Datasheet

Data Sheet ADE5166/ADE5169/ADE5566/ADE5569
Rev. D | Page 49 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.
409.60 819.22
FREQUENCY (kHz)
SAMPLING
FREQUENCY
ALIASING EFFECTS
IMAGE
FREQUENCIES
07411-022
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).