ADE7569_15 Datasheet, PDF (51/152 Pages) Analog Devices – Single-Phase Energy Measurement IC with 8052 MCU, RTC, and LCD Driver

ADE7116/ADE7156/ADE7166/ADE7169/ADE7566/ADE7569
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 com-
ponents 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
frequency 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
the example shown, 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 (low-pass filter) to attenuate high frequency (at
approximately 819.2 kHz) noise and prevents distortion in the
band of interest.
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.
ALIASING EFFECTS
IMAGE
FREQUENCIES
SAMPLING
FREQUENCY
0
2
409.6
FREQUENCY (kHz)
819.2
Figure 45. ADC and Signal Processing in Current Channel Outline Dimensions
ADC Transfer Function
Both ADCs in the ADE7116/ADE7156/ADE7166/ADE7169/
ADE7566/ADE7569 are designed to produce the same output
code for the same input signal level. With a full-scale signal on
the input of 0.4 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 0.4 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).
With the specified full-scale analog input signal of 0.4 V and
PGA1 = 1, the ADC produces an output code that is approximately
between 0x20C49B (+2,147,483d) and 0xDF3B65 (−2,147,483d).
For inputs of 0.25 V, 0.125 V, 62.5 mV, and 31.3 mV with PGA1 = 2,
4, 8, and 16, respectively, the ADC produces an output code that
is approximately between 0x28F5C2 (+2,684,354d) and 0xD70A3E
(−2,684,354d).
Rev. B | Page 51 of 152