ADE5166_08 AD [Analog Devices], ADE5166_08 Datasheet - Page 44

• Manufacturer Part Number: ADE5166_08
• Description: Single-Phase Energy Measurement IC with 8052 MCU, RTC, and LCD Driver
• Manufacturer: AD [Analog Devices]

ADE5166/ADE5169
Antialiasing Filter
Figure 41 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, which 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 42 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 42, 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.
Figure 42. ADC and Signal Processing in Current Channel Outline Dimensions
0
I
I PA
I N
I BP
FREQUENCIES
2
×1, ×2, ×4,
×8, ×16
{GAIN[2:0]}
V P
V N
PGA1
PGA1
IMAGE
PGA2
ALIASING EFFECTS
ADC
ADC
FREQUENCY (kHz)
409.6
ADC
HPF
HPF
IBGAIN[11:0]
Figure 43. ADC and Signal Processing in Current Channel with PGA1 = 2, 4, 8, or 16
HPF
Ф
INTEGRATOR
819.2
dt
x
x
FREQUENCY
2
2
SAMPLING
MULTIPLIER
VRMSOS[11:0]
IRMSOS[11:0]
LPF
LPF
π
2
Rev. 0 | Page 44 of 148
LPF2
WATTOS[15:0]
LPF2
WGAIN[11:0]
VAROS[15:0]
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 42). 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 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 equiva-
lent 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 43 shows 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).
VADIV[7:0]
VARGAIN[11:0]
VAGAIN[11:0]
%
%
VARDIV[7:0]
METERING SFRs
%
WDIV[7:0]
DFC
DFC
CF1NUM[15:0]
CF1DEN[15:0]
CF2NUM[15:0]
CF2DEN[15:0]
CF1
CF2