ade7566 Analog Devices, Inc., ade7566 Datasheet - Page 43

• Manufacturer Part Number: ade7566
• Description: Single-phase Energy Measurement Ic With 8052 Mcu, Rtc, And Lcd Driver
• Manufacturer: Analog Devices, Inc.

Antialiasing Filter
Figure 41 also shows an analog low-pass filter (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 frequency below half the sampling rate. Figure 42
illustrates the effect. Frequency components (the black arrows)
above half the sampling frequency (also know 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 (near
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 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.
*WHEN DIGITAL INTEGRATOR IS ENABLED, FULL-SCALE OUTPUT DATA IS ATTENUATED
DEPENDING ON THE SIGNAL FREQUENCY BECAUSE THE INTEGRATOR HAS A –20dB/DECADE
FREQUENCY RESPONSE. WHEN DISABLED, THE OUTPUT IS NOT FURTHER ATTENUATED.
62.5mV, 31.3mV
0.25V, 0.125V,
I
0V
I
I
AP
N
V1
ANALOG
INPUT
RANGE
×1, ×2, ×4
×8, ×16
{GAIN[2:0]}
PGA1
REFERENCE
Figure 43. ADC and Signal Processing in Current Channel with PGA1 = 2, 4, 8, or 16
ADC
HPF
0xD70A3E
0x28F5C2
0x000000
Rev. 0 | Page 43 of 136
CURRENT CHANNEL
WAVEFORM
DATA RANGE
INTEGRATOR*
DIGITAL
MODE1[5]
ADC Transfer Function
Both ADCs in the 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 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 0.4 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).
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, 82.6 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).
Figure 42. ADC and Signal Processing in Current Channel Outline Dimensions
0
dt
0xD487B0
0x2B7850
0x000000
FREQUENCIES
2
IMAGE
CURRENT CHANNEL
WAVEFORM
DATA RANGE AFTER
INTEGRATOR (60Hz)
ALIASING EFFECTS
FREQUENCY (kHz)
60Hz
409.6
CURRENT RMS (I
CALCULATION
WAVEFORM SAMPLE
REGISTER
ACTIVE AND REACTIVE
POWER CALCULATION
50Hz
0xCBD330
0x342CD0
0x000000
ADE7566/ADE7569
rms
)
CURRENT CHANNEL
WAVEFORM
DATA RANGE AFTER
INTEGRATOR (50Hz)
819.2
FREQUENCY
SAMPLING