ADE7569_15 datasheet(51/152 Pages) AD

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ADE7116/ADE7156/ADE7166/ADE7169/ADE7566/ADE7569

Rev. B | Page 51 of

152

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.

409.6

819.2

FREQUENCY (kHz)

SAMPLING

FREQUENCY

ALIASING EFFECTS

IMAGE

FREQUENCIES

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).

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