ADE7913 datasheet(19/44 Pages) AD

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Data Sheet

ADE7912/ADE7913

Rev. 0 | Page 19 of 44

Figure 26. Noise Reduction Due to Oversampling and

Noise Shaping in the Analog Modulator

The bandwidth of interest is a function of the input clock fre-

quency, the ADC output frequency (selectable by Bits[5:4]

(ADC_FREQ) in the CONFIG register; see the ADC Output

Values section for details), and Bit 7 (BW) of the CONFIG

frequency is 8 kHz, if BW is cleared to 0 (the default value)

the ADC bandwidth is 3.3 kHz. If BW is set to 1, the ADC

bandwidth is 2 kHz. Table 10 shows the ADC output frequen-

cies and the ADC bandwidth function of the input clock

(CLKIN) frequency. Three cases are shown: one for CLKIN =

4.096 MHz, the typical clock input frequency value, one for

CLKIN = 4.21 MHz, the maximum clock input frequency, and

one for CLKIN = 3.6 MHz, the minimum clock input

frequency.

Antialiasing Filter

Figure 25 also shows an analog low-pass filter (RC) on the input

to the ADC. This filter is placed outside the ADE7912/ADE7913,

and its role is to prevent aliasing. Aliasing is an artifact of all

sampled systems, as shown in Figure 27. Aliasing refers to the

frequency components in the input signal to the ADC that are

higher than half the sampling rate of the ADC and appear in the

sampled signal at a frequency below half the sampling rate.

Frequency components above half the sampling frequency (also

known as the Nyquist frequency, that is, 512 kHz) are imaged or

folded back down below 512 kHz. This happens with all ADCs,

regardless of the architecture. In Figure 27, only frequencies near

the sampling frequency of 1.024 MHz move into the bandwidth

of interest for metering, that is, 40 Hz to 3.3 kHz or 40 Hz to

2 kHz. To attenuate the high frequency noise (near 1.024 MHz)

and prevent the distortion of the bandwidth of interest, a low-

pass filer (LPF) must be introduced. It is recommended that one

RC filter with a corner frequency of 5 kHz be used for the

attenuation to be sufficiently high at the sampling frequency of

1.024 MHz. The 20 dB per decade attenuation of this filter is

usually sufficient to eliminate the effects of aliasing.

Figure 27. Aliasing Effects

Table 10. ADC Output Frequency and ADC Bandwidth as a Function of CLKIN Frequency

CLKIN

(MHz)

Bits ADC_FREQ in

CONFIG Register

ADC Output

Frequency (Hz)

ADC Bandwidth When Bit BW in

CONFIG Register Cleared to 0 (Hz)

ADC Bandwidth When Bit BW in

CONFIG Register Set to 1 (Hz)

4.096

8000

3300

2000

4000

1650

1000

2000

825

500

1000

412

250

4.21

8222

3391

2055

4111

1695

1027

2055

847

513

1027

423

256

3.6

7031

2900

1757

3515

1450

878

1757

725

439

878

362

219

NOISE

SIGNAL

NOISE

SIGNAL

3.3 4

512

FREQUENCY (kHz)

HIGH RESOLUTION

OUTPUT FROM

DIGITAL LPF

1024

3.3 4

512

FREQUENCY (kHz)

1024

DIGITAL FILTER

SHAPED NOISE

ANTIALIAS FILTER

(RC)

SAMPLING

FREQUENCY

ALIASING EFFECTS

SAMPLING

FREQUENCY

IMAGE

FREQUENCIES

512

FREQUENCY (kHz)

1024

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