AD7723 datasheet(20/33 Pages) AD

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AD7723

Rev. C | Page 19 of 32

Figure 32 shows the frequency response of the digital filter in

both low-pass and band-pass modes. Due to the sampling

nature of the converter, the pass-band response is repeated

about the input sampling frequency, fCLKIN, and at integer

multiples of fCLKIN. Out-of-band noise or signals coincident with

any of the filter images are aliased down to the pass band.

However, due to the AD7723’s high oversampling ratio, these

bands occupy only a small fraction of the spectrum, and most

broadband noise is attenuated by at least 90 dB. In addition, as

shown in Figure 33, with even a low-order filter, there is

significant attenuation at the first image frequency. This

contrasts with a normal Nyquist rate converter where a very

high-order antialias filter is required to allow most of the

bandwidth to be used while ensuring sufficient attenuation at

multiples of fCLKIN.

fCLKIN

1.0

0.5

0dB

–100dB

Figure 29. Low-Pass Filter Decimate by 16

1.0

0.5

0dB

–100dB

fCLKIN

Figure 30. Low-Pass Filter Decimate by 32

1.0

0.5

0dB

–100dB

fCLKIN

Figure 31. Band-Pass Filter Decimate by 32

fCLKIN

0dB

Figure 32. Digital Filter Frequency Response

fCLKIN/32

0dB

fCLKIN

REQUIRED

ATTENUATION

ANTIALIAS FILTER

RESPONSE

OUTPUT

DATA RATE

Figure 33. Frequency Response of Antialias Filter

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