AD7723 datasheet(16/23 Pages) AD

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AD7723

–16–

REV. 0

effectively translates the wanted band to a maximum frequency

of a little less than fCLKIN/64 as shown in Figure 25d. Halving

the output data rate reduces the work load of any following

signal processor and also allows a lower serial clock rate to be

used.

BAND LIMITED SIGNAL

0dB

fCLKIN/16

0dB

ODR

LOW PASS FILTER RESPONSE

SAMPLE

IMAGE

fCLKIN/16

LOW PASS FILTER. OUTPUT DATA RATE = fCLKIN/16

0dB

fCLKIN/16

SAMPLE

IMAGE

BAND-PASS FILTER

RESPONSE

BAND-PASS FILTER.

fCLKIN/16

LOW PASS FILTER. OUTPUT DATA RATE = fCLKIN/32

ODR

SAMPLE

IMAGE

FREQUENCY

TRANSLATED

INPUT SIGNAL

0dB

(a)

(b)

(c)

(d)

Figure 25. Band-Pass Operation

The frequency response of the three digital filter operating modes

is shown in Figures 26a, 26b, and 26c.

fCLKIN

0.0

1.0

0.5

0dB

–100dB

Figure 26a. Low-Pass Filter Decimate by 16

fCLKIN

0.0

1.0

0.5

0dB

–100dB

Figure 26b. Low-Pass Filter Decimate by 32

fCLKIN

0.0

1.0

0.5

0dB

–100dB

Figure 26c. Band-Pass Filter Decimate by 32

Figure 27a 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 mul-

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

any of the filter images are aliased down to the passband. How-

ever, due to the AD7723’s high oversampling ratio, these bands

occupy only a small fraction of the spectrum, and most broad-

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

in Figure 27b, 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 band width to be used

while ensuring sufficient attenuation at multiples of fCLKIN.

1fCLKIN

2fCLKIN

3fCLKIN

0dB

Figure 27a. Digital Filter Frequency Response

fCLKIN/32

0dB

fCLKIN

REQUIRED

ATTENUATION

ANTIALIAS FILTER

RESPONSE

OUTPUT

DATA RATE

Figure 27b. Frequency Response of Antialias Filter

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