Datasheet

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, f
CLKIN
, and at integer
multiples of f
CLKIN
. 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 f
CLKIN
.
f
CLKIN
0 1.00.5
0dB
–100dB
01186-029
Figure 29. Low-Pass Filter Decimate by 16
0 1.00.5
0dB
–
100dB
01186-030
f
CLKIN
Figure 30. Low-Pass Filter Decimate by 32
0 1.00.5
0dB
–100dB
01186-031
f
CLKIN
Figure 31. Band-Pass Filter Decimate by 32
1f
CLKIN
2f
CLKIN
3f
CLKIN
0dB
01186-032
Figure 32. Digital Filter Frequency Response
f
CLKIN
/32
0dB
f
CLKIN
REQUIRED
ATTENUATION
ANTIALIAS FILTER
RESPONSE
OUTPUT
DATA RATE
01186-033
Figure 33. Frequency Response of Antialias Filter