ADC3641 datasheet(32/77 Pages) TI

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8.3.4.4 Decimation Filter

The ADC364x supports complex decimation by 2, 4, 8, 16 and 32 with a pass-band bandwidth of ~ 80% and a

stopband rejection of at least 85dB. Table 8-3 gives an overview of the pass-band bandwidth of the different

decimation settings with respect to ADC sampling rate FS. In real decimation mode the output bandwidth is half

of the complex bandwidth.

Table 8-3. Decimation Filter Summary and Maximum Available Output Bandwidth

REAL/COMPLEX

DECIMATION

SETTING N

OUTPUT RATE

OUTPUT

BANDWIDTH

OUTPUT RATE

(FS = 65 MSPS)

OUTPUT BANDWIDTH

(FS = 65 MSPS)

Complex

FS / 2 complex

0.8 × FS / 2

32.5 MSPS complex

26 MHz

FS / 4 complex

0.8 × FS / 4

16.25 MSPS complex

13 MHz

FS / 8 complex

0.8 × FS / 8

8.125 MSPS complex

6.5 MHz

FS / 16 complex

0.8 × FS / 16

4.0625 MSPS complex

3.25 MHz

FS / 32 complex

0.8 × FS / 32

2.03125 MSPS complex

1.625 MHz

Real

FS / 2 real

0.4 × FS / 2

32.5 MSPS

13 MHz

FS / 4 real

0.4 × FS / 4

16.25 MSPS

6.5 MHz

FS / 8 real

0.4 × FS / 8

8.125 MSPS

3.25 MHz

FS / 16 real

0.4 × FS / 16

4.0625 MSPS

1.625 MHz

FS / 32 real

0.4 × FS / 32

2.03125 MSPS

0.8125 MHz

The decimation filter responses normalized tot he ADC sampling clock frequency are illustrated in . They are

interpreted as follows:

Each figure contains the filter pass-band, transition band(s) and alias or stop-band(s) as shown in Figure 8-24 to

Figure 8-33. The x-axis shows the offset frequency (after the NCO frequency shift) normalized to the ADC

sampling rate FS.

For example, in the divide-by-4 complex setup, the output data rate is FS / 4 complex with a Nyquist zone of FS /

8 or 0.125 × FS. The transition band is centered around 0.125 × FS and the alias transition band is centered at

0.375 × FS (colored in blue). The stop-bands, which alias on top of the pass-band, are centered at 0.25 × FS and

0.5 × FS and are colored in red. The stop-band attenuation is greater than 85 dB.

-120

-100

-80

-60

-40

-20

0.1

0.2

0.3

0.4

0.5

Normalized Frequency (Fs)

Passband

Transition Band

Alias Band

Attn Spec

Filter

Transition

Bands

Bands that alias on top

of signal band

Pass Band

Figure 8-23. Interpretation of the Decimation Filter Plots

ADC3643

SBAS886 – OCTOBER 2020

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