ADC3641 datasheet(30/77 Pages) TI

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8.3.4.2 Digital Filter Operation

The complex decimation operation is illustrated with an example in Figure 8-20. First the input signal (and the

negative image) are frequency shifted by the NCO frequency as shown on the left. Next a digital filter is applied

(centered around 0 Hz) and the output data rate is decimated - in this example the output data rate FS,OUT = FS/8

with a Nyquist zone of FS/16. During the complex mixing the spectrum (signal and noise) is split into real and

complex parts and thus the amplitude is reduced by 6-dB. In order to compensate this loss, there is a 6-dB

digital gain option in the decimation filter block that can be enabled via SPI write.

FS/2

-FS/2

Shifted Input

Signal (Alias)

Shifted Input Signal

Negative Image

Decimation

by 8

FS/2

-FS/2

Input Signal

(Alias)

Input Signal

Negative Image

FNCO

NCO Tuning Range

-FIN + FNCO

FIN + FNCO

FS/16

-FS/16

Figure 8-20. Complex decimation illustration

The real decimation operation is illustrated with an example in Figure 8-21. There is no frequency shift

happening and only the real portion of the complex digital filter is exercised. The output data rate is decimated -

a decimation of 8 would result in an output data rate FS,OUT = FS/8 with a Nyquist zone of FS/16.

During the real mixing the spectrum (signal and noise) amplitude is reduced by 3-dB. In order to compensate this

loss, there is a 3-dB digital gain option in the decimation filter block that can be enabled via SPI write.

FS/2

FS/4

FS/8

FS/16

FS/32

Decimation by 2

Decimation by 4

Decimation by 8

Decimation by

FS/64

Input Signal

Figure 8-21. Real decimation illustration

ADC3643

SBAS886 – OCTOBER 2020

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