ADC3683 datasheet(25/70 Pages) TI1

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7.3.4.2 Digital Filter Operation

The complex decimation operation is illustrated with an example in Figure 7-19. First, the input signal (and

the negative image) are frequency shifted by the NCO frequency as shown on the left side of the image.

Next, a digital filter is applied (centered around 0 Hz) and the output data rate is decimated by 8 complex,

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, so the amplitude is reduced by 6dB. To

compensate this loss, there is a 6dB 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 7-19. Complex decimation illustration

The real decimation operation is shown in Figure 7-20. 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 results 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 3dB. To compensate this loss,

there is a 3dB 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 7-20. Real decimation illustration

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ADC3683-SP

SBASAB5 – MARCH 2024

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