ADC3683 datasheet(26/70 Pages) TI1

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7.3.4.3 FS/4 Mixing with Real Output

In this mode, the output after complex decimation is mixed with FS/4 (FS = output data rate). Instead of a

complex output with the input signal centered around 0 Hz, the output is transmitted as a real output at twice the

data rate, and the signal is centered around FS/4 (Fout/4) as shown in Figure 7-21.

In this example, complex decimation by 8 is used. The output data is transmitted as a real output with an output

rate of Fout = FS'/4 (FS' = ADC sampling rate). The input signal is now centered around FS/4 (Fout/4) or FS'/16.

FS/2

FIN

FNCO

- FIN + FNCO

)6¶

FS/16

-FIN + FNCO + FS/4

)6¶

FS/8

Complex

Decimation /8

FS/4 mix

Fout/4 mix

Figure 7-21. FS/4 Mixing with Real Output

7.3.4.4 Numerically Controlled Oscillator (NCO) and Digital Mixer

The decimation block is equipped with a 32-bit NCO and a digital mixer to fine tune the frequency placement

prior to the digital filtering. The oscillator generates a complex exponential sequence of:

ejωn (default) or e–jωn

(1)

where: frequency (ω) is specified as a signed number by the 32-bit register setting

The complex exponential sequence is multiplied with the real input from the ADC to mix the desired carrier to

a frequency equal to fIN + fNCO. The NCO frequency can be tuned from –FS/2 to +FS/2 and is processed as a

signed, 2s complement number. After programming a new NCO frequency, the MIXER RESTART register bit or

SYNC pin has to be toggled for the new frequency to get active. Additionally the device provides the option via

SPI to invert the mixer phase.

The NCO frequency setting is set by the 32-bit register value given and calculated as:

NCO frequency = 0 to + FS/2: NCO = fNCO × 232 / FS

(2)

NCO frequency = -FS/2 to 0: NCO = (fNCO + FS) × 232 / FS

(3)

where:

• NCO = NCO register setting (decimal value)

• fNCO = Desired NCO frequency (MHz)

• FS = ADC sampling rate (MSPS)

The NCO programming is further illustrated with this example:

• ADC sampling rate FS = 65MSPS

• Input signal fIN = 10MHz

• Desired output frequency fOUT = 0MHz

ADC3683-SP

SBASAB5 – MARCH 2024

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