ADC3641 datasheet(31/77 Pages) TI

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8.3.4.2.1 FS/4 Mixing with Real Output

In this mode, the output after complex decimation gets mixed with FS/4 (FS = output data rate in this case).

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 illustrated in Figure 8-22.

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 8-22. FS/4 Mixing with real output

8.3.4.3 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

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 f IN + 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 ADC364x 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

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

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 = 65 MSPS

• Input signal fIN = 10 MHz

• Desired output frequency fOUT = 0 MHz

For this example there are actually four ways to program the NCO and achieve the desired output frequency as

shown in Table 8-2.

Table 8-2. NCO value calculations example

Alias or negative image

fNCO

NCO Value

Mixer

Phase

Frequency translation for fOUT

fIN = –10 MHz

fNCO = 10 MHz

660764199

as is

fOUT = fIN + fNCO = –10 MHz +10 MHz = 0 MHz

fIN = 10 MHz

fNCO = –10 MHz

3634203097

fOUT = fIN + fNCO = 10 MHz + (–10 MHz) = 0 MHz

fIN = 10 MHz

fNCO = 10 MHz

660764199

inverted

fOUT = fIN – fNCO = 10 MHz – 10 MHz = 0 MHz

fIN = –10 MHz

fNCO = –10 MHz

3634203097

fOUT = fIN – fNCO = –10 MHz – (–10 MHz) = 0 MHz

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