AD6673 datasheet(24/45 Pages) AD

24 / 45 page

AD6673

Data Sheet

Rev. C | Page 24 of 45

NOISE SHAPING REQUANTIZER

The AD6673 features a NSR to allow higher than 11-bit SNR to

be maintained in a subset of the Nyquist band. The harmonic

performance of the receiver is unaffected by the NSR feature. When

enabled, the NSR contributes an additional 0.6 dB of loss to the

input signal, such that a 0 dBFS input is reduced to −0.6 dBFS at

the output pins.

The NSR feature can be independently controlled per channel

via the SPI.

Two different bandwidth modes are provided; the mode can be

selected from the SPI port. In each of the two modes, the center

frequency of the band can be tuned such that IFs can be placed

anywhere in the Nyquist band.

22% BANDWIDTH MODE (>40 MHz AT

184.32 MSPS)

The first bandwidth mode offers excellent noise performance

over 22% of the ADC sample rate (44% of the Nyquist band) and

can be centered by setting the NSR mode bits in the NSR control

range can be set using the 6-bit tuning word in the NSR tuning

(TW); each step is 0.5% of the ADC sample rate. The following

three equations describe the left band edge (f0), the channel

center (fCENTER), and the right band edge (f1), respectively:

f0 = fADC × .005 × TW

fCENTER = f0 + 0.11 × fADC

f1 = f0 + 0.22 × fADC

Figure 43 to Figure 45 show the typical spectrum that can be

expected from the AD6673 in the 22% bandwidth mode for

three different tuning words.

–120

–100

–80

–60

–40

–20

100

fS = 250MSPS

fIN = 185.1MHz AT –1dBFS

SNR = 74dB

SFDR = 85dBc

Figure 43. 22% Bandwidth Mode, Tuning Word = 13

–120

–100

–80

–60

–40

–20

100

FREQUENCY (MHz)

fS = 250MSPS

fIN = 185.1MHz AT –1dBFS

SNR = 74.3dB

SFDR = 85dBc

Figure 44. 22% Bandwidth Mode, Tuning Word = 28 (fS/4 Tuning)

FREQUENCY (MHz)

100

–120

–100

–80

–60

–40

–20

fS = 250MSPS

fIN = 185.1MHz AT –1dBFS

SNR = 74.3dB

SFDR = 85dBc

Figure 45. 22% Bandwidth Mode, Tuning Word = 41

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