AD6657EBZ datasheet(22/32 Pages) AD

22 / 32 page

AD6657

Rev. 0 | Page 22 of 32

NOISE SHAPING REQUANTIZER (NSR)

The AD6657 features a noise shaping requantizer (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 or the MODE pin.

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% BW MODE (>40 MHZ @ 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 register (Address 0x3C) to 000. In this mode, the useful

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

NSR tuning register (Address 0x3E). There are 57 possible

tuning words (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 45 to Figure 47 show the typical spectrum that can be

expected from the AD6657 in the 22% BW mode for three

different tuning words.

–20

–40

–60

–80

–100

–120

020

FREQUENCY (MHz)

fS = 184.32MSPS

fIN = 140MHz @ –1.6dBFS

NSR 22% BW MODE, TW = 13

SNR = 73.4dB (75dBFS) (IN-BAND)

SFDR = 92.6dBc (IN-BAND)

Figure 45. 22% BW Mode, Tuning Word = 13

020

–20

–40

–60

–80

–100

–120

FREQUENCY (MHz)

fS = 184.32MSPS

fIN = 140MHz @ –1.6dBFS

NSR 22% BW MODE, TW = 28

SNR = 73.4dB (75dBFS) (IN-BAND)

SFDR = 93dBc (IN-BAND)

Figure 46. 22% BW Mode, Tuning Word = 28 (fS/4 Tuning)

020

–20

–40

–60

–80

–100

–120

FREQUENCY (MHz)

fS = 184.32MSPS

fIN = 140MHz @ –1.6dBFS

NSR 22% BW MODE, TW = 41

SNR = 73.4dB (75dBFS) (IN-BAND)

SFDR = 94dBc (IN-BAND)

Figure 47. 22% BW Mode, Tuning Word = 41

33% BW MODE (>60 MHZ @ 184.32 MSPS)

The second bandwidth mode offers excellent noise performance

over 33% of the ADC sample rate (66% of the Nyquist band)

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

control register (Address 0x3C) to 001. In this mode, the useful

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

NSR tuning register (Address 0x3E). There are 34 possible

tuning words (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 × 0.165 × fADC

f1 = f0 × 0.33 × fADC

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