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Data Sheet

AD6657A

NOISE SHAPING REQUANTIZER

The AD6657A features a noise shaping requantizer (NSR) to allow

higher than an 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 via 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 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 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 word 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 51 to Figure 53 show the typical spectrum that can be

expected from the AD6657A in the 22% BW mode for three

different tuning words.

Figure 51. 22% BW Mode, Tuning Word = 13

Figure 52. 22% BW Mode, Tuning Word = 28 (fS/4 Tuning)

Figure 53. 22% BW Mode, Tuning Word = 41

–20

–40

–60

–80

–100

–120

–140

9.25 18.50 27.75 37.00 46.25 55.50 64.75 74.00 83.25 92.50

FREQUENCY (MHz)

185MSPS

140.3MHz @ –1dBFS

NSR 22% BW MODE, TW = 13

SNR = 73.4dB (75.0dBFS)

SFDR = 91dBc

THIRD HARMONIC

–20

–40

–60

–80

–100

–120

–140

9.25 18.50 27.75 37.00 46.25 55.50 64.75 74.00 83.25 92.50

FREQUENCY (MHz)

185MSPS

140.3MHz @ –1dBFS

NSR 22% BW MODE, TW = 28

SNR = 73.4dB (75.0dBFS)

SFDR = 91dBc

THIRD HARMONIC

–20

–40

–60

–80

–100

–120

–140

9.25 18.50 27.75 37.00 46.25 55.50 64.75 74.00 83.25 92.50

FREQUENCY (MHz)

185MSPS

140.3MHz @ –1dBFS

NSR 22% BW MODE, TW = 41

SNR = 73.4dB (75.0dBFS)

SFDR = 91dBc

THIRD HARMONIC

Rev. A | Page 25 of 36

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