AD6657A datasheet(27/36 Pages) AD

27 / 36 page

Data Sheet

AD6657A

Rev. 0 | Page 27 of 36

36% BW MODE (>65 MHZ AT 184.32 MSPS)

The third bandwidth mode offers excellent noise performance

over 36% of the ADC sample rate (72% of the Nyquist band)

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

control register (Address 0x3C) to 010. 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 28 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 + 0.18 × fADC

f1 = f0 + 0.36 × fADC

Figure 57 to Figure 59 show the typical spectrum that can be

expected from the AD6657A in the 36% BW mode for three

different tuning words.

Figure 57. 36% BW Mode, Tuning Word = 0

Figure 58. 36% BW Mode, Tuning Word = 14 (fS/4 Tuning)

Figure 59. 36% BW Mode, Tuning Word = 28

MODE PIN

The MODE pin input allows convenient control of the NSR

feature. A logic low enables NSR mode and a logic high sets the

receiver to a straight 11-bit mode with NSR disabled. By default,

the MODE pin is pulled high internally to disable the NSR.

Each channel can be individually configured to ignore the

MODE pin state by writing to Bit 4 of the NSR control register

at Address 0x3C. Use of the NSR control register in conjunction

with the MODE pin allows for very flexible control of the NSR

feature on a per channel basis.

–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 36% BW MODE, TW = 0

SNR = 70.4dB (72.0dBFS)

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 36% BW MODE, TW = 14

SNR = 70.4dB (72.0dBFS)

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)

THIRD HARMONIC

185MSPS

140.3MHz @ –1dBFS

NSR 36% BW MODE, TW = 28

SNR = 70.4dB (72.0dBFS)

SFDR = 91dBc

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