AD6657ABBCZ datasheet(26/36 Pages) AD

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AD6657A

Data Sheet

Rev. 0 | Page 26 of 36

33% BW MODE (>60 MHZ AT 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 word 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

Figure 54 to Figure 56 show the typical spectrum that can be

expected from the AD6657A in the 33% BW mode for three

different tuning words.

Figure 54. 33% BW Mode, Tuning Word = 5

Figure 55. 33% BW Mode, Tuning Word = 17 (fS/4 Tuning)

Figure 56. 33% BW Mode, Tuning Word = 17

–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 33% BW MODE, TW = 5

SNR = 70.9dB (72.5dBFS)

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 33% BW MODE, TW = 17

SNR = 71.1dB (72.7dBFS)

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 33% BW MODE, TW = 17

SNR = 70.9dB (72.5dBFS)

SFDR = 91dBc

THIRD HARMONIC

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