AD6672 datasheet(21/32 Pages) AD

21 / 32 page

AD6672

Rev. 0 | Page 21 of 32

NOISE SHAPING REQUANTIZER

The AD6672 features a noise shaping requantizer (NSR) to

allow more 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.

Two bandwidth (BW) modes are provided; the mode can be

selected from the SPI port. In each mode, the center frequency

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

in the Nyquist band.

22% BW NSR MODE (55 MHz BW AT 250 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 (Bits[3:1]) 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 (Bits[5:0]) 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 equations describe the left

band edge (f0), the channel center (fCENTER), and the right band

edge (f1), respectively:

f0 = fADC × 0.005 × TW

fCENTER = f0 + 0.11 × fADC

f1 = f0 + 0.22 × fADC

Figure 36 to Figure 38 show the typical spectrum that can be

expected from the AD6672 in the 22% bandwidth mode for

three tuning words.

–20

–40

–60

–80

–100

–120

–140

100

125

FREQUENCY (MHz)

250MSPS

180.1MHz @ –1.6BFS

SNR = 73.4dB (75.0dBFS)

SFDR = 92dBc (IN BAND)

Figure 36. 22% Bandwidth Mode, Tuning Word = 13

–20

–40

–60

–80

–100

–120

–140

100

125

FREQUENCY (MHz)

250MSPS

180.1MHz @ –1.6BFS

SNR = 73.4dB (75.0dBFS)

SFDR = 93dBc (IN BAND)

Figure 37. 22% Bandwidth Mode, Tuning Word = 28

–20

–40

–60

–80

–100

–120

–140

100

125

FREQUENCY (MHz)

250MSPS

180.1MHz @ –1.6BFS

SNR = 73.3dB (74.9dBFS)

SFDR = 92dBc (IN BAND)

Figure 38. 22% Bandwidth Mode, Tuning Word = 41

33% BW NSR MODE (>82 MHZ BW AT 250 MSPS)

The 33% bandwidth NSR mode offers excellent noise performance

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

The fundamental can be tuned using a low-pass, band-pass, or

high-pass filter by setting the NSR tuning word bits (Bits[5:0])

in Register 0x3E.

Figure 39 to Figure 41 show the typical spectrum that can be

expected from the AD6672 with the 33% bandwidth NSR mode

enabled for three filter settings.

AD6672 Datenblatt(PDF) 21 Page - Analog Devices

AD6672 Datenblatt(HTML) 21 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datenblatt Download

Link URL