AD6672(Rev0) 查看數據表（PDF） - Analog Devices
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AD6672 Datasheet PDF : 32 Pages
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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.
0
250MSPS
180.1MHz @ –1.6BFS
–20 SNR = 73.4dB (75.0dBFS)
SFDR = 92dBc (IN BAND)
–40
–60
–80
–100
–120
–140
0
25
50
75
100
125
FREQUENCY (MHz)
Figure 36. 22% Bandwidth Mode, Tuning Word = 13
AD6672
0
250MSPS
180.1MHz @ –1.6BFS
–20 SNR = 73.4dB (75.0dBFS)
SFDR = 93dBc (IN BAND)
–40
–60
–80
–100
–120
–140
0
25
50
75
100
125
FREQUENCY (MHz)
Figure 37. 22% Bandwidth Mode, Tuning Word = 28
0
250MSPS
180.1MHz @ –1.6BFS
–20 SNR = 73.3dB (74.9dBFS)
SFDR = 92dBc (IN BAND)
–40
–60
–80
–100
–120
–140
0
25
50
75
100
125
FREQUENCY (MHz)
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.
Rev. 0 | Page 21 of 32
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