AD6643 Analog Devices, AD6643 Datasheet - Page 24

• Manufacturer Part Number: AD6643
• Description: Dual IF Receiver
• Manufacturer: Analog Devices

AD6643
NOISE SHAPING REQUANTIZER (NSR)
The AD6643 features a noise shaping requantizer (NSR) to
allow higher than 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.
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 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 (f
the channel center (f
respectively:
Figure 39 to Figure 41 show the typical spectrum that can be
expected from the AD6643 in the 22% BW mode for three
different tuning words.
f
f
f
0
CENTER
1
–100
–120
–140
= f
= f
–20
–40
–60
–80
0
ADC
0
0
+ 0.22 × f
= f
× .005 × TW
Figure 39. 22% BW Mode, Tuning Word = 13
0
10
+ 0.11 × f
20
ADC
CENTER
ADC
30
), and the right band edge (f
FREQUENCY (MHz)
40
50
200MSPS
140.1MHz @ –1.6dBFS
SNR = 73.6dB (75.2dBFS)
SFDR = 86dBc (IN BAND)
60
70
80
1
),
90
0
),
Rev. A | Page 24 of 36
–100
–120
–140
–100
–120
–140
–20
–40
–60
–80
–20
–40
–60
–80
Figure 40. 22% BW Mode, Tuning Word = 28 (f
0
0
0
0
Figure 41. 22% BW Mode, Tuning Word = 41
10
10
20
20
30
30
FREQUENCY (MHz)
FREQUENCY (MHz)
40
40
50
50
200MSPS
140.1MHz @ –1.6dBFS
SNR = 73.4dB (75dBFS)
SFDR = 86dBc (IN BAND)
200MSPS
140.1MHz @ –1.6dBFS
SNR = 73.5dB (75.1dBFS)
SFDR = 86dBc (IN BAND)
60
60
70
70
Data Sheet
S
/4 Tuning)
80
80
90
90