AD9879BS Analog Devices Inc, AD9879BS Datasheet - Page 23

• Manufacturer Part Number: AD9879BS
• Description: IC FRONT-END MIXED-SGNL 100-MQFP
• Manufacturer: Analog Devices Inc

Specifications of AD9879BS

• Rohs Status: RoHS non-compliant
• Number Of Bits: 12
• Number Of Channels: 5
• Power (watts): 587mW
• Voltage - Supply, Analog: 3.3V
• Voltage - Supply, Digital: 3.3V
• Package / Case: 100-MQFP, 100-PQFP

To keep the bandwidth of the data in the flat portion of the filter
pass band, the user must oversample the baseband data by at
least a factor of two prior to representing it to the AD9879.
Without oversampling, the Nyquist bandwidth of the baseband
data corresponds to the f
data bandwidth suffers 6 dB or more of attenuation due to the
frequency response of the digital filters.
There is an additional concern if the baseband data applied to
the AD9879 has been pulse shaped. Typically, pulse shaping is
applied to the baseband data via a filter having a raised cosine
response. In such cases, an α value is used to modify the
bandwidth of the data where the value of α is such that 0 < α <
1. A value of 0 causes the data bandwidth to correspond to the
Nyquist bandwidth. A value of 1 causes the data bandwidth to
be extended to twice the Nyquist bandwidth.
–1
–2
–3
–4
–5
–6
1
0
0
Figure 15. Cascaded Filter Pass-Band Detail (N = 4)
0.1
FREQUENCY RELATIVE TO I/Q NYQUIST BW
0.2
0.3
NYQ
. Consequently, the upper end of the
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Rev. A | Page 23 of 32
Thus, with 2× oversampling of the baseband data and α =1,
the Nyquist bandwidth of the data corresponds with the I/Q
Nyquist bandwidth.
As stated earlier, this results in problems near the upper edge of
the data bandwidth due to the frequency response of the filters.
The maximum value of α that can be implemented is 0.45. This
is because the data bandwidth becomes:
which puts the data bandwidth at the extreme edge of the flat
portion of the filter response.
If a particular application requires an α value between 0.45 and
1, then the user must oversample the baseband data by at least a
factor of four. The combined HBF1, HBF2, and CIC filter
introduces a worst-case droop of less than 0.2 dB over the
frequency range of the data to be transmitted.
1
2
(
–1
–2
–3
–4
–5
–6
1
1
0
+
0
Figure 16. Cascaded Filter Pass-Band Detail (N = 3)
α
)
0.1
f
NYQ
FREQUENCY RELATIVE TO I/Q NYQUIST BW
0.2
=
. 0
725
0.3
f
0.4
NYQ
0.5
0.6
0.7
0.8
0.9
AD9879
1.0
(16)