AD9856_15 Datasheet, PDF (22/36 Pages) Analog Devices – CMOS 200 MHz Quadrature Digital Upconverter

AD9856
0
–30
–60
–90
–120
–150
0
4
8
12
16
20
24
28
32
DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW
Figure 35. CIC Filter Frequency Response (R = 2, HFB 3 Bypassed)
0
–30
–60
–90
–120
–150
0
72
144
216
258
360
432
504
DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW
Figure 37. CIC Filter Frequency Response (R = 63, HFB 3 Bypassed)
The degree of the impact of the attenuation introduced by the
CIC filter over the Nyquist bandwidth of the data is application
specific. The user must decide how much attenuation is
acceptable. If less attenuation is desired, then additional
oversampling of the baseband data must be employed.
Alternatively, the user can precompensate the baseband data
before presenting it to the AD9856. That is, if the data is
precompensated through a filter that has a frequency response
characteristic, which is the inverse of the CIC filter response,
then the overall system response can be nearly perfectly
flattened over the bandwidth of the data.
Another issue to consider with the CIC filters is insertion loss.
Unfortunately, CIC insertion loss is not fixed, but is a function
of R, M, and N. Because M, and N are fixed for the AD9856, the
CIC insertion loss is a function of R only.
Interpolation rates that are an integer power-of-2 result in no
insertion loss. However, all noninteger power-of-2 interpolation
rates result in a specific amount of insertion loss.
0
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–3.5
–4.0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW
Figure 36. Pass-Band Detail (R = 2, HFB 3 Bypassed)
0
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–3.5
–4.0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW
Figure 38. Pass-Band Detail (R = 63, HFB 3 Bypassed)
To help overcome the insertion loss problem, the AD9856
provides the user a means to boost the gain through the CIC
stage by a factor of 2 (via the CIC Gain bit—see the Serial
Control Bus Register section). The reason for this feature is to
allow the user to take advantage of the full dynamic range of the
DAC, thus maximizing the signal-to-noise ratio (SNR) at the
output of the DAC stage. It is best to operate the DAC over its
full-scale range in order to minimize the inherent quantization
effects associated with a DAC. Any significant loss through the
CIC stage is reflected at the DAC output as a reduction in SNR.
The degradation in SNR can be overcome by boosting the CIC
output level. Table 6 tabulates insertion loss as a function of R.
The values are provided in linear and decibel form, both with
and without the factor-of-2 gain employed.
A word of caution: When the CIC Gain bit is active, ensure that
the data supplied to the AD9856 is scaled down to yield an
overall gain of unity (1) through the CIC filter stage. Gains in
excess of unity are likely to cause overflow errors in the data
path, compromising the validity of the analog output signal.
Rev. C | Page 22 of 36