AD9856/PCB Analog Devices Inc, AD9856/PCB Datasheet - Page 22

• Manufacturer Part Number: AD9856/PCB
• Description: BOARD EVAL FOR AD9856
• Manufacturer: Analog Devices Inc
• Type: DDS Modulatorsr

Specifications of AD9856/PCB

• Rohs Status: RoHS non-compliant
• For Use With/related Products: AD9856
• Lead Free Status / RoHS Status: Not Compliant

AD9856
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.
–120
–150
–120
–150
Figure 37. CIC Filter Frequency Response (R = 63, HFB 3 Bypassed)
Figure 35. CIC Filter Frequency Response (R = 2, HFB 3 Bypassed)
–30
–60
–90
–30
–60
–90
0
0
0
0
DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW
DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW
4
72
8
144
12
216
16
258
20
360
24
432
2
8
504
32
Rev. C | Page 22 of 36
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.
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–3.5
–4.0
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–3.5
–4.0
0
0
0
0
Figure 38. Pass-Band Detail (R = 63, HFB 3 Bypassed)
Figure 36. Pass-Band Detail (R = 2, HFB 3 Bypassed)
DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW
DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW
0.2
0.2
0.4
0.4
0.6
0.6
0.8
0.8
1.0
1.0
1.2
1.2
1.4
1.4
1.6
1.6
1.8
1.8
2.0
2.0