AD9856/PCB Analog Devices Inc, AD9856/PCB Datasheet - Page 21

• 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

In applications requiring both a low data rate and a high output
sample rate, a third HBF is available (HBF 3). Selecting HBF 3
offers an upsampling ratio of eight (8) instead of four (4). The
combined frequency response of HBF 1, 2, and 3 is shown in
Figure 33 and Figure 34. Comparing the pass-band detail of
HBF 1 and 2 with the pass-band detail of HBF 1, 2, and 3,
HBF 3 has virtually no impact on frequency response from
0 to 1 (where 1 corresponds to f
CASCADED INTEGRATOR COMB (CIC) FILTER
A CIC filter is unlike a typical FIR filter in that it offers the
flexibility to handle differing input and output sample rates
(only in integer ratios, however). In the purest sense, a CIC
filter can provide either an increase or a decrease in the sample
rate at the output relative to the input, depending on the archi-
tecture. If the integration stage precedes the comb stage, the
CIC filter provides sample rate reduction (decimation). When
the comb stage precedes the integrator stage the CIC filter
provides an increase in sample rate (interpolation). In the
AD9856, the CIC filter is configured as an interpolator—
a programmable interpolator—and provides a sample rate
increase, R, such that 2 ≤ R ≤ 63.
Figure 34. Pass-Band Detail: Combined Frequency Response of HBF 1 to 3
–100
–10
–20
–30
–40
–50
–60
–70
–80
–90
10
–1
–2
–3
–4
–5
–6
0
1
0
0
0
Figure 33. Half-Band 1, 2, and 3 Frequency Response
DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW
DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW
0.1
1
0.2
2
0.3
3
0.4
0.5
4
NYQ
).
0.6
5
0.7
6
0.8
7
0.9
1.0
8
Rev. C | Page 21 of 36
In addition to the ability to provide a change in sample rate
between input and output, a CIC filter also has an intrinsic low-
pass frequency response characteristic. The frequency response
of a CIC filter depends on:
•
•
•
The system function, H(z), of a CIC filter is given by:
The form on the far right has the advantage of providing a
result for z = 1 (corresponding to zero frequency or dc). The
alternate form yields an indeterminate form (0/0) for z = 1,
but is otherwise identical. The only variable parameter for
the AD9856 CIC filter is R. M and N are fixed at 1 and 4,
respectively. Thus, the CIC system function for the AD9856
simplifies to:
The transfer function is given by:
The frequency response in this form is such that f is scaled
to the output sample rate of the CIC filter. That is, f = 1
corresponds to the frequency of the output sample rate of the
CIC filter. H(f/R) yields the frequency response with respect to
the input sample of the CIC filter. Figure 35 to Figure 44 show
the CIC frequency response and pass-band detail for R = 2 and
R = 63, with HBF 3 bypassed. Figure 45 to Figure 50 are similar,
but HBF 3 is selected. Note the flatter pass-band response when
HBF 3 is employed.
As with HBFs, consideration must be given to the frequency-
dependent attenuation that the CIC filter introduces over the
frequency range of the data to be transmitted. Note that the
CIC frequency response figures have f
frequency; i.e., unity (1) on the frequency scale corresponds
to f
oversampled by a factor of 2 (as required), then the Nyquist
bandwidth of the applied data is one-half f
frequency response figures. A look at the 0.5 point on the pass-
band detail figures reveals a worst-case attenuation of about
0.25 dB (HBF 3 bypassed, R = 63). This, of course, assumes
pulse-shaped data with α = 0 (minimum bandwidth scenario).
When a value of α = 1 is used, the bandwidth of the data
corresponds to f
Thus, the worst-case attenuation for α = 1 is about 0.9 dB.
NYQ
The rate change ratio, R.
The order of the filter, N.
The number of unit delays per stage, M.
H
H
H
. If the incoming data that is applied to the AD9856 is
(
(
(
z
z
f
)
)
)
=
=
=
⎛
⎜
⎜
⎝
⎛
⎜
⎜
⎝
⎛
⎜
⎜
⎝
1
1
1
1
1
1
−
−
−
−
−
−
NYQ
z
z
z
e
e
−
z
−
−
−
−
RM
−
R
1
(the point1.0 on the CIC frequency scale).
j
1
j
2 (
2 (
⎞
⎟
⎟
⎠
π
4
π
⎞
⎟
⎟
⎠
=
fR
N
f
)
⎛
⎜
⎜
⎝
=
)
κ
⎞
⎟
⎟
⎠
R
∑
⎛
⎜
⎜
⎝
−
=
4
RM
1
0
=
κ
∑
z
=
−
⎛
⎜
⎜
⎝
−
0
κ
κ
1
R
∑
=
z
⎞
⎟
⎟
⎠
−
1
0
−
4
e
κ
−
⎞
⎟
⎟
⎠
j
N
2 (
NYQ
π
f
κ
as their reference
)
NYQ
⎞
⎟
⎟
⎠
4
on the CIC
AD9856