AD6623S/PCB Analog Devices Inc, AD6623S/PCB Datasheet - Page 26

• Manufacturer Part Number: AD6623S/PCB
• Description: BOARD EVAL SGNL PROCESSOR AD6623
• Manufacturer: Analog Devices Inc

Specifications of AD6623S/PCB

• Rohs Status: RoHS non-compliant
• Module/board Type: Evaluation Board
• For Use With/related Products: AD6623
• Lead Free Status / Rohs Status: Not Compliant

AD6623
Resampling is implemented by apparently increasing the input
sample rate by the factor L, using zero stuffing for the new data
samples. Following the resampler is a second order cascaded
integrator comb filter. Filter characteristics are determined only
by the fractional rate change (L/M).
The filter can produce output signals at the full CLK rate of
the AD6623. The output rate of this stage is given by the
equation below.
Both L
rate (L
may be between 1 and 512. The stage can be bypassed by setting
the L and M to 1.
The transfer function of the rCIC2 is given by the following
equations with respect to the rCIC2 output sample rate, f
Frequency Response of rCIC2
The frequency response of the rCIC2 can be expressed as
follows. The maximum gain is L
M
appropriate when the samples are destined for a DAC with a
zero order hold output.
The pass-band droop of CIC5 should be calculated using this
equation and can be compensated for in the RCF stage. The
gain should be calculated from the CIC scaling section above.
Programming Guidelines for AD6623 CIC Filters
The values M
grammed for each channel at locations 0xn07, 0xn08. While
these control registers are nine bits and 12 bits wide respec-
tively, M
shown by Table XIII according to the interpolation factor of the
CIC5. Exceeding the recommended guidelines may result in
overflow for input sequences at or near full scale. While rela-
tively large ratios of L
interpolations with minimal power consumption, L
should be minimized to achieve the best overall image rejection.
rCIC2
rCIC z
rCIC
f
OUT
rCIC2
/L
rCIC2
2
2
=
rCIC2
( )
rCIC2
( ) =
f
M
) may be from 1 to 4096 and the decimation (M
L
and M
rCIC
=
rCIC
–1 and L
factor normalizes for the increased rate, which is
rCIC2



M
2
L
1
2
rCIC
–
1
rCIC
rCIC2
f
rCIC
–1, L
–
z
2
–
z
2
L
rCIC2
2
–
rCIC2
rCIC






are unsigned integers. The interpolation
1
sin
rCIC2
2



–1 should be confined to the ranges
/M
sin



2
π
–1 can be independently pro-
rCIC2



L
π
rCIC
rCIC2
f
f
out
out
f
allow for the larger overall
2
×



at baseband. The initial
f









2
rCIC2
/M
OUT
rCIC2
rCIC2
(19)
(20)
(21)
.
)
–26–
As an example, consider an input from the CIC5 whose bandwidth
is 0.0033 of the CIC5 rate, centered at baseband. Interpolation
by a factor of five reveals five images, as shown below.
The rCIC2 rejects each of the undesired images while passing
the image at baseband. The images of a pure tone at channel
center (DC) are nulled perfectly, but as the bandwidth increases
the rejection is diminished. The lower band edge of the first
image always has the least rejection. In this example, the rCIC2
is interpolating by a factor of five and the input signal has a
bandwidth of 0.0033 of the CIC5 output sample rate. Figure 32
shows –110 dBc rejection of the lower band edge of the first image.
All other image frequencies have better rejection.
Table XIV lists maximum bandwidth that will be rejected to
various levels for CIC2 interpolation factors from 1 to 32. The
example above corresponds to the listing in the –110 dB column
and the L
the interpolation factor increases.
–110
–130
–150
–110
–130
–150
–10
–30
–50
–70
–90
–10
–30
–50
–70
–90
10
10
rCIC2
–3
–3
Figure 31. Unfiltered rCIC2 Images
Figure 32. Filtered rCIC2 Images
= 5 row. The rejection of the CIC2 improves as
–2
–2
–1
–1
0
0
1
1
2
2
REV. A
3
3