AD6623S/PCB Analog Devices Inc, AD6623S/PCB Datasheet - Page 24

• 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
Sync pulses from Sync1, 2, and 3 pins are not masked in any fash-
ion and directly connect to all Sync multiplexers of all channels. The
Sync Control Block Diagram, Figure 37, in the Synchronization
section of this data sheet provides an overview of all sync signal
routing for one channel.
CASCADED INTEGRATOR COMB (CIC) INTERPOLATING
FILTERS
The I and Q outputs of the RCF stage are interpolated by two
cascaded integrator comb (CIC) filters. The CIC section is
separated into three discrete blocks: a fifth order filter (CIC5), a
second order resampling filter (rCIC2), and a scaling block (CIC
Scaling). The CIC5 and rCIC2 blocks each exhibit a gain that
changes with respect to their rate change factors, L
and L
in a shared CIC Scaling block and can be done to within 6 dB.
The remaining compensation can come from the RCF (in the
form of coefficient scaling) or the fine scaling unit.
CIC Scaling
The scale factor S
between 4 and 32. This is a combined scaler for the CIC5 and
rCIC2 stages. The overall gain of the CIC section is given by
the equation below
CIC5
The first CIC filter stage, the CIC5, is a fifth order interpolating
cascaded integrator comb whose impulse response is completely
defined by its interpolation factor, L
be independently programmed for each channel at location 0xn09.
While this control register is 8 bits wide, L
to the range from 1 to 32 to avoid the possibility of internal
overflow for full scale inputs. The output rate of this stage is given
by the equation below.
The transfer function of the CIC5 is given by the following
equations with respect to the CIC5 output sample rate, f
The SCIC value can be independently programmed for each
channel at Control Register 0xn06. S
according to equation (13) below to ensure the net gain through
the CIC stages.
SCIC serves to frame which bits of the CIC output are transferred
to the NCO stage. This results in controlling the data out of the
CIC stages in 6 dB increments. For the best dynamic range, S
should be set to the smallest value possible (lowest attenuation)
without creating an overflow condition. This can be safely
accomplished using the equation below. To ensure the CIC
CIC Gain
CIC z
f
CIC
CIC5
2
5
_
( )
=
CIC_SCALE
. The product of these gains must be compensated for
f
CIC
=
2
–SCIC



5
=
1
×
L
–
1
L
CIC
–
CIC
z
CIC
–
z
L
5
–
CIC
4
5
1
is a programmable unsigned integer
Figure 28. CIC5
×
5



L
5
CIC5
rCIC
L
CIC5
2
×
2
–
S
CIC5
CIC
CIC
. The value L
may be safely calculated
CIC5
rCIC2
L
M
rCIC 2
rCIC 2
should be confined
rCIC2
CIC5
, M
SAMP5
–1 can
rCIC2
(10)
(11)
(12)
CIC
,
.
–24–
output data is in range, Equation 13 must always be met. The
maximum total interpolation rate may be limited by the amount
of scaling available.
This polynomial fraction can be completely reduced as follows
demonstrating a finite impulse response with perfect phase
linearity for all values of L
The frequency response of the CIC5 can be expressed as follows.
The initial 1/L
which is appropriate when the samples are destined for a DAC
with a zero order hold output. The maximum gain is L
baseband, but internal registers peak in response to various
dynamic inputs. As long as L
is no possibility of overflow at any register.
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.
As an example, consider an input from the RCF whose bandwidth
is 0.141 of the RCF output rate, centered at baseband. Interpolation
by a factor of five reveals five images, as shown below.
The CIC5 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 CIC5 is
interpolating by a factor of five and the input signal has a bandwidth
of 0.141 of the RCF output sample rate. The plot below shows
–110 dBc rejection of the lower band edge of the first image. All
other image frequencies have better rejection.
S
0
CIC z
CIC f
CIC
≤
S
5
5
≥
CIC
–110
–130
–150
( )
( ) =
–10
–30
–50
–70
–90
ceil
10
–3
≤
=
(
Figure 29. Unfiltered CIC5 Images
4
58



CIC5
L
L
×
k
CIC
CIC
1
∑
=
log
5
–2
0
–
5
factor normalizes for the increased rate,
1






z
2
sin
(
–
L
k

 =

sin
CIC
5



–1
CIC5
π
5



)
L
π
L
CIC5
+
CIC
k
CIC
.
∑
f
f
=
log
5
CIC
CIC
1
–
f
5
1
is confined to 32 or less, there



0
×
5
5
2
z



(
f
L
–
1



CIC
–






5
e
2
1
j
)
2
)
π
L
CIC
k
5



5
2
CIC5
3
REV. A
4
at
(13)
(14)
(15)
(16)