AD6622AS Analog Devices Inc, AD6622AS Datasheet - Page 13

• Manufacturer Part Number: AD6622AS
• Manufacturer: Analog Devices Inc

Specifications of AD6622AS

• Lead Free Status / RoHS Status: Not Compliant

CASCASDED INTEGRATOR COMB (CIC)
INTERPOLATING FILTER
The I and Q outputs of the RCF stage are interpolated in inte-
ger factors by two cascaded integrator comb (CIC) filters. The
CIC section is separated into three discrete blocks: a fifth order
filter (CIC5), a second order filter (CIC2), and a scaling block
(CIC Scaling). The CIC5 and CIC2 blocks each exhibit a gain
that increases with respect to their interpolation factors, L
and L
in a shared CIC Scaling block.
CIC Scaling
The CIC5 and CIC2 stages have a baseband gain of L
L
in the CIC stages. The CIC scale block reduces the signal level
without truncation or loss of resolution. The overall gain of the
CIC section is given by Equation 9.
The value CIC_Scale may range from 0 to 25, and can be inde-
pendently programmed for each channel at Control Register
0x06. CIC_Scale may be safely calculated according Equation 10
to ensure the net gain through the CIC stages.
The ceil function is the next highest integer. While this normally
constitutes a small loss, it can be recovered in the RCF scaling.
Likewise, if the RCF output level is known to be less than full
scale, the CIC gain can be increased by reducing CIC_Scale.
CIC5
The CIC5 is a fifth order interpolating cascaded integrator comb
whose impulse response is completely defined by its interpola-
tion factor, L
programmed for each channel at location 0x09. While this con-
trol register is 8-bits wide, L
from 1 to 32 to avoid the possibility of internal overflow for
full-scale inputs. The transfer function of the CIC5 is given
by the following equations with respect to the CIC5 output
sample rate, f
This polynomial fraction can be completely reduced as follows,
demonstrating a finite impulse response with perfect phase lin-
earity for all values of L
CIC2
CIC z
CIC Gain
CIC Scale
CIC z
. The CIC scaling block is used to avoid numeric overflow
CIC2
_
5
5
. The product of these gains must be compensated for
_
( )
( )
CIC_SCALE
2
SAMP5
–CIC_SCALE
CIC5
=
=
=



=

 

1
. The value L
ceil
L
.
1
k
L
–
∑
CIC
=
CIC
–
5 1
z
(log (
0
–
z
–
5
CIC5
z
L
–
4
CIC
1
−
×
2
k
5
.




 

L
L
CIC5
5
5
CIC
CIC
L
CIC5
=
CIC5
2
5
CIC5
should be confined to the range
4
L
×
k
∑
CIC
×
=
2
5 1
L
–1 can be independently
1
–
–
CIC Scale
CIC




z
2
_
−
))
1
−
L
CIC2
CIC2
e
j
2
π
L
CIC
k
5




5
CIC5
CIC5
4
(10)
(11)
(12)
×
(9)
The frequency response of the CIC5 can be expressed as follows.
The initial 1/L
appropriate when the samples are destined for a DAC with a
zero order hold output. The maximum gain is (L
band, but internal registers peak in response to various dynamic
inputs. As long as L
possibility of overflow at any register.
As an example, we will consider an input from the RCF whose
bandwidth is 0.141 of the RCF output rate, centered at base-
band. Interpolation by a factor of five reveals five images, as
shown in Figure 14.
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 band-
width 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.
CIC
–110
–130
–150
–110
–130
–150
–10
–30
–50
–70
–90
–10
–30
–50
–70
–90
5
10
10
( )
f
CIC5
=
–2
–2
L
factor normalizes for the increased rate, which is
CIC
1
CIC5
5






sin
–1
–1
is confined to 32 or less, there is no
sin



π



L
π
CIC
L
f
CIC
CIC
f
5
0
0
×
5
5



f









5
1
1
AD6622
CIC5
)
2
4
2
at base-
(13)