AD6654/PCB Analog Devices Inc, AD6654/PCB Datasheet - Page 38

• Manufacturer Part Number: AD6654/PCB
• Description: BOARD EVALUATION FOR AD6654
• Manufacturer: Analog Devices Inc

Specifications of AD6654/PCB

• Module/board Type: Evaluation Board
• For Use With/related Products: AD6554
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

AD6654
FIR HALF-BAND BLOCK
The output of the CIC filter is pipelined into the FIR HB (half-
band) block. Each channel has two sets of cascading fixed
coefficient FIR and fixed coefficient half-band filters. The half-
band filters decimate by 2. Each of these filters (FIR1, HB1,
FIR2, and HB2) is described in the following sections.
3-TAP FIXED COEFFICIENT FILTER (FIR1)
The 3-tap FIR filter is useful in certain filter configurations in
which extra alias protection is needed for the decimating HB1
filter. It is a simple sum-of-products FIR filter with three filter
taps and 2-bit fixed coefficients. Note that this filter does not
decimate. The coefficients of this symmetric filter are {1, 2, 1}.
The normalized coefficients used in the implementation are
{0.25, 0.5, 0.25}.
The user can either use or bypass this filter. Writing Logic 0 to
the FIR1 enable bit in the FIR-HB control register bypasses this
fixed coefficient filter. The filter is useful only in certain filter
configurations and bypassing it for other applications results in
power savings.
This filter runs at the same sample rate as the CIC filter output
rate and is given by
where:
f
M
The maximum input and output rates for this filter are
150 MHz.
IN
–100.00
CIC
is the input rate to the channel.
–16.67
–25.00
–33.33
–41.67
–50.00
–58.33
–66.67
–75.00
–83.33
–91.67
–8.33
is the decimation ratio in the CIC filter stage.
f
FIR1
0
Figure 48. FIR1 Filter Response to the Input Rate of the Filter
0
=
0.1
M
f
FRACTION OF FIR1 INPUT SAMPLE RATE
IN
CIC
0.2
0.3
0.34
FIR1 RESPONSE
0.4
0.5
0.6
0.66
0.7
0.8
0.9
–81
Rev. 0 | Page 38 of 88
DECIMATE-BY-2 HALF-BAND FILTER (HB1)
The next stage of the FIR-HB block is a decimate-by-2 half-
band filter. The 11-tap, symmetrical, fixed coefficient HB1 filter
has low power consumption due to its polyphase implementa-
tion. The filter has 22 bits of input and output data with 10-bit
coefficients. Table 16 lists the coefficients of the half-band filter.
The normalized coefficients used in the implementation and
the 10-bit decimal equivalent value of the coefficients are also
listed. Other coefficients are 0s.
Table 16. Fixed Coefficients for HB1 Filter
Coefficient
Number
C1, C11
C3, C9
C5, C7
C6
Similar to the FIR1 filter, this filter can be used or bypassed.
Writing Logic 0 to the HB1 enable bit in the FIR-HB control
register bypasses this fixed coefficient HB filter. The filter is
useful only in certain filter configurations and bypassing it for
other applications results in power savings. For example, it is
useful in narrow-band and wideband output applications in
which more filtering is required, as compared to very wide
bandwidth applications in which a higher output rate might
prohibit the use of a decimating filter. The response of the filter
is shown in Figure 49.
The input sample rate of this filter is the same as the CIC filter
output rate and is given by
where:
f
M
IN
CIC
is the input rate to the channel.
–100
–110
–120
is the decimation ratio in the CIC filter stage.
–10
–20
–30
–40
–50
–60
–70
–80
–90
f
HB1
0
Figure 49. HB1 Filter Response to the Input Rate of the Filter
0
=
HB1 RESPONSE
0.1
M
f
IN
CIC
FRACTION OF HB1 INPUT SAMPLE RATE
0.2
Normalized
Coefficient
0.013671875
−0.103515625
0.58984375
1
0.3
0.43
0.4
0.5
0.6
0.57
0.7
Decimal Coefficient
(10-Bit)
7
−53
302
512
0.8
0.9
–77