AD6654/PCB Analog Devices Inc, AD6654/PCB Datasheet - Page 39

• 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

The filter has a maximum input sample rate of 150 MHz and,
when filter is not bypassed, the maximum output rate is
75 MHz.
The filter has a ripple of 0.0012 dB and rejection of 77 dB. For
an alias rejection of 77 dB, the alias-protected bandwidth is 14%
of the filter input sample rate. The bandwidth of the filter for a
ripple of 0.00075 dB is also the same as the alias-protected
bandwidth, due to the nature of half-band filters. The 3 dB
bandwidth of this filter is 44% of the filter input sample rate.
For example, if the sample rate into the filter is 50 MHz, then
the alias-protected bandwidth of the HB1 filter is 7 MHz. If the
bandwidth of the required carrier is greater than 7 MHz, then
HB1 might not be useful.
6-TAP FIXED COEFFICIENT FILTER (FIR2)
Following the first cascade of the FIR1 and HB1 filters is the
second cascade of the FIR2 and HB2 filters. The 6-tap, fixed
coefficient FIR2 filter is useful in providing extra alias
protection for the decimating HB2 filter in certain filter
configurations. It is a simple sum-of-products FIR filter with
six filter taps and 5-bit fixed coefficients. Note that this filter
does not decimate. The normalized coefficients used in the
implementation and the 5-bit decimal equivalent value of the
coefficients are listed in Table 17.
Table 17. 6-Tap FIR2 Filter Coefficients
Coefficient
Number
C0, C5
C1, C4
C2, C3
The user can either use or bypass this filter. Writing Logic 0 to
FIR2 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. The filter is especially useful in increasing the
stop-band attenuation of the HB2 filter that follows. Therefore,
it is optimal to use both FIR2 and HB2 in a configuration.
Figure 50. Composite Response of FIR1 and HB1 Filters to Their Input Rate
–100
–110
–120
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
0
0.1
FRACTION OF HB1 INPUT SAMPLE RATE
0.2
Normalized
Coefficient
−0.125
0.1875
0.9375
0.3
0.43
0.4
0.5
0.6
0.57
FIR1 + HB1 RESPONSE
0.7
Decimal Coefficient
(5-Bit)
−2
3
15
0.8
0.9
–107
Rev. 0 | Page 39 of 88
This filter runs at a sample rate given by one of the following
equations:
where:
f
f
The maximum input and output rate for this filter is 75 MHz.
The response of the FIR2 filter is shown in Figure 51.
DECIMATE-BY-2 HALF-BAND FILTER (HB2)
The second stage of the second cascade of the FIR-HB block is a
decimate-by-2 half-band filter. The 27-tap, symmetric, fixed
coefficient HB2 filter has low power consumption due to its
polyphase implementation. The filter has 20 bits of input and
output data with 12-bit coefficients. The normalized
coefficients used in the implementation and the 10-bit decimal
equivalent value of the coefficients are listed in Table 18. Other
coefficients are 0s.
Table 18. HB2 Filter Fixed Coefficients
Coefficient
Number
C1, C27
C3, C25
C5, C23
C7, C21
C9, C19
C11, C17
C13, C15
C14
Similar to the HB1 filter, this filter can either be used or
bypassed. Writing Logic 0 to the HB1 enable bit in the FIR-HB
HB1
FIR2
–100.00
–16.67
–25.00
–33.33
–41.67
–50.00
–58.33
–66.67
–75.00
–83.33
–91.67
is the input rate of the HB1 filter.
is the input rate of the FIR2 filter.
–8.33
f
f
FIR2
FIR2
0
Figure 51. FIR2 Filter Response to the Input Rate of the Filter
0
= f
=
FIR2 RESPONSE
HB1
f
0.1
HB1
2
, if HB1 is bypassed
FRACTION OF FIR2 INPUT SAMPLE RATE
0.2
, if HB1 is not bypassed
Normalized
Coefficient
0.00097656
−0.00537109
0.015
−0.0380859
0.0825195
−0.1821289
0.6259766
1
0.3
0.39
0.4
0.5
0.6
0.61
0.7
Decimal Coefficient
(12-Bit)
2
−11
32
−78
169
−373
1282
2048
0.8
0.9
AD6654
–30