AD6649 Analog Devices, AD6649 Datasheet - Page 24

• Manufacturer Part Number: AD6649
• Description: IF Diversity Receiver
• Manufacturer: Analog Devices

AD6649
FIR FILTERS
The two FIR filters that can be used are either a 47-tap, high
performance, fixed-coefficient FIR filter or a 21-tap, low latency,
fixed-coefficient FIR filter. These filters are useful in providing
alias protection at the device output. The high performance FIR
is a simple sum-of-products FIR filter with 47 filter taps and
21-bit fixed coefficients. Note that this filter does not decimate.
The normalized coefficients used in the implementation and the
decimal equivalent value of the coefficients are listed in Table 12.
Table 12. High Performance FIR Filter Coefficients
Coefficient
Number
C0, C46
C1, C45
C2, C44
C3, C43
C4, C42
C5, C41
C6, C40
C7, C39
C8, C38
C9, C37
C10, C36
C11, C35
C12, C34
C13, C33
C14, C32
C15, C31
C16, C30
C17, C29
C18, C28
C19, C27
C20, C26
C21, C25
C22, C24
C23
FIR SYNCHRONIZATION
The AD6649 filters within a single part or across multiple parts can
be synchronized using the external SYNC input. The filters can
be configured to be resynchronized on every SYNC signal or only
on the first SYNC signal after the SPI control register is written.
A valid SYNC causes the FIR filter to restart at the programmed
decimation phase value. Bit 4 and Bit 5 of Register 0x58 allow
the FIR to be resynchronized on every SYNC signal or only on
the first SYNC signal after the register is written.
FILTER PERFORMANCE
When using the fixed-frequency NCO and a 95 MHz FIR filter,
the output rate is equal to the sample clock rate. The composite
response of this mode is shown in Figure 41. The detailed pass-
band response for this mode is shown in Figure 42. To place the
Normalized
Coefficient
−0.0001335
−0.0009689
−0.0024185
−0.0019341
0.0023584
0.0051260
−0.0009680
−0.0086231
−0.0011368
0.0142097
0.0064697
−0.0207596
−0.0161047
0.0274601
0.0310631
−0.0348339
−0.0557785
0.0415993
0.0986786
−0.0463982
−0.1893501
0.0505829
0.6113434
0.9171314
Decimal Coefficient
(21-Bit)
−140
−1016
−2536
−2028
2473
5375
−1015
−9042
−1192
14900
6784
−21768
−16887
28794
32572
−36526
−58488
43620
103472
−48652
−198548
53040
641040
961682
Rev. A | Page 24 of 40
part in this mode, set SPI Register 0x50 to 0xB0. When operating
in this mode, the NCO must be placed at f
NCO select bit (Bit 0) in Register 0x5A must be set. It is important
to note that a −1.0 dBFS input level at the analog inputs corresponds
to an output level of −2.5 dBFS when using the low latency FIR
filter. This output level reduction is a result of the −1.5 dB pass-
band attenuation in the FIR filter in this mode and does not
result in loss in the dynamic range of the converter.
When using the tunable-frequency NCO and 100 MHz FIR filter,
the output rate is equal to the sample clock rate. The response of
the high performance FIR filter is shown in Figure 43. The detailed
pass-band response for this mode is shown in Figure 44. To place
the part into this mode, set SPI Register 0x50 to 0xA0. When
using the high performance FIR filter, the output level is −1.3 dBFS
for a corresponding input level of −1.0 dBFS at the analog inputs.
This is a result of the −0.3 dB pass-band attenuation of the FIR
filter in this mode and does not result in loss in the dynamic range
of the converter.
Figure 41. Low Latency FIR Filter Composite Response at 245.76 MSPS
Figure 42. Low Latency FIR Filter Pass-Band Response at 245.76 MSPS
–1.000
–1.125
–1.250
–1.375
–1.500
–1
–2
–3
–4
0
0
0
(Fixed-Frequency NCO, 95 MHz FIR Filter Mode)
(Fixed-Frequency NCO, 95 MHz FIR Filter Mode)
30.72
30.72
FILTER RESPONSE (MHz)
FILTER RESPONSE (MHz)
61.44
61.44
S
/4, and the low latency
92.16
92.16
Data Sheet
122.88
122.88