AD6654BBC datasheet(39/88 Pages) AD

39 / 88 page

AD6654

Rev. 0 | Page 39 of 88

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.

FRACTION OF HB1 INPUT SAMPLE RATE

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

–20

–10

–40

–30

–60

–50

–80

–70

–100

–90

–120

–110

–107

FIR1 + HB1 RESPONSE

0.43

0.57

Figure 50. Composite Response of FIR1 and HB1 Filters to Their Input Rate

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

Normalized

Coefficient

Decimal Coefficient

(5-Bit)

C0, C5

−0.125

−2

C1, C4

0.1875

C2, C3

0.9375

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.

This filter runs at a sample rate given by one of the following

equations:

fFIR2 = fHB1,

if HB1 is bypassed

fFIR2 =

HB1

if HB1 is not bypassed

where:

fHB1

is the input rate of the HB1 filter.

fFIR2

is the input rate of the FIR2 filter.

The maximum input and output rate for this filter is 75 MHz.

The response of the FIR2 filter is shown in Figure 51.

FRACTION OF FIR2 INPUT SAMPLE RATE

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

–8.33

–16.67

–25.00

–33.33

–41.67

–50.00

–58.33

–66.67

–75.00

–83.33

–91.67

–100.00

–30

0.39

FIR2 RESPONSE

0.61

Figure 51. FIR2 Filter Response to the Input Rate of the Filter

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

Normalized

Coefficient

Decimal Coefficient

(12-Bit)

C1, C27

0.00097656

C3, C25

−0.00537109

−11

C5, C23

0.015

C7, C21

−0.0380859

−78

C9, C19

0.0825195

169

C11, C17

−0.1821289

−373

C13, C15

0.6259766

1282

C14

2048

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

AD6654BBC Datenblatt(PDF) 39 Page - Analog Devices

AD6654BBC Datenblatt(HTML) 39 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88

Datenblatt Download

Link URL