AD6636PCB datasheet(31/72 Pages) AD

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AD6636

Rev. 0 | Page 31 of 72

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 15.

Table 15. 6-Tap FIR1 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.

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

The response of the FIR2 filter is shown in Figure 34.

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 34. 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 16. Other coefficients

are zeros.

Table 16. 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, the user can either use or bypass this

filter. 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,

the filter is useful in narrow-band applications in which more

filtering is required, as compared to wide-band applications, in

which a higher output rate might prohibit the use of a decimat-

ing filter. The response of the HB2 filter is shown in Figure 35.

FRACTION OF HB2 INPUT SAMPLE RATE

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.01

–19.99

–29.99

–9.99

–39.99

–49.99

–60.00

–70.00

–80.00

–90.00

–100.00

–110.00

–120.00

–65

0.66

0.34

HB2 RESPONSE

Figure 35. HB2 Filter Response to the Input Rate of the Filter

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