AD6654BBCZ datasheet(38/88 Pages) AD

38 / 88 page

AD6654

Rev. 0 | Page 38 of 88

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.

FRACTION OF FIR1 INPUT SAMPLE RATE

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

–16.67

–8.33

–33.33

–25.00

–50.00

–41.67

–66.67

–58.33

–83.33

–75.00

–100.00

–91.67

0.34

0.66

–81

FIR1 RESPONSE

Figure 48. FIR1 Filter Response to the Input Rate of the Filter

This filter runs at the same sample rate as the CIC filter output

rate and is given by

CIC

FIR1

where:

fIN

is the input rate to the channel.

MCIC

is the decimation ratio in the CIC filter stage.

The maximum input and output rates for this filter are

150 MHz.

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

Normalized

Coefficient

Decimal Coefficient

(10-Bit)

C1, C11

0.013671875

C3, C9

−0.103515625

−53

C5, C7

0.58984375

302

512

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

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

CIC

HB1

where:

fIN

is the input rate to the channel.

MCIC

is the decimation ratio in the CIC filter stage.

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

–110

–100

–90

–120

HB1 RESPONSE

0.43

0.57

–77

Figure 49. HB1 Filter Response to the Input Rate of the Filter

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