AD6654BBCZ datasheet(40/88 Pages) AD

40 / 88 page

AD6654

Rev. 0 | Page 40 of 88

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 52.

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 52. HB2 Filter Response to the Input Rate of the Filter

The filter input sample rate is the same as the FIR2 filter output

rate and is given by one of the following equations:

fHB2 = fFIR2 = fHB1,

if HB1 is bypassed

fHB2 = fFIR2 =

HB1

if HB1 is not bypassed

where:

fFIR2

is the input rate of the FIR2 filter.

fHB2

is the input rate of the HB2 filter.

The input to the filter has a maximum of 75 MHz. The

maximum output rate when not bypassed is 37.5 MHz.

The filter has a ripple of 0.00075 dB and rejection of 81 dB. For

an alias rejection of 81 dB, the alias-protected bandwidth is 33%

of the filter input sample rate. The bandwidth of the filter for a

ripple of 0.00075 dB is the same as alias-protected bandwidth,

due to the nature of half-band filters. The 3 dB bandwidth of

this filter is 47% of the filter input sample rate. For example, if

the sample rate into the filter is 25 MHz, then the alias-

protected bandwidth of the HB2 filter is 8.25 MHz (33% of

25 MHz). If the bandwidth of the required carrier is greater

than 8.25 MHz, then HB2 might not be useful.

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

–90

0.66

0.34

FIR2 + HB2

RESPONSE

Figure 53. Composite Response of FIR2 and HB2 Filters to Their Input Rates

fHB1

is the input rate of the HB1 filter.

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