AD6655 datasheet(39/84 Pages) AD

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AD6655

Rev. 0 | Page 39 of 84

DECIMATING HALF-BAND FILTER AND FIR FILTER

The goal of the AD6655 half-band digital filter is to allow the

sample rate to be reduced by a factor of 2 while rejecting aliases

that fall into the band of interest. This filter is designed to operate

as either a low-pass or a high-pass filter and to provide >100 dB

of alias protection for 11% of the input rate of the structure.

Used in conjunction with the NCO and the FIR filter, the half-

band filter can provide an effective band-pass. For an ADC

sample rate of 150 MSPS, this provides a maximum usable

bandwidth of 33 MHz.

HALF-BAND FILTER COEFFICIENTS

The 19-tap, symmetrical, fixed-coefficient half-band filter has low

power consumption due to its polyphase implementation. Table 19

lists the coefficients of the half-band filter. The normalized

coefficients used in the implementation and the decimal

equivalent value of the coefficients are also listed. Coefficients

not listed in Table 19 are 0s.

Table 19. Fixed Coefficients for Half-Band Filter

Coefficient

Number

Normalized

Coefficient

Decimal Coefficient

(20-Bit)

C0, C18

0.0008049

844

C2, C16

−0.0059023

−6189

C4, C14

0.0239182

25080

C6, C12

−0.0755024

−79170

C8, C10

0.3066864

321584

0.5

524287

HALF-BAND FILTER FEATURES

In the AD6655, the half-band filter cannot be disabled. The

filter can be set for a low-pass or high-pass response. For a high-

pass filter, Bit 1 of Register 0x103 should be set; for a low-pass

response, this bit should be cleared. The low-pass response of the

filter with respect to the normalized output rate is shown in

Figure 72, and the high-pass response is shown in Figure 73.

–110

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

0.1

0.2

0.3

0.4

FRACTION OF INPUT SAMPLE RATE

Figure 72. Half-Band Filter Low-Pass Response

–110

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

0.1

0.2

0.3

0.4

FRACTION OF INPUT SAMPLE RATE

Figure 73. Half-Band Filter High-Pass Response

The half-band filter has a ripple of 0.000182 dB and a rejection

of 100 dB. For an alias rejection of 100 dB, the alias protected

bandwidth is 11% of the input sample rate. If both the I and the

Q paths are used, a complex bandwidth of 22% of the input rate

is available.

In the event of even Nyquist zone sampling, the half-band filter

can be configured to provide a spectral reversal. Setting Bit 2

high in Address 0x103 enables the spectral reversal feature.

The half-band decimation phase can be selected such that

the half-band filter starts on the first or second sample following

synchronization. This shifts the output from the half-band between

the two input sample clocks. The decimation phase can be set to

0 or 1, using Bit 3 of Register 0x103.

FIXED-COEFFICIENT FIR FILTER

Following the half-band filters is a 66-tap, fixed-coefficient FIR

filter. This filter is useful in providing extra alias protection for

the decimating half-band filter. It is a simple sum-of-products

FIR filter with 66 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 20.

The user can either select or bypass this filter, but the FIR filter

can be enabled only when the half-band filter is enabled. Writing

Logic 0 to the enable FIR filter bit (Bit 0) in Register 0x102

bypasses this fixed-coefficient filter. The filter is necessary when

using the final NCO with a real output; bypassing it when using

other configurations results in power savings.

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