AD6679 datasheet(49/81 Pages) AD

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Data Sheet

AD6679

Rev. B | Page 49 of 81

FIR FILTERS

OVERVIEW

There are four sets of decimate by 2, low-pass, half-band, FIR

filters (labeled HB1 FIR, HB2 FIR, HB3 FIR, and HB4 FIR in

Figure 63) following the frequency translation stage. After the

carrier of interest is tuned down to dc (carrier frequency =

0 Hz), these filters efficiently lower the sample rate, while

providing sufficient alias rejection from unwanted adjacent

carriers around the bandwidth of interest.

HB1 FIR is always enabled and cannot be bypassed. The HB2,

HB3, and HB4 FIR filters are optional and can be bypassed for

higher output sample rates.

Table 20 shows the different bandwidths selectable by including

different half-band filters. In all cases, the DDC filtering stage

on the AD6679 provides <−0.001 dB of pass-band ripple and

>100 dB of stop band alias rejection.

Table 21 shows the amount of stop-band alias rejection for

multiple pass-band ripple/cutoff points. The decimation ratio of

the filtering stage of each DDC can be controlled individually

through Bits[1:0] of the DDC control registers (Register 0x310,

HALF-BAND FILTERS

The AD6679 offers four half-band filters to enable digital signal

processing of the ADC converted data. These half-band filters

are bypassable and can be individually selected.

HB4 Filter

The first decimate by 2, half-band, low-pass, FIR filter (HB4)

uses an 11-tap, symmetrical, fixed coefficient filter implementa-

tion that is optimized for low power consumption. The HB4

filter is used only when complex outputs (decimate by 16) or

real outputs (decimate by 8) are enabled; otherwise, it is

bypassed. Table 19 and Figure 67 show the coefficients and

response of the HB4 filter.

Table 19. HB4 Filter Coefficients

HB4 Coefficient

Number

Normalized

Coefficient

Decimal

Coefficient (15-Bit)

C1, C11

0.006042

C2, C10

C3, C9

−0.049316

−808

C4, C8

C5, C7

0.293273

4805

0.500000

8192

–120

–100

–80

–60

–40

–20

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

NORMALIZED FREQUENCY (× π RAD/SAMPLE)

Figure 67. HB4 Filter Response

Table 20. DDC Filter Characteristics

ADC

Sample

Rate

(MSPS)

Half Band Filter

Selection

Real Output

Complex (I/Q) Output

Alias

Protected

Bandwidth

(MHz)

Ideal SNR

Improve-

ment1 (dB)

Pass-

Band

Ripple

(dB)

Alias

Rejection

(dB)

Decima-

tion Ratio

Output

Sample Rate

(MSPS)

Decima-

tion

Ratio

Output Sample

Rate (MSPS)

500

HB1

500

250 (I) + 250 (Q)

192.5

<−0.001

>100

HB1 + HB2

250

125 (I) + 125 (Q)

96.3

HB1 + HB2 + HB3

125

62.5 (I) + 62.5 (Q)

48.1

HB1 + HB2 + HB3

+ HB4

62.5

31.25 (I) + 31.25 (Q)

24.1

Ideal SNR improvement due to oversampling and filtering = 10log(bandwidth/(fS/2)).

Table 21. DDC Filter Alias Rejection

Alias Rejection

(dB)

Pass-Band Ripple/Cutoff

Point (dB)

Alias Protected Bandwidth for Real

(I) Outputs1

Alias Protected Bandwidth for Complex

(I/Q) Outputs

>100

<−0.001

<38.5% × fOUT

<77% × fOUT

<−0.001

<38.7% × fOUT

<77.4% × fOUT

<−0.001

<38.9% × fOUT

<77.8% × fOUT

63.3

<−0.006

<40% × fOUT

<80% × fOUT

−0.5

44.4% × fOUT

88.8% × fOUT

19.3

−1.0

45.6% × fOUT

91.2% × fOUT

10.7

−3.0

48% × fOUT

96% × fOUT

fOUT = ADC input sample rate ÷ DDC decimation.

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