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AD6674

Data Sheet

FIR FILTERS

GENERAL DESCRIPTION

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

impulse response (FIR) filters (labeled HB1 FIR, HB2 FIR, HB3

FIR, and HB4 FIR in Figure 99) 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 17 shows the different bandwidths selectable by including

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

on the AD6674 provides <−0.001 dB of pass-band ripple and

>100 dB of stop-band alias rejection.

Table 18 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,

Table 17. DDC Filter Characteristics

ADC

Sample

Rate

(MSPS)

Half Band

Filter

Selection

Real Output

Complex (I/Q) Output

Alias

Protected

Bandwidth

(MHz)

Ideal SNR

Improvement1

(dB)

Pass-

Band

Ripple

(dB)

Alias

Rejection

(dB)

Decimation

Ratio

Output

Sample

Rate

(MSPS)

Decimation

Ratio

Output Sample Rate

(MSPS)

1000

HB1

1000

500 (I) + 500 (Q)

385.0

<−0.001

>100

HB1 + HB2

500

250 (I) + 250 (Q)

192.5

HB1 + HB2 +

HB3

250

125 (I) + 125 (Q)

96.3

HB1 + HB2 +

HB3 + HB4

125

62.5 (I) + 62.5 (Q)

48.1

750

HB1

750

375 (I) + 375 (Q)

288.8

HB1 + HB2

375

187.5 (I) + 187.5 (Q)

144.4

HB1 + HB2 +

HB3

187.5

93.75 (I) + 93.75 (Q)

72.2

HB1 + HB2 +

HB3 + HB4

93.75

46.875 (I) + 46.875 (Q)

36.1

500

HB1

500

250 (I) + 250 (Q)

192.5

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 18. 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.

Rev. B | Page 46 of 91

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