AD9671KBCZ datasheet(26/61 Pages) AD

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Data Sheet

AD9671

Rev. A| Page 25 of 60

Antialiasing Filter (AAF)

The filter that the signal reaches prior to the ADC is used to

reject dc signals and to band limit the signal for antialiasing.

The antialiasing filter is a combination of a single-pole, high-pass

filter and a second-order, low-pass filter. Configure the high-

pass filter as a ratio of the low-pass filter cutoff frequency using

Address 0x02B, Bits[1:0].

The filter uses on-chip tuning to trim the capacitors and, in turn, to

set the desired low-pass cutoff frequency and reduce variations.

The default −3 dB low-pass filter cutoff is 1/3, 1/4.5, or 1/6 of the

ADC sample clock rate. The cutoff can be scaled to 0.75, 0.8, 0.9,

1.0, 1.13, 1.25, or 1.45 times this frequency using Address 0x00F.

The cutoff tolerance (±10%) is maintained from 8 MHz to 18 MHz

for low bandwidth mode or 13.5 MHz to 30 MHz for high

bandwidth mode.

Table 11 and Table 12 calculate the valid SPI-selectable low-pass

filter settings and expected cutoff frequencies for the low band-

width and high bandwidth modes at the minimum sample

frequency and the maximum sample frequency in each speed

mode.

Tuning is normally off to avoid changing the capacitor settings

during critical times. The tuning circuit is enabled through the

SPI. It is disabled automatically after 512 cycles of the ADC sample

clock. Initialize the tuning of the filter after initial power-up and

after reprogramming of the filter cutoff scaling or the ADC

sample rate. The tuning is initiated using Address 0x02B, Bit 6.

Four SPI-programmable settings allow users to vary the high-

pass filter cutoff frequency as a function of the low-pass cutoff

frequency. Two examples are shown in Table 13: an 8 MHz low-

pass cutoff frequency and an 18 MHz low-pass cutoff frequency. In

both cases, as the ratio decreases, the amount of rejection on the

low end frequencies increases. Therefore, making the entire AAF

frequency pass band narrow can reduce low frequency noise or

maximize dynamic range for harmonic processing.

Table 11. SPI-Selectable Low-Pass Filter Cutoff Options for Low Bandwidth Mode at Example Sampling Frequencies

Address

0x00F,

Bits[7:3]

LPF Cutoff

Frequency (MHz)

Sampling Frequency (MHz)

20.5

125

0 0000

1.45 × (1/3) × fSAMPLE

9.91

Out of tunable filter

range

Out of tunable filter

range

Out of tunable filter

range

Out of tunable filter

range

0 0001

1.25 × (1/3) × fSAMPLE

8.54

16.67

Out of tunable filter

range

Out of tunable filter

range

Out of tunable filter

range

0 0010

1.13 × (1/3) × fSAMPLE

Out of tunable filter

range

15.00

Out of tunable filter

range

Out of tunable filter

range

Out of tunable filter

range

0 0011

1.0 × (1/3) × fSAMPLE

Out of tunable filter

range

13.33

Out of tunable filter

range

Out of tunable filter

range

Out of tunable filter

range

0 0100

0.9 × (1/3) × fSAMPLE

Out of tunable filter

range

12.00

Out of tunable filter

range

Out of tunable filter

range

Out of tunable filter

range

0 0101

0.8 × (1/3) × fSAMPLE

Out of tunable filter

range

10.67

17.33

Out of tunable filter

range

Out of tunable filter

range

0 0110

0.75 × (1/3) × fSAMPLE

Out of tunable filter

range

10.00

16.25

16.82

Out of tunable filter

range

0 1000

1.45 × (1/4.5) × fSAMPLE

Out of tunable filter

range

12.89

20.94

Out of tunable filter

range

Out of tunable filter

range

0 1001

1.25 × (1/4.5) × fSAMPLE

Out of tunable filter

range

11.11

18.06

Out of tunable filter

range

Out of tunable filter

range

0 1010

1.13 × (1/4.5) × fSAMPLE

Out of tunable filter

range

10.00

16.25

Out of tunable filter

range

Out of tunable filter

range

0 1011

1.0 × (1/4.5) × fSAMPLE

Out of tunable filter

range

8.89

14.44

17.78

Out of tunable filter

range

0 1100

0.9 × (1/4.5) × fSAMPLE

Out of tunable filter

range

8.00

13.00

16.00

Out of tunable filter

range

0 1101

0.8 × (1/4.5) × fSAMPLE

Out of tunable filter

range

Out of tunable filter

range

11.56

14.22

Out of tunable filter

range

0 1110

0.75 × (1/4.5) × fSAMPLE

Out of tunable filter

range

Out of tunable filter

range

10.83

13.33

17.50

AD9671KBCZ Datasheet(PDF) 26 Page - Analog Devices

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