AD8284WCSVZ datasheet(14/28 Pages) AD

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AD8284

Data Sheet

Rev. D | Page 14 of 28

Antialiasing Filter

The AAF uses a combination of poles and zeros to create a

third-order elliptic filter. An elliptic filter is used to achieve

a sharp roll-off after the cutoff frequency. This architecture

achieves a −30 dB per octave roll-off in the first octave after

the cutoff frequency.

The filter uses on-chip tuning to trim the internal resistors and

capacitors to set the desired cutoff frequency. The tuning method

reduces variations in the cutoff frequency due to standard IC

process tolerances of resistors and capacitors.

The default tuning settings for a −3 dB low-pass filter cutoff is

1/3 × 1.125 × the ADC sample clock frequency. This setting can

be changed to 1/4 the ADC sample clock frequency. The cutoff

can also be scaled from 0.75 to 1.25 (in 0.0625 increments)

times these frequencies through the SPI.

Tuning is normally off and is initiated by the user via the SPI

port. After the filter is tuned to a specific frequency, it remains

at that frequency until another tuning sequence is initiated. The

tuning process can take up to 2048 clock cycles.

The filter defaults to its highest frequency setting before it is

tuned. To maintain the expected ratio of clock frequency to

cutoff frequency, tune the filter after initial power-up, after

reprogramming the filter cutoff scaling via the SPI, or after

changing the ADC sample rate. Occasional retuning during an

idle time is recommended to compensate for temperature drift.

A cutoff frequency range of 9 MHz to 15 MHz is possible, for

example

•

ADC clock: 40 MHz

•

Default tuned cutoff frequency = (40 MHz ÷ 3) × 1.125 =

15 MHz

The autotune cycle takes several clock cycles to complete. During

this time, the mux channels, A to D, are not operational; however,

the AUX input can be used during the autotuning cycle.

Saturation Flag

The saturation flag function detects overvoltage conditions that

may push the LNA or PGA out of their linear regions. The flag

is set when the PGA output voltage exceeds 2.0 V p-p or the

LNA output voltage exceeds 4.0 V p-p. This function is particularly

useful for detecting saturation events that may be filtered out by

the AAF and are, therefore, undetectable by monitoring the

ADC output.

When the saturation flag trips, it remains on for a minimum of

25 ns after the saturation event has ended.

Figure 16. Simplified Block Diagram of the Analog Channel

INx+

INx–

MUX

VMID

SATURATION DETECTION

ADC

200Ω

20kΩ

200kΩ

200Ω

200kΩ

50Ω

2pF

LNA

PGA

AAF

+REF

–REF

+REF

–REF

AD8284

AD8284WCSVZ Datasheet(PDF) 14 Page - Analog Devices

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