AD7761 datasheet(44/76 Pages) AD

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Data Sheet

AD7761

Rev. A | Page 43 of 75

Wideband Low Ripple Filter

The wideband filter, referred to as a brick wall filter, has a low ripple

pass band, within ±0.005 dB of ripple, of 0.4 × ODR. The wideband

filter has full attenuation at 0.499 × ODR (Nyquist), maximizing

antialias protection. The wideband filter has a pass-band ripple of

±0.005 dB and a stop band attenuation of 105 dB from Nyquist

out to fCHOP. For more information on antialiasing and fCHOP

aliasing, see the Antialiasing section.

The wideband filter is a very high order digital filter with a group

delay of approximately 34/ODR. After a synchronization pulse,

there is an additional delay from the SYNC_IN rising edge to

fully settled data. The settling times for the AD7761 when using

the wideband filter are shown in Table 25. See Table 10 for the

noise performance of the wideband filter across power modes

and decimation rates.

Filter Settling Time

The AD7761 digital filters are resynchronized on the rising edge

of the SYNC_IN signal. This resynchronization should be

provided after power-up in pin control mode or SPI control

mode, and after any reconfiguration of the device in SPI control

mode, prior to capturing ADC samples. Once the SYNC_IN

rising edge is provided, there is a deterministic delay until the

first new conversion result is available, and until the first settled

data is available.

Table 25 and Table 26 provide these delays, measured in MCLK

cycles, for wideband and sinc5 filters, respectively, for each

possible setting of MCLK_DIV. Each table provides the delays

for configurations in which all channels are using the exact

same configuration (Group B unused), and for configurations

in which one or more channels have a different decimation rate

applied (Group B is used).

For example, if a user configures channels with the wideband

filter and MCLK_DIV = MCLK/4, and assigns some channels

to Group A with decimate x32 and others to Group B with

decimate x64, then the delay until the first DRDY after the

SYNC_IN is 758 MCLK periods. All active channels then output

the first data after 758 MCLK periods. However, due to differing

decimation rates across channels, in this case the first settled

data becomes available for the Group A channels 8,822 MCLK

periods after SYNC_IN, and after 17,014 MCLK periods for

Group B channels.

–10

–30

–50

–70

–90

–100

–110

–120

–130

–140

0.2

0.1

0.3

0.4

0.5

0.6

0.7

0.9

0.8

1.0

NORMALIZED INPUT FREQUENCY (

fIN/fODR)

–20

–40

–60

–80

Figure 64. Wideband Filter Frequency Response

–0.010

0.010

0.008

–0.008

0.006

–0.006

0.004

–0.004

0.002

–0.002

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

NORMALIZED INPUT FREQUENCY (

fIN/fODR)

Figure 65. Wideband Filter Pass-Band Ripple

1.2

1.0

0.8

0.6

0.4

0.2

–0.2

OUTPUT DATA RATE SAMPLES

Figure 66. Wideband Filter Step Response

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