AD7725 datasheet(19/28 Pages) AD

19 / 28 page

REV. A

AD7725

–19–

Figure 20 shows an example of a filtering function implemented

on the postprocessor. Figure 20a shows the data path representa-

tion of an FIR filter, while Figure 20b shows how this algorithm

would be implemented on the AD7725. Because the postprocessor

can implement three filter taps per MAC block, 1.3 MAC

blocks are required to implement a 4-tap FIR filter. This is a

useful guideline when calculating the design requirements for a

new application.

SIGNAL

Z–1

SIGNAL

OUT

Z–1

a) FIR Data Path Representation

SIGNAL

OUT

Z–1

1 MAC BLOCK

b) FIR Postprocessor Implementation

Figure 20. AD7725 Postprocessor Mapping

PROGRAMMING THE POSTPROCESSOR

The postprocessor is programmed by loading a user-defined

filter in the form of a configuration file into the device.

Generating a Configuration File to Load into the Postprocessor

A user-defined configuration file can be generated to load into

the postprocessor on the AD7725 to program the multipliers

and accumulators to perform user-specific filtering require-

ments. The configuration file can be generated using a digital

filter design package called Filter Wizard, which is available

from the Analog Devices website.

Filter Wizard

This digital filter design package allows the user to design differ-

ent filter types and then generates the appropriate configuration

file to be loaded into the postprocessor. This application includes

the ability to specify a range of different filter options including

single or multistage; normalized or user-specified output

frequency; FIR or IIR; low-pass, band-pass; Window type;

pass-band frequency and ripple; stop-band frequency, attenua-

tion and ripple; daisy-chaining and interlacing. It also informs

the user of the power dissipation of the AD7725 associated

with the particular filter designed. This is to avoid filters being

designed that result in the device exceeding its maximum power

specifications. The magnitude, phase, and impulse responses

can be plotted so that the user knows the filter response (cutoff

frequency, transition width, attenuation) before generating the

coefficients. Once the filter characteristics have been decided,

the configuration file is generated and will be ready for loading

into the postprocessor.

Filter Configuration File Format

The configuration file that is generated by the Filter Wizard is

made up of 8272 bits of data. The first word in the file is called

the ID word, and the device will accept the configuration file only

if this is 0x7725. The rest of the configuration data is split into 12

blocks of 672 bits. The AD7725 postprocessor therefore accepts

672 bits at a time (42, 16-bit words). Each block of 672 bits is

followed by a cyclic redundancy check (CRC) word. The ID

word and the CRC words are used by the device to check for

errors in the configuration file and are not actually written to the

postprocessor. The postprocessor therefore holds 8064 bits of

data (672

12). See the Serial Mode and Parallel Mode sections

for further information on how configuration errors are detected

and handled. The filter coefficients in the configuration file that

are loaded into the postprocessor have 24-bit precision and have

a value in the range –8

≤ coefficient < +8. The coefficients are

made up of 1 sign bit, 3 magnitude bits left of the decimal point,

and 20 right of the decimal point.

Using the Internal Default Filter

The AD7725 has a default filter stored in internal ROM that

can be loaded into the postprocessor. This functionality allows

the user to evaluate the device without having to download a

configuration file. The default filter is a two-stage, low-pass, FIR

filter whose specifications are directly related to the CLKIN

frequency. With a CLKIN frequency of 9.6 MHz, the default

filter has a cutoff frequency of 49 kHz and a stop-band frequency

of 72.7 kHz. This filter has a total decimation by 4, which occurs

in the first stage, resulting in the output data being available to the

interface at a frequency of CLKIN/32. For more detailed specifica-

tions on this filter see the Preset Filter, Default Filter, and

Postprocessor Characteristics section. When powered up in

boot-from-ROM mode, the AD7725 will automatically load the

default filter characteristic into the postprocessor. Figure 21

shows the default filter response, when operating with a 9.6 MHz

CLKIN frequency.

FREQUENCY – kHz

150

100

–140

–120

–100

–80

–60

–40

–20

–160

Figure 21. Default Filter Response for CLKIN = 9.6 MHz

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