AD7763 datasheet(24/32 Pages) AD

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AD7763

Rev. 0 | Page 24 of 32

PROGRAMMABLE FIR FILTER

As discussed in the Theory of Operation section, the third FIR

filter on the AD7763 can be programmed by the user. The default

coefficients that are loaded on reset are shown in Table 12. This

gives the frequency response shown in Figure 37. The frequencies

shown in Figure 37 scale directly with the output data rate.

Table 12. Default Filter Coefficients

Decimal

Value

Hex

Value

Decimal

Value

Hex

Value

+53656736

332BCA0

+700847

AB1AF

+25142688

17FA5A0

−70922

401150A

−4497814

444A196

−583959

408E917

−11935847

4B62067

−175934

402AF3E

−1313841

4140C31

+388667

5EE3B

+6976334

6A734E

+294000

47C70

+3268059

31DDDB

−183250

402CBD2

−3794610

439E6B2

−302597

4049E05

−3747402

4392E4A

+16034

3EA2

+1509849

1709D9

+238315

3A2EB

+3428088

344EF8

+88266

158CA

+80255

1397F

−143205

4022F65

−2672124

428C5FC

−128919

401F797

−1056628

4101F74

+51794

CA52

+1741563

1A92FB

+121875

1DC13

+1502200

16EBF8

+16426

402A

−835960

40CC178

−90524

401619C

−1528400

4175250

−63899

400F99B

+93626

16DBA

+45234

B0B2

+1269502

135EFE

+114720

1C020

+411245

6466D

+102357

18FD5

−864038

40D2F26

+52669

CDBD

−664622

40A242E

+15559

3CC7

+434489

6A139

+1963

7AB

The default filter should be sufficient for most applications.

It is a standard brick wall filter with a symmetrical impulse

response. The default filter has a length of 96 taps and is

nonaliasing, with 120 dB of attenuation at Nyquist. This filter

not only performs signal antialiasing but also suppresses out-of-

band quantization noise produced by the analog-to-digital

conversion process. Any significant relaxation in the stop-band

attenuation or transition bandwidth relative to the default filter

can result in failure to meet the SNR specifications.

To create a user-defined filter, note the following:

• The filter must be even, symmetrical FIR.

• The coefficients are 27 bits in length. All coefficients are

in sign-and-magnitude format. The sign bit coded as

positive = 0 is followed by 26 magnitude bits.

• The filter length must be between 12 taps and 96 taps in

steps of 12.

• Because the filter is symmetrical, the number of coefficients

that must be downloaded is half the filter length. The default

filter coefficients are an example of this, with only 48

coefficients listed for a 96-tap filter.

• Coefficients are written from the center of impulse response

(adjacent to the point of symmetry) outward.

• The coefficients are scaled so that the in-band gain

of the filter is equal to 134217726, with the coefficients

rounded to the nearest integer. For a low-pass filter, this is the

equivalent of having the coefficients sum arithmetically

(including sign) to +67108863 (0x3FFFFFF) positive value

over the half-impulse-response coefficient set (maximum 48

coefficients). Any deviation from this results in the

introduction of a gain error.

–20

–40

–60

–80

–100

–120

–140

–160

100

400

500

300

200

600

FREQUENCY (kHz)

–0.1dB FREQUENCY = 251kHz

PASS-BAND RIPPLE = 0.05dB

STOP BAND = 312.5kHz

–3dB FREQUENCY = 256kHz

Figure 37. Default Filter Frequency Response (625 kHz ODR)

To download a user-defined filter, see the Downloading a User-

Defined Filter section.

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