AD7760BCP datasheet(17/22 Pages) AD

17 / 22 page

Preliminary Technical Data

AD7760

Rev. PrN | Page 17 of 22

PROGRAMMABLE FIR FILTER

As previously mentioned, the third FIR filter on the AD7760 is

user programmable. The default coefficients that are loaded on

reset are given in Table 8. This gives a frequency response

shown in Figure 19. The frequencies quoted in Figure 19 scale

directly with the Output Data Rate.

Table 8. Default Filter Coefficients

Dec. Value

Hex

Value

Dec.

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 almost all applications.

It is a standard brick wall filter with a symmetrical impulse

response. The default filter has a length of 96, in non-aliasing

with 120dB of attenuation at Nyquist. This filter not only

performs signal anti-aliasing but also suppresses out-of-band

quantization noise produced by the A-D conversion process.

Any significant relaxation in the stop-band attenuation or

transition band width relative to the default filter may result in a

failure to meet the SNR specifications.

If a user does wish to create their own filter then the following

should be noted:

•

The filter must be even, symmetrical FIR.

•

The coefficients are in sign-and-magnitude format

with 26 magnitude bits and sign coded as positive=0.

•

The filter length must be between 12 and 96 in steps

of 12.

•

As the filter is symmetrical, the number of coefficients

that must be downloaded will be 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)

outwards.

•

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 (0x3FF

FFFF) positive value over the half-impulse-response

coefficient set (max 48 coefficients). Any deviation

from this will result in a gain error being introduced.

-160

-140

-120

-100

-80

-60

-40

-20

500

1000

1500

2000

2500

Frequency (kHz)

Passband Ripple = 0.05 dB

-0.1dB Frequency = 1.004MHz

-3dB Frequency = 1.06MHz

Stopband = 1.25MHz

Figure 19. Default Filter Frequency Response (2.5MHz ODR)

The procedure for downloading a user-defined filter is detailed

in the Downloading a User-Defined Filter section.

AD7760BCP датащи(PDF) 17 Page - Analog Devices

AD7760BCP датащи(HTML) 17 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

датащи скачать

ссылки URL