EVAL-AD7762EB View Datasheet(PDF) - Analog Devices

Part Name

Description

MFG CO.

EVAL-AD7762EB

625 kSPS, 24-Bit, 109 dB Σ−Δ ADC With On-Chip Buffer

Analog Devices 

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AD7762

PROGRAMMABLE FIR FILTER

As previously mentioned, the third FIR filter on the AD7762 is

user programmable. The default coefficients that are loaded on

reset are given in Table 10 and the frequency responses are

shown in Figure 31. The frequencies quoted in Figure 31 scale

directly with the output data rate.

Table 10. Default Filter Coefficients

Dec.

No. Value

Hex

Value

No.

0 53656736 332BCA0 24

1 25142688 17FA5A0 25

2 −4497814 444A196 26

3 −11935847 4B62067 27

4 −1313841 4140C31 28

5 6976334

6A734E 29

6 3268059

31DDDB 30

7 −3794610 439E6B2 31

8 −3747402 4392E4A 32

9 1509849

1709D9 33

10 3428088

344EF8 34

11 80255

1397F

35

12 −2672124 428C5FC 36

13 −1056628 4101F74 37

14 1741563

1A92FB 38

15 1502200

16EBF8 39

16 −835960

40CC178 40

17 −1528400 4175250 41

18 93626

16DBA

42

19 1269502

135EFE

43

20 411245

6466D

44

21 −864038

40D2F26 45

22 −664622

40A242E 46

23 434489

6A139

47

Dec.

Value

700847

−70922

−583959

−175934

388667

294000

−183250

−302597

16034

238315

88266

−143205

−128919

51794

121875

16426

−90524

−63899

45234

114720

102357

52669

15559

1963

Hex

Value

AB1AF

401150A

408E917

402AF3E

5EE3B

47C70

402CBD2

4049E05

3EA2

3A2EB

158CA

4022F65

401F797

CA52

1DC13

402A

401619C

400F99B

B0B2

1C020

18FD5

CDBD

3CC7

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 taps in non-

aliasing 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 a failure to meet the SNR specifications.

To create a filter, note the following:

• 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 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 exemplify 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 a 67108863 (0x3FF FFFF) positive

value over the half-impulse response coefficient set

(maximum 48 coefficients). Any deviation from this

introduces a gain error.

0

PASS-BAND RIPPLE = 0.05dB

–0.1dB FREQUENCY = 251kHz

–20

–3dB FREQUENCY = 256kHz

STOP BAND = 312.5kHz

–40

–60

–80

–100

–120

–140

–160

0

100

200

300

400

500

600

FREQUENCY (kHz)

Figure 31. Default Filter Frequency Response (625 kHz ODR)

The procedure for downloading a user-defined filter is detailed

in the Downloading a User-Defined Filter section.

Rev. 0 | Page 20 of 28

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