EVAL-AD7763EB View Datasheet(PDF) - Analog Devices

Part Name

Description

MFG CO.

EVAL-AD7763EB

24-Bit, 625 kSPS, 109 dB Σ-Δ ADC with On-Chip Buffers, Serial Interface

Analog Devices 

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AD7763

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

0 +53656736 332BCA0 24 +700847

1 +25142688 17FA5A0 25 −70922

2 −4497814 444A196 26 −583959

3 −11935847 4B62067 27 −175934

4 −1313841 4140C31 28 +388667

5 +6976334 6A734E 29 +294000

6 +3268059 31DDDB 30 −183250

7 −3794610 439E6B2 31 −302597

8 −3747402 4392E4A 32 +16034

9 +1509849 1709D9 33 +238315

10 +3428088 344EF8 34 +88266

11 +80255

1397F 35 −143205

12 −2672124 428C5FC 36 −128919

13 −1056628 4101F74 37 +51794

14 +1741563 1A92FB 38 +121875

15 +1502200 16EBF8 39 +16426

16 −835960

40CC178 40 −90524

17 −1528400 4175250 41 −63899

18 +93626

16DBA 42 +45234

19 +1269502 135EFE 43 +114720

20 +411245

6466D 44 +102357

21 −864038

40D2F26 45 +52669

22 −664622

40A242E 46 +15559

23 +434489

6A139 47 +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 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.

Data Sheet

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.

0

PASS-BAND RIPPLE = 0.05dB

–0.1dB FREQUENCY = 251kHz

–20

–3dB FREQUENCY = 265kHz

STOP BAND = 312.5kHz

–40

–60

–80

–100

–120

–140

–160

0

100

200

300

400

500

600

FREQUENCY (kHz)

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

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

Defined Filter section.

Rev. B | Page 24 of 32

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