AD7763 datasheet(26/32 Pages) AD

26 / 32 page

AD7763

Rev. 0 | Page 26 of 32

EXAMPLE FILTER DOWNLOAD

The following is an example of downloading a short, user-defined

filter with 24 taps. The frequency response is shown in Figure 38.

–80

600

FREQUENCY (kHz)

–10

–20

–30

–40

–50

–60

–70

100

200

300

400

500

Figure 38. 24-Tap FIR Frequency Response

The coefficients for the filter in Table 14 are shown from the center

of symmetry outward; that is, Coefficient 1 is the coefficient at the

center of symmetry. The raw coefficients were generated using a

commercial filter design tool and scaled appropriately so their

sum equals 67108863 (0x3FF FFFF).

Table 14. 24-Tap FIR Coefficients

Coefficient

Raw

Scaled

+0.365481974

+53188232

+0.201339905

+29300796

+0.009636604

+1402406

−0.075708848

−11017834

−0.042856209

−6236822

+0.019944246

+2902466

+0.036437914

+5302774

+0.007592007

+1104856

−0.021556583

−3137108

−0.024888355

−3621978

−0.012379538

−1801582

−0.001905756

−277343

Table 15 shows the 32-bit word (as per the format shown in the

Downloading a User-Defined Filter section) in hexadecimal for

each of the coefficients that must be written to the AD7763 to

realize this filter. The table is also split into the bytes that are all

summed to produce the checksum. The checksum generated

from these coefficients is 0x0E6B.

Table 15. Filter Hex Values1

32-Bit Word Written to Download Coefficient

Coefficient

Byte 1

Byte 2

Byte 3

Byte 4

1 All values of words listed are with reference to writing to one device only

(ALL = 0) with Address 000 (as assigned to the device using the ADR[2:0]

pins).

Table 16 lists in hexadecimal format the sequence of 32-bit

words the user writes to the AD7763 to set up the ADC and

download this filter, assuming selection of an output data rate

of 625 kHz.

Table 16.

Word1

Description

0x0001807A

Address of Control Register 1. Control register

data. DL filter, set filter length = 24, set output

data rate = 625 kHz.

0x032B9688

First coefficient.

0x01BF183C

Second coefficient.

…

Other coefficients.

0x04043B5F

Twelfth (final) coefficient.

0x00000E6B

Checksum. Wait (0.5 × tICLK × number of unused

coefficients) for AD7763 to fill remaining unused

coefficients with 0s.

0x0001087A

Address of Control Register. Control register data.

Set read status and maintain filter length and

decimation settings. Read contents of status

the filter downloaded correctly.

1 All values of words listed are with reference to writing to one device only

(ALL = 0) with Address 000 (as assigned to the device using the ADR[2:0]

pins).

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