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AD7762

Rev. 0 | Page 21 of 28

DOWNLOADING A USER-DEFINED FILTER

EXAMPLE FILTER DOWNLOAD

As previously mentioned, the filter coefficients are 27 bits in

length; 1 sign and 26 magnitude bits. Because the AD7762

has a 16-bit parallel bus, the coefficients are padded with

5 MSB 0s to generate a 32-bit word and split into two 16-bit

words for downloading. The first 16-bit word for each coefficient

becomes (00000, Sign bit, Magnitude [25:16]), while the second

word becomes (Magnitude [15:0]). To ensure that a filter is

down-loaded correctly, a checksum must also be generated and

then downloaded following the final coefficient. The checksum

is a 16-bit word generated by splitting each 32-bit word into

4 bytes and summing all bytes from all coefficients up to a

maximum of 192 bytes (48 coefficients × 4 bytes). The same

checksum is generated internally in the AD7762 and compared

with the checksum downloaded. The DL_OK bit in the status

The following is an example of downloading a short user-

defined filter with 24 taps. The frequency response is shown

in Figure 32.

–80

600

FREQUENCY (kHz)

–10

–20

–30

–40

–50

–60

–70

100

200

300

400

500

To download a user filter:

Write to Control Register 1, setting the DL_Filt bit and

also the correct filter length bits corresponding to the

length of the filter to be downloaded (see Table 11).

Figure 32. 24-Tap FIR Frequency Response

Write the first half of the current coefficient data

(00000, Sign bit, Magnitude [25:16]). The first coefficient

to be written must be the one adjacent to the point of filter

symmetry.

The coefficients for the filter are listed in Table 12 and are

shown from the center of symmetry outwards. The raw

coefficients were generated using a commercial filter

design tool and scaled appropriately so their sum equals

67108863 (0x3FF FFFF).

Write the second half of the current coefficient data

(Magnitude [15:0]).

Table 12. 24-Tap FIR Coefficients

Coefficient

Raw

Scaled

Repeat Step 2 and Step 3 for each coefficient.

0.365481974

53188232

0.201339905

29300796

Write the 16-bit checksum.

0.009636604

1402406

Use these methods to verify that the filter coefficients are

downloaded correctly:

−0.075708848

−11017834

−0.042856209

−6236822

0.019944246

2902466

a. Read the status register, checking the DL_OK bit.

0.036437914

5302774

b. Read data and observe the status of the DL_OK bit.

0.007592007

1104856

−0.021556583

−3137108

Note that because the user coefficients are stored in RAM, they

are cleared after a

−0.024888355

−3621978

RESET operation or a loss of power.

−0.012379538

−1801582

Table 11. Filter Length Values

−0.001905756

−277343

FLEN[3:0]

Number of Coefficients

Filter Length

0000

Default

0001

0011

0101

0111

1001

1011

1101

1111

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