AD7760BCP datasheet(18/22 Pages) AD

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AD7760

Preliminary Technical Data

Rev. PrN | Page 18 of 22

DOWNLOADING A USER-DEFINED FILTER

As previously mentioned, the filter coefficients are 27 bits in

length; one sign and 26 magnitude bits. Since the AD7760 has a

16-bit parallel bus, the coefficients are padded with 5 MSB zeros

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

downloaded correctly, a checksum must also be generated and

downloaded following the final coefficient. The checksum is a

16-bit word generated by splitting each 32-bit word mentioned

above 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 AD7760 and

compared with the checksum downloaded. The DL_OK bit in

the Status Register is set if these two checksums agree. The

following is the procedure for downloading 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 about to be downloaded (See Table

9).

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.

Write the second half of the current coefficient data

(Magnitude [15:0]).

Repeat Steps 2 and 3 for each coefficient.

Write the 16-bit checksum.

There are two methods to verify that the filter

coefficients have been downloaded correctly:

Read the Status Register checking the

DL_OK bit.

Start reading data and observe the status of

the DL_OK bit.

Table 9. Filter Length Values

FLEN[3:0]

Num Coeffs

Filter Length

0000

Default

0001

0011

0101

0111

1001

1011

1101

1111

It should be borne in mind that since the user coefficients are

stored in RAM, they will be cleared after a RESET operation or

a loss of power..

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 20.

-80

-70

-60

-50

-40

-30

-20

-10

100

200

300

400

500

600

Frequency (kHz)

Figure 20. 24-Tap FIR Frequency Response

The coefficients for the filter are listed in Table 10. The

coefficients are in shown from the center of symmetry

outwards. The raw coefficients were generated using a

commercially available filter design tool and scaled

appropriately so their sum equals +67108863 (0x3FF FFFF).

Table 10. 24-Tap FIR Coefficients

Coeff

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

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