EVAL-AD7760/AD7762EDZ

Rev. PrB | Page 11 of 23

DOWNLOADING A USER-DEFINED FILTER

The final stage of the AD7760/2 FIR filter can be programmed

to suit the user’s specific requirements. The filter designed must

correspond with the requirements listed in the datasheet – it

must be a symmetrical filter with an even number of

coefficients. The number of coefficients can be from 12 to 96.

• Due to the symmetry of the filter, the coefficients will

repeat. Thus, only half of the coefficients are required to

be sent to the AD7760/2 device.

• The AD7760/2 software reads the coefficients from a

text file and writes each coefficient to the AD7760/2.

The filter file should contain the correct checksum,

which is also written to the ADC, and the file must be in

the correct format.

14. An example of this format is shown in Figure 12. This

example corresponds to the example digital filter in the

Downloading A User Defined Filter section in the

AD7760/2 datasheet.

15. Each of the 32 bit words (in this case there are 12 words for

the coefficients + the checksum) are in a Hexadecimal

format – there must also be no spaces between the

characters and no text, spaces or notation before or after

any of the hex words to be written. The software

implements a text read from each line of the file and then

translates this value to the correct binary 32 –bit word to

be written to the AD7760/2 as per the description in the

datasheet.

Figure 12.Example of format required for filter coefficients to be downloaded

correctly.

16. To download a filter,

a)

Select the number of coefficients that will be

downloaded – this is half the filter length – so if you

have a 24 tap filter you select 12 in the drop down

selection shown in Figure 13.

Figure 13.Number of coefficients to be sent to the AD7760/2 device.

b)

Now click on the folder icon and browse to the

location of the filter text file. The install will have

placed a copy of the datasheet example at Program

files/Analog Device/ AD7760_2 this file is called

“filter_12c.txt”

Figure 14.Browse to select the specific text file.

c)

Finally click the “Configure Filter” button to

download the user-defined filter.

d)

Once this is done then, click the “Sample” button to

acquire a batch of samples from the AD7760/2 device.

e)

The filter example used in the AD7760/2 datasheet

will show a slower transition band. The FFT of this

filter implementation is shown in Figure 15. As

highlighted, the status bits output by the ADC will

show if the filter file has been downloaded correctly.

The indicators for the DL_OK, FILT_OK and UFILT

are all asserted to show a successful filter download.