Datasheet

AD7763 Data Sheet
Rev. B | Page 24 of 32
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
Hex
Value
0 +53656736 332BCA0 24 +700847 AB1AF
1 +25142688 17FA5A0 25 −70922 401150A
2 −4497814 444A196 26 −583959 408E917
3 −11935847 4B62067 27 −175934 402AF3E
4 −1313841 4140C31 28 +388667 5EE3B
5 +6976334 6A734E 29 +294000 47C70
6 +3268059 31DDDB 30 −183250 402CBD2
7 −3794610 439E6B2 31 −302597 4049E05
8 −3747402 4392E4A 32 +16034 3EA2
9 +1509849 1709D9 33 +238315 3A2EB
10 +3428088 344EF8 34 +88266 158CA
11 +80255 1397F 35 −143205 4022F65
12 −2672124 428C5FC 36 −128919 401F797
13 −1056628 4101F74 37 +51794 CA52
14 +1741563 1A92FB 38 +121875 1DC13
15 +1502200 16EBF8 39 +16426 402A
16 −835960 40CC178 40 −90524 401619C
17 −1528400 4175250 41 −63899 400F99B
18 +93626 16DBA 42 +45234 B0B2
19 +1269502 135EFE 43 +114720 1C020
20 +411245 6466D 44 +102357 18FD5
21 −864038 40D2F26 45 +52669 CDBD
22 −664622 40A242E 46 +15559 3CC7
23 +434489 6A139 47 +1963 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.
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.
–20
–40
–60
–80
–100
–120
–140
–160
0 100 400 500300200 600
FREQUENCY (kHz)
AMPLITUDE (dB)
0
05476-022
–0.1dB FREQUENCY = 251kHz
PASS-BAND RIPPLE = 0.05dB
STOP BAND = 312.5kHz
–3dB FREQUENCY = 265kHz
Figure 37. Default Filter Frequency Response (625 kHz ODR)
To download a user-defined filter, see the Downloading a User-
Defined Filter section.