Datasheet

AD7760
Rev. A | Page 30 of 36
PROGRAMMABLE FIR FILTER
As previously mentioned, the third FIR filter on the AD7760
is user programmable. The default coefficients that are loaded
upon reset are given in
Table 10, and the frequency responses
are shown in
Figure 57. The frequencies quoted in Figure 57
scale directly with the output data rate.
Table 10. Default Filter Coefficients
No.
Dec
Value
Hex
Value
No.
Dec
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, is nonaliasing, and provides
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 a failure to
meet the SNR specifications.
The default filter characteristics scale with both the MCLK
frequency applied and the decimation rate chosen by the user.
To create a filter, note the following:
• The filter must be an even, symmetrical FIR.
• The coefficients are in sign-and-magnitude format, with
26 magnitude bits and sign coded as positive = 0.
• 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 exemplify this with
only 48 coefficients listed for a 96-tap filter.
• Coefficients are written from the center of the impulse
response (adjacent to the point of symmetry) outwards.
• The coefficients are scaled so that the in-band gain of the
filter is equal to 134,217,726, with the coefficients rounded
to the nearest integer. For a low-pass filter, this is the
equivalent of having the coefficients summed arithmetically
(including sign) to a +67,108,863 (0x3FF FFFF) positive
value over the half-impulse-response coefficient set (a
maximum of 48 coefficients). Any deviation from this
introduces a gain error.
–20
–40
–60
–80
–100
–120
–140
–160
0 500 200015001000 2500
FREQUENCY (kHz)
AMPLITUDE (dB)
0
–0.1dB FREQUENCY = 1.004MHz
PASS-BAND RIPPLE = 0.05dB
STOP BAND = 1.25MHz
–3dB FREQUENCY = 1.06MHz
04975-044
Figure 57. Default Filter Frequency Response (2.5 MHz ODR)
The procedure for downloading a user-defined filter is detailed
in the
Downloading a User-Defined Filter section.