
Preliminary Technical Data
AD7760
PROGRAMMABLE FIR FILTER
As previously mentioned, the third FIR filter on the AD7760 is
user programmable. The default coefficients that are loaded on
reset are given in Table 8. This gives a frequency response
shown in Figure 19. The frequencies quoted in Figure 19 scale
directly with the Output Data Rate.
Rev. PrN | Page 17 of 22
Table 8. Default Filter Coefficients
#
Dec. Value
Hex
Value
332BCA0
17FA5A0
444A196
4B62067
4140C31
6A734E
31DDDB
439E6B2
4392E4A
1709D9
344EF8
1397F
428C5FC
4101F74
1A92FB
16EBF8
40CC178
4175250
16DBA
135EFE
6466D
40D2F26
40A242E
6A139
#
Dec.
Value
700847
-70922
-583959
-175934
388667
294000
-183250
-302597
16034
238315
88266
-143205
-128919
51794
121875
16426
-90524
-63899
45234
114720
102357
52669
15559
1963
Hex Value
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
53656736
25142688
-4497814
-11935847
-1313841
6976334
3268059
-3794610
-3747402
1509849
3428088
80255
-2672124
-1056628
1741563
1502200
-835960
-1528400
93626
1269502
411245
-864038
-664622
434489
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
AB1AF
401150A
408E917
402AF3E
5EE3B
47C70
402CBD2
4049E05
3EA2
3A2EB
158CA
4022F65
401F797
CA52
1DC13
402A
401619C
400F99B
B0B2
1C020
18FD5
CDBD
3CC7
7AB
The default filter should be sufficient for almost all applications.
It is a standard brick wall filter with a symmetrical impulse
response. The default filter has a length of 96, in non-aliasing
with 120dB of attenuation at Nyquist. This filter not only
performs signal anti-aliasing but also suppresses out-of-band
quantization noise produced by the A-D conversion process.
Any significant relaxation in the stop-band attenuation or
transition band width relative to the default filter may result in a
failure to meet the SNR specifications.
If a user does wish to create their own filter then the following
should be noted:
The filter must be 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 and 96 in steps
of 12.
As the filter is symmetrical, the number of coefficients
that must be downloaded will be 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)
outwards.
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 (0x3FF
FFFF) positive value over the half-impulse-response
coefficient set (max 48 coefficients). Any deviation
from this will result in a gain error being introduced.
-160
-140
-120
-100
-80
-60
-40
-20
0
0
500
1000
1500
2000
2500
Frequency (kHz)
A
Passband Ripple = 0.05 dB
-0.1dB Frequency = 1.004MHz
-3dB Frequency = 1.06MHz
Stopband = 1.25MHz
Figure 19. Default Filter Frequency Response (2.5MHz ODR)
The procedure for downloading a user-defined filter is detailed
in the Downloading a User-Defined Filter section.