AD7762 Analog Devices, AD7762 Datasheet - Page 20

• Manufacturer Part Number: AD7762
• Description: Parallel Interface, 625 kSPS, 24-Bit Sigma-Delta A/D Converter
• Manufacturer: Analog Devices

Specifications of AD7762

• Resolution (bits): 24bit
• # Chan: 1
• Sample Rate: 40MSPS
• Interface: Par
• Analog Input Type: Diff-Uni
• Ain Range: (1.6Vref) p-p,4 V p-p,6.5 V p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: QFP

AD7762
PROGRAMMABLE FIR FILTER
As previously mentioned, the third FIR filter on the AD7762 is
user programmable. The default coefficients that are loaded on
reset are given in
shown in Figure 31. The frequencies quoted in Figure 31 scale
directly with the output data rate.
Table 10. Default Filter Coefficients
No.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
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 taps in non-
aliasing 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 a failure to meet the SNR specifications.
Dec.
Value
53656736
25142688
−4497814
−11935847
−1313841
6976334
3268059
−3794610
−3747402
1509849
3428088
80255
−2672124
−1056628
1741563
1502200
−835960
−1528400
93626
1269502
411245
−864038
−664622
434489
Table 10 and the frequency responses are
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
No.
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
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
AB1AF
401150A
408E917
402AF3E
5EE3B
47C70
402CBD2
4049E05
3EA2
3A2EB
158CA
4022F65
401F797
CA52
1DC13
402A
401619C
400F99B
B0B2
1C020
18FD5
CDBD
3CC7
7AB
Rev. 0 | Page 20 of 28
To create a filter, note the following:
•
•
•
•
•
•
The procedure for downloading a user-defined filter is detailed
in the Downloading a User-Defined Filter section.
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 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 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 a 67108863 (0x3FF FFFF) positive
value over the half-impulse response coefficient set
(maximum 48 coefficients). Any deviation from this
introduces a gain error.
–100
–120
–140
–160
–20
–40
–60
–80
Figure 31. Default Filter Frequency Response (625 kHz ODR)
0
0
100
200
FREQUENCY (kHz)
300
–0.1dB FREQUENCY = 251kHz
PASS-BAND RIPPLE = 0.05dB
–3dB FREQUENCY = 256kHz
STOP BAND = 312.5kHz
400
500
600