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參數資料
| 型號： | AD7760BCP |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | 2.5 MSPS, 20-Bit ADC |
| 中文描述： | 1-CH 20-BIT DELTA-SIGMA ADC, PARALLEL ACCESS, QCC48 |
| 封裝： | LFCSP-48 |
| 文件頁數： | 18/22頁 |
| 文件大小： | 868K |
| 代理商： | AD7760BCP |
AD7760
Preliminary Technical Data
DOWNLOADING A USER-DEFINED FILTER
As previously mentioned, the filter coefficients are 27 bits in
length; one sign and 26 magnitude bits. Since the AD7760 has a
16-bit parallel bus, the coefficients are padded with 5 MSB zeros
to generate a 32-bit word and split into two 16-bit words for
downloading. The first 16-bit word for each coefficient becomes
(00000, Sign bit, Magnitude[25:16]), while the second word
becomes (Magnitude [15:0]). To ensure that a filter is
downloaded correctly, a checksum must also be generated and
downloaded following the final coefficient. The checksum is a
16-bit word generated by splitting each 32-bit word mentioned
above into 4 bytes and summing all bytes from all coefficients
up to a maximum of 192 bytes (48 coefficients × 4 bytes). The
same checksum is generated internally in the AD7760 and
compared with the checksum downloaded. The DL_OK bit in
the Status Register is set if these two checksums agree. The
following is the procedure for downloading a user filter:
Rev. PrN | Page 18 of 22
1.
Write to Control Register 1 setting the DL_Filt bit and
also the correct filter length bits corresponding to the
length of the filter about to be downloaded (See Table
9).
2.
Write the first half of the current coefficient data
(00000, Sign bit, Magnitude[25:16]). The first
coefficient to be written must be the one adjacent to
the point of filter symmetry.
3.
Write the second half of the current coefficient data
(Magnitude [15:0]).
4.
Repeat Steps 2 and 3 for each coefficient.
5.
Write the 16-bit checksum.
6.
There are two methods to verify that the filter
coefficients have been downloaded correctly:
a.
Read the Status Register checking the
DL_OK bit.
b.
Start reading data and observe the status of
the DL_OK bit.
Table 9. Filter Length Values
FLEN[3:0]
0000
0001
0011
0101
0111
1001
1011
1101
1111
Num Coeffs
Default
6
12
18
24
30
36
42
48
Filter Length
Default
12
24
36
48
60
72
84
96
It should be borne in mind that since the user coefficients are
stored in RAM, they will be cleared after a RESET operation or
a loss of power..
EXAMPLE FILTER DOWNLOAD
The following is an example of downloading a short user
defined filter with 24-taps. The frequency response is shown in
Figure 20.
-80
-70
-60
-50
-40
-30
-20
-10
0
10
0
100
200
300
400
500
600
Frequency (kHz)
A
Figure 20. 24-Tap FIR Frequency Response
The coefficients for the filter are listed in Table 10. The
coefficients are in shown from the center of symmetry
outwards. The raw coefficients were generated using a
commercially available filter design tool and scaled
appropriately so their sum equals +67108863 (0x3FF FFFF).
Table 10. 24-Tap FIR Coefficients
Coeff
1
2
3
4
5
6
7
8
9
10
11
12
Raw
Scaled
53188232
29300796
1402406
-11017834
-6236822
2902466
5302774
1104856
-3137108
-3621978
-1801582
-277343
0.365481974
0.201339905
0.009636604
-0.075708848
-0.042856209
0.019944246
0.036437914
0.007592007
-0.021556583
-0.024888355
-0.012379538
-0.001905756
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