AD7762 Analog Devices, AD7762 Datasheet - Page 21

• 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

DOWNLOADING A USER-DEFINED FILTER
As previously mentioned, the filter coefficients are 27 bits in
length; 1 sign and 26 magnitude bits. Because the AD7762
has a 16-bit parallel bus, the coefficients are padded with
5 MSB 0s 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
down-loaded correctly, a checksum must also be generated and
then downloaded following the final coefficient. The checksum
is a 16-bit word generated by splitting each 32-bit word 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 AD7762 and compared
with the checksum downloaded. The DL_OK bit in the status
register is set if these two checksums agree.
To download a user filter:
Table 11. Filter Length Values
FLEN[3:0]
0000
0001
0011
0101
0111
1001
1011
1101
1111
1.
2.
3.
4.
5.
6.
Note that because the user coefficients are stored in RAM, they
are cleared after a
downloaded correctly:
Write to Control Register 1, setting the DL_Filt bit and
also the correct filter length bits corresponding to the
length of the filter to be downloaded (see Table 11).
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.
Write the second half of the current coefficient data
(Magnitude [15:0]).
Repeat Step 2 and Step 3 for each coefficient.
Write the 16-bit checksum.
Use these methods to verify that the filter coefficients are
b. Read data and observe the status of the DL_OK bit.
a. Read the status register, checking the DL_OK bit.
RESET operation or a loss of power.
Number of Coefficients
Default
6
12
18
24
30
36
42
48
Filter Length
Default
12
24
36
48
60
72
84
96
Rev. 0 | Page 21 of 28
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 32.
The coefficients for the filter are listed in Table 12 and are
shown from the center of symmetry outwards. The raw
coefficients were generated using a commercial filter
design tool and scaled appropriately so their sum equals
67108863 (0x3FF FFFF).
Table 12. 24-Tap FIR Coefficients
Coefficient
1
2
3
4
5
6
7
8
9
10
11
12
–10
–20
–30
–40
–50
–60
–70
–80
10
0
0
Figure 32. 24-Tap FIR Frequency Response
100
Raw
0.365481974
0.201339905
0.009636604
−0.075708848
−0.042856209
0.019944246
0.036437914
0.007592007
−0.021556583
−0.024888355
−0.012379538
−0.001905756
200
FREQUENCY (kHz)
300
400
Scaled
53188232
29300796
1402406
−11017834
−6236822
2902466
5302774
1104856
−3137108
−3621978
−1801582
−277343
500
AD7762
600