AD7763 Analog Devices, AD7763 Datasheet - Page 25

• Manufacturer Part Number: AD7763
• Description: 24-Bit, 625 kSPS, 109 dB Sigma-Delta ADC with On-Chip Buffers, Serial Interface
• Manufacturer: Analog Devices

Specifications of AD7763

• Resolution (bits): 24bit
• # Chan: 1
• Sample Rate: 40MSPS
• Interface: I2S,Ser
• Analog Input Type: Diff-Bip
• Ain Range: 4 V p-p,6.5 V p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: QFP

DOWNLOADING A USER-DEFINED FILTER
As discussed in the Programmable FIR Filter section, each of
the filter coefficients is 27 bits in length: one sign bit and 26 magni-
tude bits. To download coefficients for a user-specific FIR filter, a
32-bit word is written to the AD7763 for each coefficient.
D31
ALL
When a user writes coefficients to one device, the address of that
particular device (as assigned by the ADR[2:0] pins) must be
specified in the bits labeled ADR[2:0].
In a configuration where more than one device shares the same SDI
line, setting the ALL bit to logic high and leaving Address Bits
ADR[2:0] logic low enables the user to write each coefficient to all
devices simultaneously.
To ensure that a filter is downloaded correctly, a checksum must
be generated and downloaded following the download of 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 (maximum number
of coefficients = 48 bytes × 4 bytes written for each coefficient).
The checksum is written to the device in the form of a 32-bit word
in the following format:
D31
ALL
Note that when writing the checksum, the addressing requirements
are as before, and Bit 27 to Bit 16 are all set to 0.
The same checksum is generated internally in the AD7763 and
compared with the checksum downloaded. The DL_OK bit in
the status register is set if these two checksums agree.
D30
ADR2
D30
ADR2
D29
ADR1
D29
ADR1
D28
ADR0
D28
ADR0
D27
0
D[27:16]
0
D26
Sign
D[25:0]
Magnitude
D[15:0]
Checksum
Rev. A | Page 25 of 32
To download a user-defined filter:
Note that because the user coefficients are stored in RAM, they
are cleared after a RESET operation or a loss of power.
Table 13. Filter Length Values
FLEN[3:0]
0000
0001
0011
0101
0111
1001
1011
1101
1111
1.
2.
3.
4.
5.
Write to Control Register 1, setting the DL Filt bit. The
correct Filter Length Bits FLEN[3:0] correspond
to the length of the filter about to be downloaded
(see Table 13) and the correct decimation rate.
Write the 32-bit word (as per format specified). The
first coefficient to be written must be the one adjacent
to the point of filter symmetry.
Repeat Step 2 for each coefficient.
Implement the checksum write as per the specified
format.
Use the following methods to verify that the filter
coefficients have been downloaded correctly:
•
•
Read the status register, checking the DL_OK bit.
Start reading data and observe the status of the
DL_OK bit.
Number of Coefficients
Default
6
12
18
24
30
36
42
48
Filter Length
Default
12
24
36
48
60
72
84
96
AD7763