ADS1259
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SBAS424C –JUNE 2009–REVISED MARCH 2010
DATA FORMAT
DATA CHECKSUM BYTE AND FLAG BIT
The ADS1259 outputs 24 bits of conversion data in
binary twos complement format, MSB first. The data
LSB has a weight of VREF/(223 – 1). A positive
full-scale input produces an output code of 7FFFFFh
and the negative full-scale input produces an output
An optional checksum byte can be appended to the
conversion data bytes. The checksum makes the
data word length four bytes in length instead of three.
The checksum byte is enabled by the register bit
CHKSUM. The checksum itself is the least significant
byte sum of the three conversion data bytes, offset by
9Bh. Note that the checksum byte option only applies
to the readback conversion data, not to register data.
The checksum is either seven bits or eight bits,
depending if the FLAG register bit is enabled. If the
FLAG bit is enabled the checksum is seven bits, with
bit 7 replaced by the out-of-range flag. Figure 57 and
Table 17 describe the combinations of the FLAG and
CHKSUM register bits.
code of 800000h. The output clips at these codes for
Table 16
signals that exceed full-scale.
summarizes the
ideal output codes for different input signals.
Table 16. Ideal Output Code versus Input Signal
DIFFERENTIAL INPUT SIGNAL VIN
(AINP – AINN)
IDEAL OUTPUT
CODE(1)
≥ VREF
7FFFFFh
+VREF
Checksum = MSB data byte + Mid data byte + LSB
data byte + 9Bh.
000001h
(223 - 1)
0
000000h
FFFFFFh
32-Bit Conversion Data (CHKSUM = 1)
24-Bit Conversion Data (CHKSUM = 0)
-VREF
(223 - 1)
MSB
MID
LSB
CHECKSUM
Bit 7 of Checksum
223
223 - 1
Flag = 1; Bit 0 of LSB Conversion Data
800000h
£ -VREF
or
Figure 57. Checksum Byte and Out-of-Range Flag
(1) Excludes effects of noise, linearity, offset, and gain errors.
DATA INTEGRITY
Table 17. Checksum Byte and Over-Range Flag
Data readback integrity is augmented by a checksum
byte and redundant data read capability. The
checksum byte is the sum of three data conversion
bytes, offset by 9Bh. Additionally, the data conversion
bytes may be read multiple times by continuing to
shift data past the initial read of 24 bits (32 bits if
checksum is enabled).
FLAG
CHKSUM
REGISTER REGISTER
BIT
BIT
DESCRIPTION
No checksum byte, no out-of-range
flag
0
0
8-bit checksum byte, no
out-of-range flag
0
1
1
0
No checksum byte, out-of-range
flag replaces LSB (bit 0) of
conversion data
7-bit checksum byte, out-of-range
replaces MSB (bit 7) of checksum
byte.
1
1
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