Datasheet

+V
REF
(2 1)-
23
-V
REF
(2 1)-
23
MSB MID LSB CHECKSUM
Bit7ofChecksum
or
Flag=1;Bit0ofLSBConversionData
24-BitConversionData(CHKSUM=0)
32-BitConversionData (CHKSUM=1)
£ -V
REF
2
23
2 1-
23
ADS1259
www.ti.com SBAS424D –JUNE 2009– REVISED AUGUST 2011
DATA FORMAT DATA CHECKSUM BYTE AND FLAG BIT
The ADS1259 outputs 24 bits of conversion data in An optional checksum byte can be appended to the
binary twos complement format, MSB first. The data conversion data bytes. The checksum makes the
LSB has a weight of V
REF
/(2
23
– 1). A positive data word length four bytes in length instead of three.
full-scale input produces an output code of 7FFFFFh The checksum byte is enabled by the register bit
and the negative full-scale input produces an output CHKSUM. The checksum itself is the least significant
code of 800000h. The output clips at these codes for byte sum of the three conversion data bytes, offset by
signals that exceed full-scale.
Table 16
summarizes the 9Bh. Note that the checksum byte option only applies
ideal output codes for different input signals. to the readback conversion data, not to register data.
The checksum is either seven bits or eight bits,
Table 16. Ideal Output Code versus Input Signal depending if the FLAG register bit is enabled. If the
FLAG bit is enabled the checksum is seven bits, with
DIFFERENTIAL INPUT SIGNAL V
IN
IDEAL OUTPUT
bit 7 replaced by the out-of-range flag. Figure 58 and
(AINP – AINN) CODE
(1)
Table 17 describe the combinations of the FLAG and
≥ V
REF
7FFFFFh
CHKSUM register bits.
Checksum = MSB data byte + Mid data byte + LSB
000001h
data byte + 9Bh.
0 000000h
FFFFFFh
800000h
Figure 58. 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
FLAG CHKSUM
byte and redundant data read capability. The
REGISTER REGISTER
checksum byte is the sum of three data conversion
BIT BIT DESCRIPTION
bytes, offset by 9Bh. Additionally, the data conversion
No checksum byte, no out-of-range
0 0
bytes may be read multiple times by continuing to
flag
shift data past the initial read of 24 bits (32 bits if
8-bit checksum byte, no
0 1
checksum is enabled).
out-of-range flag
No checksum byte, out-of-range
1 0 flag replaces LSB (bit 0) of
conversion data
7-bit checksum byte, out-of-range
1 1 replaces MSB (bit 7) of checksum
byte.
Copyright © 2009–2011, Texas Instruments Incorporated 29