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SBAS288D − JUNE 2003 − REVISED AUGUST 2004
System Calibration
SERIAL INTERFACE
System calibration corrects both internal and external
offset and gain errors using the SYSOCAL and SYSGCAL
commands. During system calibration, the appropriate
calibration signals must be applied by the user to the
inputs.
The SPI-compatible serial interface consists of four
signals: CS, SCLK, DIN, and DOUT, and allows a
controller to communicate with the ADS1255/6. The
programmable functions are controlled using a set of
on-chip registers. Data is written to and read from these
registers via the serial interface
SYSOCAL performs a system offset calibration. The user
must supply a zero input differential signal. The
ADS1255/6 then computes a value that will nullify the
offset in the system. Table 22 shows the time required for
system offset calibration for the different data rate settings.
Note this timing is the same for the self offset calibration.
System offset calibration updates the OFC register.
The DRDY output line is used as a status signal to indicate
when a conversion has been completed. DRDY goes low
when new data is available. The Timing Specification
shows the timing diagram for interfacing to the
ADS1255/6.
SYSGCAL performs a system gain calibration. The user
must supply a full-scale input signal to the ADS1255/6.
The ADS1255/6 then computes a value to nullify the gain
error in the system. System gain calibration can correct
inputs that are 80% of the full-scale input voltage and
larger. Make sure not to exceed the full-scale input voltage
when using system gain calibration. Table 22 shows the
time required for system gain calibration for the different
data rate settings. System gain calibration updates the
FSC register.
CHIP SELECT (CS)
The chip select (CS) input allows individual selection of a
ADS1255/6 device when multiple devices share the serial
bus. CS must remain low for the duration of the serial
communication. When CS is taken high, the serial
interface is reset and DOUT enters a high impedance
state. CS may be permanently tied low.
SERIAL CLOCK (SCLK)
The serial clock (SCLK) features a Schmitt-triggered input
and is used to clock data on the DIN and DOUT pins into
and out of the ADS1255/6. Even though the input has
hysteresis, it is recommended to keep SCLK as clean as
possible to prevent glitches from accidentally shifting the
data. If SCLK is held low for 32 DRDY periods, the serial
interface will reset and the next SCLK pulse will start a new
communication cycle. This timeout feature can be used to
recover communication when a serial interface transmis-
sion is interrupted. A special pattern on SCLK will reset the
chip; see the RESET section for more details on this
procedure.
Table 22. System Gain Calibration Timing
DATA RATE
SYSTEM GAIN CALIBRATION TIME
(SPS)
30,000
15,000
7500
3750
2000
1000
500
100
60
417µs
484µs
617µs
884µs
1.4ms
2.4ms
4.4ms
20.4ms
33.7ms
40.4ms
67.0ms
80.4ms
133.7ms
200.4ms
400.4ms
800.4ms
DATA INPUT (DIN) AND DATA OUTPUT (DOUT)
The data input pin (DIN) is used along with SCLK to send
data to the ADS1255/6. The data output pin (DOUT) along
with SCLK is used to read data from the ADS1255/6. Data
on DIN is shifted into the part on the falling edge of SCLK
while data is shifted out on DOUT on the rising edge of
SCLK. DOUT is high impedance when not in use to allow
DIN and DOUT to be connected together and be driven by
a bi-directional bus. Note: the RDATAC command must
not be issued while DIN and DOUT are connected
together.
50
30
25
15
10
5
2.5
NOTE: For f
= 7.68MHz.
CLKIN
Auto-Calibration
Auto-calibration can be enabled (ACAL bit in ADCON
register) to have the ADS1255/6 automatically initiate a
self-calibration at the completion of a write command
(WREG) that changes the data rate, PGA setting, or Buffer
status.
26
BB [ BURR-BROWN CORPORATION ]
