Datasheet

CalibrationTime=
50
f
OSC
32
f
MOD
16
f
DATA
+
+
ADS1246
ADS1247
ADS1248
SBAS426G –AUGUST 2008–REVISED OCTOBER 2011 www.ti.com
Calibration Commands
disconnected from the internal circuitry and a zero
differential signal is applied internally. With both offset
The ADS1246/7/8 provide commands for three types
calibrations the offset calibration register (OFC) is
of calibration: system gain calibration, system offset
updated afterwards. When either offset calibration
calibration and self offset calibration. Where absolute
command is issued, the ADS1246/7/8 stop the
accuracy is needed, it is recommended that
current conversion and start the calibration procedure
calibration be performed after power on, a change in
immediately.
temperature, a change of PGA and in some cases a
change in channel. At the completion of calibration,
Calibration Timing
the DRDY signal goes low indicating the calibration is
finished. The first data after calibration are always
When calibration is initiated, the device performs 16
valid. If the START pin is taken low or a SLEEP
consecutive data conversions and averages the
command is issued after any calibration command,
results to calculate the calibration value. This
the devices goes to sleep after completing calibration.
provides a more accurate calibration value. The time
required for calibration is shown in Table 14 and can
It is important to allow a pending system calibration to
be calculated using Equation 9:
complete before issuing any other commands.
Issuing commands during a calibration can result in
corrupted data. If this occurs either resend the
(9)
calibration command that was aborted or issue a
device reset.
ADC POWER-UP
When DVDD is pulled up, the internal power-on reset
System Gain Calibration
module generates a pulse that resets all digital
System gain calibration corrects for gain error in the
circuitry. All the digital circuits are held in a reset
signal path. The system gain calibration is initiated by
state for 2
16
system clocks to allow the analog circuits
sending the SYSGCAL command while applying a
and the internal digital power supply to settle. SPI
full-scale input to the selected analog inputs.
communication cannot occur until the internal reset is
Afterwards the full-scale calibration register (FSC) is
released.
updated. When a system gain calibration command is
issued, the ADS1246/7/8 stop the current conversion
ADC SLEEP MODE
and start the calibration procedure immediately.
Power consumption can be dramatically reduced by
System Offset and Self Offset Calibration placing the ADS1246/7/8 into sleep mode. There are
two ways to put the device into sleep mode: the sleep
System offset calibration corrects both internal and
command (SLEEP) and through the START pin.
external offset errors. The system offset calibration is
initiated by sending the SYSGOCAL command while During sleep mode, the internal reference status
applying a zero differential input (V
IN
= 0) to the depends on the setting of the VREFCON bits in the
selected analog inputs. The self offset calibration is MUX1 register; see the Register Descriptions section
initiated by sending the SELFOCAL command. for details.
During self offset calibration, the selected inputs are
Table 14. Calibration Time versus Data Rate
DATA RATE (SPS) CALIBRATION TIME (ms)
(1)
5 3201.01
10 1601.01
20 801.012
40 400.26
80 200.26
160 100.14
320 50.14
640 25.14
1000 16.14
2000 8.07
(1) For f
OSC
= 4.096MHz.
32 Submit Documentation Feedback Copyright © 2008–2011, Texas Instruments Incorporated
Product Folder Link(s): ADS1246 ADS1247 ADS1248