Also, during this cycle, the sampling capacitor is discon-
nected from the converter’s analog input and is connected
across REFIN. A gain calibration is initiated and proceeds
over the next three conversions. After this, the input capaci-
tor is once again connected to the analog input. Conversions
proceed as usual over the next three cycles in order to fill the
digital filter. DRDY remains HIGH during this time. On the
next cycle, the DRDY signal goes LOW indicating valid
data, the input to the sampling capacitor is shorted, and an
offset calibration is initiated. At this point, the Background
Calibration sequence repeats.
This will be an important consideration in many systems
which use a 2.5V or greater reference, as the input range is
constrained by the expected power supply variations. In
addition, the expected full-scale voltage will impact the
allowable offset voltage (and vice-versa) as the combination
of the two must remain within the power supply and ground
potentials, regardless of the results obtained via the range
calculation shown previously.
There are only two solutions to this constraint: either the
system design must ensure that the full-scale and offset
voltage variations will remain within the power supply and
ground potentials, or the part must be used in a gain of 2 or
greater.
In essence, the Background Calibration Mode performs
continuous self-calibration where the offset and gain cali-
brations are interleaved with regular conversions. Thus, the
data rate is reduced by a factor of 6. The advantage is that
the converter is continuously adjusting to environmental
changes such as ambient or component temperature (due to
airflow variations).
SLEEP MODE
The Sleep Mode is entered after the bits 110 have been
written to the Command Register Operation Mode bits
(MD2 through MD0). This mode is exited by entering a new
mode into the MD2-MD0 bits.
The ADS1212/13 will remain in the Background Calibra-
tion Mode indefinitely. To move to any other mode, the
Command Register Operation Mode bits (MD2 through
MD0) must be set to the appropriate values.
The Sleep Mode causes the analog section and a good deal
of the digital section to power down. For full analog power
down, the VBIAS generator and the internal reference must
also be powered down by setting the BIAS and REFO bits
in the Command Register accordingly. The power dissipa-
tion shown in the Specifications Table is with the internal
reference and the VBIAS generator disabled.
System Calibration Offset and Full-Scale
Calibration Limits
The System Offset and Full-Scale Calibration range of the
ADS1212/13 is limited and is listed in the Specifications
Table. The range is specified as:
To establish serial communication with the converter while
it is in Sleep Mode, one of the following procedures must be
used: If CS is being used, simply taking CS LOW will
enable serial communication to proceed normally. If CS is
not being used (tied LOW) and the ADS1212/13 is in the
Master Mode, then a falling edge must be produced on the
SDIO line. If SDIO is LOW, the SDIO line must be taken
HIGH for 4 • tXIN periods (minimum) and then taken LOW.
Alternatively, SDIO can be forced HIGH after putting the
ADS1212/13 to “sleep” and then taken LOW when the
Sleep Mode is to be exited. Finally, if CS is not being used
(tied LOW) and the ADS1212/13 is in the Slave Mode, then
simply sending a normal Instruction Register command will
re-establish communication.
(VFS – | VOS |) < 1.3 • (2 • REFIN)/GAIN
(VFS – | VOS |) > 0.7 • (2 • REFIN)/GAIN
where VFS is the system full-scale voltage and | VOS | is the
absolute value of the system offset voltage. In the following
discussion, keep in mind that these voltages are differential
voltages.
For example, with the internal reference (2.5V) and a gain of
two, the previous equations become (after some manipulation):
VFS – 3.25 < VOS < VFS – 1.75
If VFS is perfect at 2.5V (positive full-scale), then VOS must
be greater than –0.75V and less than 0.75V. Thus, when offset
calibration is performed, the positive input can be no more
than 0.75V below or above the negative input. If this range is
exceeded, the ADS1212/13 may not calibrate properly.
Once serial communication is resumed, the Sleep Mode is
exited by changing the MD2-MD0 bits to any other mode.
When a new mode (other than Sleep) has been entered, the
ADS1212/13 will execute a very brief internal power-up
sequence of the analog and digital circuitry. Once this has
been done, one normal conversion cycle is performed before
the new mode is actually entered. At the end of this conversion
cycle, the new mode takes effect and the converter will
respond accordingly. The DRDY signal will remain HIGH
through the first conversion cycle. It will also remain HIGH
through the second, even if the new mode is the Normal Mode.
This calculation method works for all gains other than one.
For a gain of one and the internal reference (2.5V), the
equation becomes:
VFS – 6.5 < VOS < VFS – 3.5
With a 5V positive full-scale input, VOS must be greater than
–1.5V and less than 1.5V. Since the offset represents a
common-mode voltage and the input voltage range in a gain
of one is 0V to 5V, a common-mode voltage will cause the
actual input voltage to possibly go below 0V or above 5V.
The specifications also show that for the specifications to be
valid, the input voltage must not go below AGND by more
than 30mV or above AVDD by more than 30mV.
If the VBIAS generator and/or the internal reference have
been disabled, then they must be manually re-enabled via the
appropriate bits in the Command Register. In addition, the
internal reference will have to charge the external bypass
capacitor(s) and possibly other circuitry. There may also be
ADS1212, 1213
16
SBAS064A
BB [ BURR-BROWN CORPORATION ]
