Datasheet

Ch Seq Control
Reg
AIN0
AIN1
AIN2
AIN3/7
AINGND
ADC
AVDD AGND
Config
Reg
Temp
Sensor
ADS8634
ADS8638
www.ti.com SBAS541A –MAY 2011– REVISED AUGUST 2011
These devices allow the use of an external reference in the range of 2.0V to 3.0V. The nominal input ranges
±10V, ±5V, ±2.5V, 0V to 5V, and 0V to 10V assume a 2.5V reference; a different reference voltage scales the
full-scale ranges proportionately. For example, if a 3.0V reference is used and the ±10V range is selected, the
actual input range is scaled by (3.0/2.5) for a full-scale range of ±12V.
The internal reference can be enabled/disabled through the configuration register. The reference block is
powered down when the internal reference is disabled. Ensure that the internal reference is disabled when the
external reference is connected. The external reference is the default selection after power-on or reset.
TEMPERATURE SENSOR
The ADS8634/8 feature an on-chip temperature sensoras shown in Figure 75. The device temperature can be
read at any time during a scan, either in auto or manual mode.There are three registers associated with the
temperature sensor operation. The temperature sensor can be enabled/disabled through the Aux-Config
configuration register. Disabling the temperature sensor powers down the temperature block. It is necessary to
enable (power up) the temperature sensor at least one cycle before the device temperature sensor is selected
with the channel sequencing control registers (manual/auto). This selection overrides the input channel scan
sequence and range selection and connects the ADC input to an internal temperature sensor. The temperature
sensor must be deselected with channel sequencing control registers (manual/auto) to resume normal scanning.
In case of auto-sequencing, the device starts scanning from where it left off before the temperature
measurement. The temperature sensor is disabled by default after power-on or reset.
Figure 75. Reading the ADS8634/8 Temperature
(Refer to Table 11 for more details on configuration register settings)
The temperature sensor transfer function follows a straight line, as shown in Equation 1:
Output Code = m
REF
× Device Temperature in °C + C
REF
(1)
Equation 1 can be re-written as Equation 2:
Device Temperature in °C = (Output Code – C
REF
)/m
REF
where:
m
REF
= the slope,
and C
REF
= the offset (in ADC output code) of the temperature sensor transfer function (2)
Both m
REF
and C
REF
change with the reference voltage. The initial values of m
REF
and C
REF
at a 2.5V reference
are: m
REF_2.5
= 0.47 and C
REF_2.5
= 3777.2
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Product Folder Link(s): ADS8634 ADS8638