Datasheet

AD7291 Data Sheet
Rev. C | Page 14 of 28
Temperature Value Format
One LSB of the ADC corresponds to 0.25°C. The temperature
reading from the ADC is stored in a 12-bit twos complement
format, to accommodate both positive and negative tempera-
ture measurements. Sample temperature values are listed in
Table 7. The temperature conversion formulas are as follows:
Positive Temperature = ADC Code/4
Negative Temperature = (4096 − ADC Code)/4
The previous formulae are for a V
REF
of 2.5 V only. If an external
reference is used, the temperature sensor requires an external
reference of between 2 V and 2.5 V for correct operation. The
temperature results (in Celsius) are calculated using the
following formula, where V
EXT_REF
is the value of the external
reference voltage.
15.2733.109
10
_
−






+=
ADCCode
VeTemperatur
REFEXT
Table 7. Temperature Data Format
Temperature (°C) Digital Output
−40 1111 0110 0000
−25 1111 1001 1100
−10 1111 1101 1000
−0.25 1111 1111 1111
0 0000 0000 0000
+0.25 0000 0000 0001
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+25
0000 0110 0100
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V
DRIVE
V
DRIVE
controls the voltage at which the serial interface operates.
V
DRIVE
allows the ADC to easily interface to both 1.8 V and 3 V
processors. For example, if the AD7291 is operated with a V
DD
of 3.3 V, t h e V
DRIVE
pin can be powered from a 1.8 V supply.
This enables the AD7291 to operate with a larger dynamic
range with a V
DD
of 3.3 V while still being able to interface to
1.8 V processors. Take care to ensure that V
DRIVE
does not
exceed V
DD
by more than 0.3 V (see the Absolute Maximum
Ratings section).
THE INTERNAL OR EXTERNAL REFERENCE
The AD7291 can operate with either the internal 2.5 V on-chip
reference or an externally applied reference. The EXT_REF bit
in the command register is used to determine whether the internal
reference is used. If the EXT_REF bit is selected in the command
register, an external reference can be supplied through the V
REF
pin.
On power-up, the internal reference is enabled. Suitable external
reference sources for the AD7291 include AD780, AD1582,
ADR431, REF193, and ADR391.
The internal reference circuitry consists of a 2.5 V band gap
reference and a reference buffer. When the AD7291 operates in
internal reference mode, the 2.5 V internal reference is available
at the V
REF
pin, which must be decoupled to GND1 using a 10 μF
capacitor. It is recommended that the internal reference be
buffered before applying it elsewhere in the system.
The internal reference is capable of sourcing up to 2 mA of
current when the converter is static. The reference buffer
requires 5.5 ms to power up and charge the 10 μF decoupling
capacitor during the power-up time.
RESET
The AD7291 includes a reset feature, which can be used to reset
the device and the content of all internal registers including the
command register to their default state. To activate the reset
operation, the
PD
/
RST
pin must be brought low for a minimum of
1 ns and a maximum of 100 ns and be asynchronous to the clock;
therefore, it can be triggered at any time. If the
PD
/
RST
pin is held
low for greater than 100 ns, the device enters full power-down
mode. It is imperative that the
PD
/
RST
pin be held at a stable logic
level at all times to ensure normal operation.