AD7291 Analog Devices, AD7291 Datasheet - Page 14

• Manufacturer Part Number: AD7291
• Description: 8-Channel, I2C, 12-Bit SAR ADC with Temperature Sensor
• Manufacturer: Analog Devices

Specifications of AD7291

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 22.2kSPS
• Interface: I²C/Ser 2-Wire,Ser
• Analog Input Type: SE-Uni
• Ain Range: Uni (Vref)
• Adc Architecture: SAR
• Pkg Type: CSP

AD7291
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:
The previous formulae are for a V
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
external reference voltage.
Table 7. Temperature Data Format
Temperature (°C)
−40
−25
−10
−0.25
0
+0.25
+10
+25
+50
+75
+100
+105
+125
Positive Temperature = ADC Code/4
Negative Temperature = (4096 − ADC Code)/4
Temperatur
e
=
V
EXT
_
REF
⎛
⎜
⎝
ADCCode
10
REF
Digital Output
1111 0110 0000
1111 1001 1100
1111 1101 1000
1111 1111 1111
0000 0000 0000
0000 0000 0001
0000 0010 1000
0000 0110 0100
0000 1100 1000
0001 0010 1100
0001 1001 0000
0001 1010 0100
0001 1111 0100
of 2.5 V only. If an
EXT_REF
+
109
3 .
is the value of the
⎞
⎟
⎠
−
273
.
15
Rev. B | Page 14 of 28
V
V
V
processors. For example, if the AD7291 is operated with a V
of 3.3 V, the V
This enables the AD7291 to operate with a larger dynamic
range with a V
1.8 V processors. Take care to ensure that V
exceed V
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
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
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 should 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 part 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.
DRIVE
DRIVE
DRIVE
controls the voltage at which the serial interface operates.
allows the ADC to easily interface to both 1.8 V and 3 V
REF
DD
pin, which should be decoupled to GND1 using a
by more than 0.3 V (see the Absolute Maximum
DRIVE
REF
DD
pin. On power-up, the internal reference is
of 3.3 V while still being able to interface to
pin can be powered from a 1.8 V supply.
DRIVE
Data Sheet
does not
DD