EVAL-AD7291SDZ Analog Devices Inc, EVAL-AD7291SDZ Datasheet - Page 13

• Manufacturer Part Number: EVAL-AD7291SDZ
• Description: Evaluation Control Board
• Manufacturer: Analog Devices Inc
• Series: -r

Specifications of EVAL-AD7291SDZ

• Rohs Compliant: YES
• Silicon Manufacturer: Analog Devices
• Kit Application Type: Interface
• Application Sub Type: ADC
• Features: Channel Sequencer Operation, Alert Function, Autocycle Mode
• Number Of Adc's: 8
• Number Of Bits: 12
• Sampling Rate (per Second): 22.22k
• Data Interface: I²C
• Input Range: -
• Power (typ) @ Conditions: -
• Operating Temperature: -
• Utilized Ic / Part: AD7291
• Silicon Core Number: AD7291
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

ADC TRANSFER FUNCTION
The output coding of the AD7291 is straight binary for the
analog input channel conversion results and twos complement
for the temperature conversion result. The designed code tran-
sitions occur at successive LSB values (that is, 1 LSB, 2 LSBs,
and so forth). The LSB size is V
ideal transfer characteristic for the AD7291 for straight binary
coding is shown in Figure 21.
TEMPERATURE SENSOR OPERATION
The AD7291 contains one local temperature sensor. The on-chip,
band gap temperature sensor measures the temperature of the
AD7291 die.
The temperature sensor module on the AD7291 is based on the
three current principle (see Figure 22), where three currents are
passed through a diode and the forward voltage drop is
measured, allowing the temperature to be calculated free of
errors caused by series resistance.
NOTES
1. V
I
111...111
111...110
111...000
011...111
REF
TRANSISTOR
000...010
000...001
000...000
Figure 22. Top Level Structure of Internal Temperature Sensor
INTERNAL
IS 2.5V.
SENSE
Figure 21. Straight Binary Transfer Characteristic
4 × I
0V
1LSB
8 × I
I
BIAS
ANALOG INPUT
BIAS
DIODE
REF
1LSB = V
/4096 for the AD7291. The
V
DD
+V
REF
REF
/4096
– 1LSB
V
TO ADC
V
OUT+
OUT–
Rev. 0 | Page 13 of 28
Each input integrates, in turn, over a period of several hundred
microseconds. This takes place continuously in the background,
leaving the user free to perform conversions on the other
channels. When integration is complete, a signal passes to the
control logic to initiate a conversion automatically.
If the ADC is in command mode and performing a voltage
conversion, the AD7291 waits for it to complete and then
initiates a temperature sensor conversion. If the ADC is not
performing voltage conversions, temperature conversions occur
at 5 ms intervals.
In autocycle mode, the conversion is inserted into an
appropriate place in the current sequence. If the ADC is idle,
the conversion takes place immediately. The T
result register stores the result of the last conversion on the
temperature channel; this can be read at any time.
Theoretically, the temperature measuring circuit can measure
temperatures from −512°C to +511°C with a resolution of
0.25°C. However, temperatures outside T
perature range for the AD7291) are outside the guaranteed
operating temperature range of the device. The temperature
sensor is enabled by setting the TSENSE bit in the command
register.
TEMPERATURE SENSOR AVERAGING
The AD7291 incorporates a temperature sensor averaging
feature to enhance the accuracy of the temperature measure-
ments. The temperature averaging feature is performed
continuously in the background provided the TSENSE bit
in the command register is enabled. The temperature is
measured each time a T
moving average method is used to determine the result in the
T
following equation:
The average result is then available in the T
register whose content is updated after every T
The first T
temperature sensor has been selected in the command register
(Bit D7) is the actual first T
result remains valid until the next T
completed and the result register is updated.
T
SENSE
SENSE
AVG
average result register. The average result is given by the
SENSE
=
7
8
(
Previous
conversion result given by the AD7291 after the
_
SENSE
Average
SENSE
conversion is performed and a
conversion result, and this
_
Result
SENSE
)
+
conversion is
A
1
8
(the specified tem-
SENSE
(
Current
SENSE
SENSE
average result
conversion
conversion.
AD7291
_
Result
)