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AD7291 Fiches technique(PDF) 8 Page - Analog Devices

No de pièce AD7291
Description  8-Channel, I2C, 12-Bit SAR ADC with Temperature Sensor
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Fabricant  AD [Analog Devices]
Site Internet  http://www.analog.com
Logo AD - Analog Devices

AD7291 Fiches technique(HTML) 8 Page - Analog Devices

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AD7291
Preliminary Technical Data
Rev. PrC | Page 8 of 21
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
transitions occur at successive LSB values (that is, 1 LSB, 2 LSBs,
and so forth). The LSB size is VREF/4096 for the AD7291. The
ideal transfer characteristic for the AD7291 for straight binary
coding is shown in Figure 7.
000…000
0V
ANALOG INPUT
111…111
000…001
000…010
111…110
111…000
011…111
1LSB
1LSB = VREF/4096
+VREF – 1LSB
NOTES
VREF IS EITHER REFIN OR 2 × REFIN.
Figure 7. Straight Binary Transfer Characteristic
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 8),
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.
Figure 8. Top Level Structure of Internal Temperature Sensor
The temperature conversion consists of two phases, the
integration followed by the conversion. The TSENSE integrates in
turn, over a period of one hundred microseconds once the TSENSE
bit is selected in the Command Register. 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 internally to initiate a
conversion automatically. It takes a period of
approximately100μs to complete the integration and conversion
of the temperature result. If the ADC is in command mode, the
temperature conversion is performed as soon as the next
conversion is completed. 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 TSENSE
Result Register stores the result of the last conversion on the
temperature channel; these 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 TA (the specified
temperature range for the AD7291) are outside the guaranteed
operating temperature range of the device.
Temperature Sensor Averaging
The AD7291 incorporates a temperature sensor averaging
feature to enhance the accuracy of the temperature
measurements. The temperature averaging is performed
continuously in the background once the TSENSE bit in the
command register is enabled. The temperature is measured
each time a TSENSE conversion is performed and a moving
average method is used to determine the result in the TSENSE
Average Result Register. The average result is given by the
following equation;
(
) (
)
sult
Current
sult
evious
AVG
TSENSE
Re
_
8
1
Re
_
Pr
8
7
_
+
=
The average result is then available in the TSENSE Average Result
Register whose content is updated after every TSENSE conversion.
The first TSENSE conversion result given by the AD7291 after the
temperature sensor has been selected in the command register
(bit D7) is the actual first TSENSE conversion result and this result
will remain valid until the next TSENSE conversion is completed
and the result register updated. If the status of the TSENSEAVG bit
is not changed on successive writes to the command register,
the averaging function will not be reinitialized and will
continue calculating the cumulative average. If the command
register is written to and the content of the TSENSEAVG bit
changed the averaging function is reset and the next TSENSE
average conversion result is the current temperature conversion
result.
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
temperature measurements. The temperature data format is
provided in Table 10.


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