Datasheet

Data Sheet AD7291
Rev. C | Page 13 of 28
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
REF
/4096 for the AD7291. The
ideal transfer characteristic for the AD7291 for straight binary
coding is shown in Figure 21.
111...111
111...110
111...000
011...111
000...010
000...001
000...000
1LSB = V
REF
/4096
ANALOG INPUT
NOTES
1. V
REF
IS 2.5V.
ADC CODE
+V
REF
– 1LSB
1LSB
0V
08711-007
Figure 21. 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 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.
I 4 × I
INTERNAL
SENSE
TRANSISTOR
BIAS
DIODE
V
DD
8 × I I
BIAS
V
OUT+
V
OUT–
TO ADC
08711-008
Figure 22. Top Level Structure of Internal Temperature Sensor
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
SENSE
conversion
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
A
(the specified tem-
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
SENSE
conversion is performed and a
moving average method is used to determine the result in the
T
SENSE
average result register. The average result is given by the
following equation:
(
) ( )
ResultCurrent
Result
AveragePreviousAVGT
SENSE
_
8
1
__
8
7
+
=
The average result is then available in the T
SENSE
average result
register whose content is updated after every T
SENSE
conversion.
The first T
SENSE
conversion result given by the AD7291 after the
temperature sensor is selected in the command register (Bit D7)
is the actual first T
SENSE
conversion result, and this result remains
valid until the next T
SENSE
conversion is completed and the result
register is updated.