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| 型號： | EVAL-AD7291SDZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 6/29頁 |
| 文件大?。?/td> | 0K |
| 描述： | BOARD EVAL FOR AD7291 |
| 標(biāo)準(zhǔn)包裝： | 1 |
| ADC 的數(shù)量： | 8 |
| 位數(shù)： | 12 |
| 采樣率（每秒）： | 22.22k |
| 數(shù)據(jù)接口： | I²C |
| 已用 IC / 零件： | AD7291 |
| 已供物品： | 板 |
| 相關(guān)產(chǎn)品： | AD7291BCPZ-RL7-ND - IC ADC I2C/SRL 22.22K 20LFCSP AD7291BCPZ-ND - IC ADC 12BIT SAR 8CH 20-LFCSP |
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Data Sheet
AD7291
Rev. B | 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 VREF/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 = VREF/4096
ANALOG INPUT
NOTES
1. VREF IS 2.5V.
ADC
CO
DE
+VREF – 1LSB
1LSB
0V
0
87
1
1-
00
7
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.
I4 × I
INTERNAL
SENSE
TRANSISTOR
BIAS
DIODE
VDD
8 × I
IBIAS
VOUT+
VOUT–
TO ADC
0
871
1-
00
8
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 TSENSE 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 TA (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 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:
() ()
Result
Current
Result
Average
Previous
AVG
T
SENSE
_
8
1
_
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 remains valid until the next TSENSE conversion is
completed and the result register is updated.
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