Datasheet AD7291 (Analog Devices) - 14

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Data Sheet AD7291 ADC TRANSFER FUNCTION
Each input integrates, in turn, over a period of several hundred The output coding of the AD7291 is straight binary for the microseconds. This takes place continuously in the background, analog input channel conversion results and twos complement leaving the user free to perform conversions on the other for the temperature conversion result. The designed code tran- channels. When integration is complete, a signal passes to the sitions occur at successive LSB values (that is, 1 LSB, 2 LSBs, control logic to initiate a conversion automatically. and so forth). The LSB size is VREF/4096 for the AD7291. The If the ADC is in command mode and performing a voltage ideal transfer characteristic for the AD7291 for straight binary conversion, the AD7291 waits for it to complete and then coding is shown in Figure 21. initiates a temperature sensor conversion. If the ADC is not performing voltage conversions, temperature conversions occur at 5 ms intervals.
111...111 111...110
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
111...000 DE
result register stores the result of the last conversion on the
011...111 1LSB = V
temperature channel; this can be read at any time.
REF/4096 ADC CO
Theoretically, the temperature measuring circuit can measure temperatures from −512°C to +511°C with a resolution of
000...010 000...001
0.25°C. However, temperatures outside TA (the specified tem-
000...000
perature range for the AD7291) are outside the guaranteed
0V1LSB +VREF – 1LSB
operating temperature range of the device. The temperature
ANALOG INPUT
007
NOTES
1- sensor is enabled by setting the TSENSE bit in the command 1
1. VREF IS 2.5V.
087 register. Figure 21. Straight Binary Transfer Characteristic
TEMPERATURE SENSOR AVERAGING TEMPERATURE SENSOR OPERATION
The AD7291 incorporates a temperature sensor averaging The AD7291 contains one local temperature sensor. The on-chip, feature to enhance the accuracy of the temperature measure- band gap temperature sensor measures the temperature of the ments. The temperature averaging feature is performed AD7291 die. continuously in the background provided the TSENSE bit The temperature sensor module on the AD7291 is based on the in the command register is enabled. The temperature is three current principle (see Figure 22), where three currents are measured each time a TSENSE conversion is performed and a passed through a diode and the forward voltage drop is moving average method is used to determine the result in the measured, allowing the temperature to be calculated free of TSENSE average result register. The average result is given by the errors caused by series resistance. following equation:
VDD
7 1
I 4 × I 8 × I I
T = _ _ + _ SENSE AVG (Previous Average Result) (Current Result)
BIAS
8 8 The average result is then available in the TSENSE average result register whose content is updated after every TSENSE conversion.
VOUT+
The first TSENSE conversion result given by the AD7291 after the temperature sensor is selected in the command register (Bit D7)
TO ADC
is the actual first TSENSE conversion result, and this result remains
INTERNAL VOUT–
valid until the next TSENSE conversion is completed and the result
SENSE TRANSISTOR
register is updated.
BIAS DIODE
008 1- 1 087 Figure 22. Top Level Structure of Internal Temperature Sensor Rev. C | Page 13 of 28 Document Outline FEATURES FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION PRODUCT HIGHLIGHTS REVISION HISTORY SPECIFICATIONS I2C TIMING SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS TERMINOLOGY CIRCUIT INFORMATION CONVERTER OPERATION ANALOG INPUT ADC TRANSFER FUNCTION TEMPERATURE SENSOR OPERATION TEMPERATURE SENSOR AVERAGING Temperature Value Format VDRIVE THE INTERNAL OR EXTERNAL REFERENCE RESET INTERNAL REGISTER STRUCTURE ADDRESS POINTER REGISTER COMMAND REGISTER (0x00) Sample Delay and Bit Trial Delay VOLTAGE CONVERSION RESULT REGISTER (0x01) Temperature Value Format TSENSE CONVERSION RESULT REGISTER (0x02) TSENSE AVERAGE RESULT REGISTER (0X03) LIMIT REGISTERS (0X04 TO 0X1E) DATAHIGH Register DATALOW Register HYSTERESIS REGISTER ALERT STATUS REGISTER A AND ALERT STATUS REGISTER B (0x1F AND 0x20) I2C INTERFACE SERIAL BUS ADDRESS BYTE GENERAL I2C TIMING WRITING TO THE AD7291 WRITING TWO BYTES OF DATA TO A 16-BIT REGISTER WRITING TO MULTIPLE REGISTERS READING DATA FROM THE AD7291 READING TWO BYTES OF DATA FROM A 16-BIT REGISTER MODES OF OPERATION COMMAND MODE AUTOCYCLE MODE OUTLINE DIMENSIONS ORDERING GUIDE