ADS1118_13 Datasheet, PDF (18/38 Pages) Texas Instruments – Ultra-Small, Low-Power, SPI™-Compatible, 16-Bit Analog-to-Digital Converter and Temperature Sensor with Internal Reference

ADS1118
SBAS457C – OCTOBER 2010 – REVISED FEBRUARY 2013
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TEMPERATURE SENSOR
The temperature measurement mode of the ADS1118 is configured as a 14-bit result when enabled. Two bytes
must be read to obtain data. The first byte is the most significant byte (MSB), followed by a second byte, the
least significant byte (LSB). The first 14 bits are used to indicate temperature. That is, the 14-bit temperature
result is left-justified within the 16-bit result register and the last two bits always read back as '0'. One 14-bit LSB
equals 0.03125°C. Negative numbers are represented in binary twos complement format.
Table 4. 14-bit Temperature Data Format
TEMPERATURE (°C)
128
127.96875
100
80
75
50
25
0.25
0
–0.25
–25
–55
DIGITAL OUTPUT (BINARY)
01 0000 0000 0000
00 1111 1111 1111
00 1100 1000 0000
00 1010 0000 0000
00 1001 0110 0000
00 0110 0100 0000
00 0011 0010 0000
00 0000 0000 1000
00 0000 0000 0000
11 1111 1111 1000
11 1100 1110 0000
11 1001 0010 0000
HEX
1000
0FFF
0C80
0A00
0960
0640
0320
0008
0000
3FF8
3CE0
3920
Converting from Temperature to Digital Codes
For positive temperatures (for example, +50°C):
Twos complement is not performed on positive numbers. Therefore, simply convert the number to binary
code in a 14-bit left justified format with the MSB = 0 to denote the positive sign.
Example: (+50°C)/(0.03125°C/count) = 1600 = 0640h = 00 0110 0100 0000
For negative temperatures (for example –25°C):
Generate the twos complement of a negative number by complementing the absolute binary number and
adding 1. Then denote the negative sign with the MSB = 1.
Example:(|–25°C|)/(0.03125°C/count) = 800 = 0320h = 00 0011 0010 0000
Twos complement format: 11 1100 1101 1111 + 1 = 11 1100 1110 0000
Converting from Digital Codes to Temperature
To convert from digital codes to temperature, first check whether the MSB is a '0' or a '1'. If the MSB is a '0',
simply multiply the decimal code by 0.03125°C to obtain the result. If the MSB = 1, subtract '1' from the result
and complement all of the bits. Then multiply the result by –0.03125°C.
Example: ADS1118 reads back 0960h: 0960h has an MSB = 0.
(0960h) × (0.03125°C) = (2400) × (0.03125°C) = +75°C
Example: ADS1118 reads back 3CE0h: 3CE0h has an MSB = 1.
Complement the result: 3CE0h → 0320h
(0320h) × (–0.03125°C) = (800) × (–0.03125°C) = –25°C
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