260
7647H–AVR–03/12
Atmel ATmega16/32/64/M1/C1
Note: 1. 5V range: Max R
load
30KΩ
3V range: Max R
load
15KΩ
19.2.2 Current Source for Low Cost Traducer
An external transducer based on variable resistor can be connected to the Current Source. This
ca be for instance:
• A thermistor, or temperature-sensitive resistor, used as a temperature sensor
• A CdS photoconductive cell, or luminosity-sensitivity resistor, used as a luminosity sensor.
Using the Current Source with this type of transducer eliminates the need for additional parts
otherwise required in resistor network or Wheatstone bridge.
19.2.3 Voltage Reference for External Devices
An external resistor used in conjunction with the Current Source can be used as voltage refer-
ence for external devices. Using a resistor in serie with a lower tolerance than the Current
Source accuracy (≤2%) is recommended. Table 19-2 gives an example of voltage references
using standard values of resistors.
Table 19-2. Example of Resistor Values(±1%) for a 16-address System (AV
CC
= 5V
(1)
)
Physical
Address
Resistor Value
R
load
(Ohm)
Typical
Measured
Voltage (V)
Minimum
Reading with
a 2.56V ref
Typical
Reading with
a 2.56V ref
Miximum
Reading with
a 2.56V ref
0 1 000 0.1 38 40 45
11 2000.12464854
215000.15576068
318000.18697281
422000.22848899
5 2700 0.27 104 108 122
6 3300 0.33 127 132 149
7 4700 0.47 181 188 212
8 6 800 0.68 262 272 306
9 8 200 0.82 316 328 369
10 10 000 1.0 386 400 450
11 12 000 1.2 463 480 540
12 15 000 1.5 579 600 675
13 18 000 1.8 694 720 810
14 22 000 2.2 849 880 989
15 27 000 2.7 1023 1023 1023
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