Book Analog Interfacing to Embedded Microprocessors
two resistors had different temperature coefficients. Using resistors with lower
temperature coefficients will reduce temperature-induced errors overall.
Voltage References
All ADC systems require some kind of voltage reference. All voltage references
have some nominal value, but they also have a tolerance that specifies how
much they can vary from this value. Since references are semiconductor
devices, they are susceptible to temperature effects as well.
The LM336A-2.5 is a 2.5v reference diode that is used much like a zener
(Figure 9.4). When operated within its specified current range and at 25°C,
it has a voltage range between 2.44 and 2.54 volts (the B version has a wider
range). If an LM336 is used as a reference to measure voltage with an 8-bit
ADC, an input of 1v will result in an output value between 100 (at 2.54v) and
104 (at 2.44v). In a 10-bit system, the same 1v input will result in an output
value between 403 and 419.
Figure 9
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