Book Analog Interfacing to Embedded Microprocessors
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 9.4 shows what happens in an ADC system using an LM336 with
nominal, maximum, and minimum values. At an input voltage of zero, the
output code will be zero. As the measured voltage rises, the code read from
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