ADC fundamentals

What Is an ADC?
• Mixed-Signal Device
– Analog Input
– Digital Output
• May be Considered to be a Divider
– Output says: Input is What Fraction of VREF?
– Output = 2n x G x AIN / VREF
• n = # of Output Bits (Resolution)
• G = Gain Factor (usually “1”)
• AIN = Analog Input Voltage (or Current)
• VREF (IREF)= Reference Voltage (or Current)
Because the Analog-to-Digital Converter (A/D Converter or ADC) has both analog and digital
functions, it is a mixed-signal device. Many of us consider the ADC to be a mysterious device.
It can, however, be considered very simply to be the instrument that it is: a device that
provides an output that digitally represents the input voltage or current level.
Notice I said voltage or current. Most ADCs convert an input voltage to a digital word, but the
true definition of an ADC does include the possibility of an input current.
An ADC has an analog reference voltage or current against which the analog input is
compared. The digital output word tells us what fraction of the reference voltage or current is
the input voltage or current. So, basically, the ADC is a divider.
The Input/Output transfer function is given by the formula indicated here. If you have seen this
formula before, you probably did not see the “G” term (gain factor). This is because we
generally consider this to be unity. However, National Semiconductor has introduced ADCs
with other gain factors, so it is important to understand that this factor is present.

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