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Digital-to-Analog Conversion


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Objectives

– Understand how a weighted-resister DAC can be
used to convert numbers with binary or non-binary bit
weightings
– Understand the meaning of the terms used to specify
DAC accuracy
– Understand how an R-2R ladder can be used to
convert both unsigned and signed binary numbers
– Understand the offset binary representation of
negative numbers


Adding an op-amp:

– The voltage at the junction of all the resistors is now
held constant by the feedback
• Hence current drawn from V3 is independent of the other
voltages V2, …, V0
• Hence any gate non-linearity has no effect  more
accurate.
– Lower output impedance
– Much slower: op-amp slew rate  1 V/μs.



Bipolar DAC

A bipolar DAC is one that can give out both positive and
negative voltages according to the sign of its input. There
are two aspects of the circuit that we need to change:



Positive and Negative Currents

We need to alter our R-2R ladder circuit so that we can get
an output current that can be positive or negative
according to the sign of the input number.