
DAC Ladder Networks
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So now look at inset B. The inverting () input will be driven to ground potential. Because the inverting input is at ground potential but not physically wired to ground, it is said to be a "virtual ground." If the output is at a potential V_{out}, the current through the resistor R is just V_{out}/R. But where does that current come from? It all must come from the input current I_{in} since ideal operational amplifiers draw no current through their inputs. Thus, the current through R must be I_{in}. Using a consistent sign convention, if the input current comes from a positive voltage source, then passes the inverting input at V=0, the output must have a negative potential, so V_{out} = I_{in} R.
