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DAC Ladder Networks

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Op Amp diagrams

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 Vout, the current through the resistor R is just Vout/R. But where does that current come from? It all must come from the input current Iin since ideal operational amplifiers draw no current through their inputs. Thus, the current through R must be Iin. 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 Vout = -Iin R.



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