Figure 1.  Circuit Diagram for an Op Amp Summer Circuit used as a DAC

The circuit shown in Figure 1 is a simple 4-bit digital-to-analog converter.  It is actually just a simple op amp summer circuit, i.e., an operational amplifier configured to output a voltage that is proportional to the sum of the input voltages.

The op-amp summer circuit above works as a DAC because its input voltages are binary weighted with respect to each other, as set by the resistors (10K, 20K, 40K, 80K) at the inputs.  

The output Vo of this summer circuit w is:    

Vo = -V_Ref (5K) (S3/10K + S2/20K + S1/40K + S0/80K) = -V_Ref (S3/2 + S2/4 + S1/8 + S0/16) wherein S3, S2, S1, and S0 are the logic inputs ('1' or '0'). The number of bits of this DAC may be increased by connecting more switches with corresponding binary-weighted resistors to the inputs.

A similar but better-designed and more commonly used DAC is the R-2R Ladder DAC.

See Also:  Digital-to-Analog Converter;  Operational Amplifiers

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