# Weighted Resistor DAC and its Operation

HI friends, today we are going to learn weighted resistor DAC. Many times we require to convert a digital output into its equivalent analog voltage. To perform this operation we are going to use weighted resistor digital to analog converter.

The following figure shows the circuit diagram of weighted resistor DAC. This DAC circuit uses weighted values of resistor like 2R, 4R, 6R, 8R and so on depending on the digital inputs available therefore such type of network is known as weighted resistor DAC.

This circuit consists of a transistor switch (shown by the upward arrow) which turns on the switch when the digital input is ‘1’ and if digital input becomes ‘0’ it will open the switch. When transistor switch gets closed, current flows through the weighted resistor due to the reference voltage as shown in circuit diagram.

When all such currents from different weighted resistors get added at summing point (which is also known as a virtual ground) of the operational amplifier it will produce a proportional voltage as its output.

For a 4 bit DAC, the output V0 is given as follows

Where S3, S2, S1 and S0 represents the status of the switches i.e. on or off (1 or 0).

If resistors are in binary weights i.e. R3=2Rf, R2=4Rf, R1=8Rf and R0=16Rf, the above equation can be written as,

From the above discussion, we can say that for a 4 bit DAC 4 switches produces 16 different combinations of output and hence produces 16 different output voltage.in general n-bit DAC produces 2^n different discrete analog voltages.