Digital to Analog (D/A) Converter

 

Digital to Analog Converter (DAC) is a device that transforms digital data into an analog signal.  A DAC can reconstruct sampled data into an analog signal with precision. The digital data may be produced from a microprocessor, Application Specific Integrated Circuit (ASIC), or Field Programmable Gate Array (FPGA), but ultimately the data requires the conversion to an analog signal in order to interact with the real world.

There are two methods commonly used for digital to analog conversion: Weighted Resistors method and the other one is using the R-2R ladder network method.

D/A Converter using Weighted Resistors method

The basic operation of DAC is the ability to add inputs that will ultimately correspond to the contributions of the various bits of the digital input. In the voltage domain, that is if the input signals are voltages, the addition of the binary bits can be achieved using the inverting summing amplifier shown in the diagram below:

In the voltage domain, that is if the input signals are voltages, the addition of the binary bits can be achieved using the inverting summing amplifier shown in the above figure.

The input resistors of the op-amp have their resistance values weighted in a binary format. When the receiving binary 1 the switch connects the resistor to the reference voltage. When the logic circuit receives binary 0, the switch connects the resistor to ground. All the digital input bits are simultaneously applied to the DAC.

The DAC generates analog output voltage corresponding to the given digital data signal. For the DAC the given digital voltage is b3 b2 b1 b0 where each bit is a binary value (0 or 1). The output voltage produced at output side is

V0=R0/R (b3+b2/2+b1/4+b0/8) Vref

As the number of bits is increasing in the digital input voltage, the range of the resistor values becomes large and accordingly, the accuracy becomes poor.

R-2R Ladder Digital to Analog Converter (DAC)

The R-2R ladder DAC constructed as a binary-weighted DAC that uses a repeating cascaded structure of resistor values R and 2R. This improves the precision due to the relative ease of producing equal valued-matched resistors (or current sources).

The above figure shows the 4-bit R-2R ladder DAC. In order to achieve high-level accuracy, we have chosen the resistor values as R and 2R. Let the binary value B3 B2 B1 B0, if b3=1, b2=b1=b0=0, then the circuit is shown in the figure below it is a simplified form of the above DAC circuit. The output voltage is V0=3R(i3/2)= Vref/2

Similarly, If b2=1, and b3=b1=b0=0, then the output voltage is V0=3R(i2/4)=Vref/4 and the circuit is simplified as below

If b1=1 and b2=b3=b0=0, then the circuit shown in the figure below it is a simplified form of the above DAC circuit. The output voltage is V0=3R(i1/8)= Vref/8

Finally, the circuit is shown in below corresponding to the case where b0=1 and b2=b3=b1=0. The output voltage is V0=3R(i0/16) = Vref/16

In this way, we can find that when the input data is b3b2b1b0 (where individual bits are either 0 or 1), then the output voltage is

Applications of Digital to Analog Converter

• Audio Amplifier
• Video Encoder
• Display Electronics
• Data Acquisition Systems
• Calibration
• Motor Control
• Data Distribution System
• Digital Potentiometer
• Software Radio