Digital-to-Analog Conversion or simply DAC, is a device that is used to convert a digital (usually binary) code into an analog signal (current, voltage, or electric charge). Digital-to-analog conversion is the primary means by which digital equipment such as computer-based systems are able to translate digital data into real-world signals that are more understandable to or useable by humans, such as music, speech, pictures, video. It also allows digital control of machines, equipment, household appliances. When data is in binary form, the 0’s and 1’s may be of several forms such as the TTL form where the logic zero may be a value up to 0.8 volts and the 1 may be a voltage from 2 to 5 volts. The data can be converted to clean digital form using gates which are designed to be on or off depending on the value of the incoming signal. Data in clean binary digital form can be converted to an analog form by using a summing amplifier. Here is a simplified functional diagram of an 8-bit DAC.

Digital to Analog Conversion Techniques

1. Weighted Summing Amplifier
2. R-2R Network Approach

Weighted Sum DAC One way to achieve D/A conversion is to use a summing amplifier. This approach is not satisfactory for a large number of bits because it requires too much precision in the summing resistors. This problem is overcome in the R-2R network DAC.

Weighted Sum DAC One way to achieve D/A conversion is to use a summing amplifier. This approach is not satisfactory for a large number of bits because it requires too much precision in the summing resistors.

 

As seen an N-bit weighted sum DAC would require largest resistance to be 2^N time the value of smallest resistance on the summing input, which becomes impractical as the number of bits increases. For example for a 8-bit weighted sum DAC the largest resistance would be 256 times the smallest resistance and for 12-bit it will be 2048 times.

This problem is overcome in the R-2R network DAC

R-2R ladder DAC

The summing amplifier with the R-2R ladder of resistances shown produces the output where the D’s take the value 0 or 1. The digital inputs could be TTL voltages which close the switches on a logical 1 and leave it grounded for a logical 0. This is illustrated for 4 bits, but can be extended to any number with just the resistance values R and 2R.

Vout= Rf/R*Vref (D₀/2^N + D₁/2^N-1 + ………..D_N-1/2¹)

 

PIN diagram of DAC

Interfacing of DAC with 8086 Microprocessor

DAC accepts digital data on its B0-B8 lines and convert it to its analog equivalent at the Iout pit. We need an I-to-V converter such as built using an OPAMP LM741.

B8-B0 are connected on port A of 8255. Note that B8 is LSB and B0 is MSB on DAC-0800.

Programs:

WAP in 8086 AL to generate a square wave pulse at the output using DAC-0800

Solution:

Initialize 8255

MOV AL, 80h