Lab #14:

Digital-to-Analog Conversion


Digital-to-analog converters (DACs) are widely employed to convert digitally encoded information into analog form. Typically one uses standard digital signals (e.g. TTL) to encode in binary form the amplitude of the desired analog signal. DACs will convert these digital inputs (ranging from 8 to 20 bits, depending upon the model) into a continuous analog output signal proportional to the binary number represented by the digital input values. The output signal is of course not really continuous, as there are only a finite number of slices available - 8 input bits yield 256 different output values, 16 bits gets you 65,536. The output signal may be either a voltage or a current. Current output DACs are common because many DACs operate by using the digital inputs to turn on or off a set of scaled current sources (I0, I0/2, I0/4, etc). The output current is simply the sum of the selected internal currents. Since one usually wants an output voltage rather than a current, additional circuitry (usually op-amp based) is required to convert the current to a voltage.

This lab explores the DAC0808 chip. This is an 8-bit converter with a 150 ns settling time and a current output. The IC requires an externally supplied reference current, generally set by a reference voltage and a resistor. A usual reference current is 2 mA and should not exceed 5 mA. Look at the data sheets for all the specs and limits. Use the metal-film resistors in this circuit for precision.


1) The first task is to get the DAC0808 functioning using manually controlled digital inputs, a suitable reference input (2 mA), and a simple arrangement to convert the output current to a voltage. Try a single op-amp in a current-to-voltage setup (H&H p.184). After building the circuit, test it out. Determine the output produced by each input bit set individually (with the others set to zero), as well as the output from the all-zero and all-one states. Determine the smallest V that you can achieve with this circuit. Measure the V and V at five different places across the output range, such as (0-1), (64-65), (128-129), (192-193), and (254-255). How linear is the your circuits output? What is the resolution, and how does it compare to what you expect? Does the resolution remain the same over the output range?


2) Now modify the output section of your circuit to provide a bipolar output range of 5 V. You will need to use more op-amps for this, configured as a voltage adder, in order to get the appropriate voltage offset. Have this circuit give -5 V for all zeros on the input, and +5 V for all ones. Document the proper operation of this circuit.


3) Automate your system to produce a ramp or sawtooth wave from -5 V to +5V. Use a counter built from two 74LS193's to provide the digital input. Show complete circuit diagrams and explain your design. Sketch the output waveforms. What is the maximum frequency sawtooth your circuit could generate, based upon the DAC speed, the op-amp slew rate, and the counter's max speed? Test to see if your estimate is valid.