Lab 9
Digital-to-Analog Converters

In this exercise you will investigate Digital-to-Analog Converter (DAC). You will construct a 4 Bit DAC from a summing amplifier. You will also test a commercial 8 Bit DAC.

Reading Assignment

Building Scientific Apparatus. Section 6.6.1 (Binary Counting). Section 6.6.7 (Digital-to-Analog Conversion).

Supplies and Materials

Student Workstation. Operational amplifier: OP97FP. Digital-to-analog converter: AD557. Resistors, 1/4 W 5%: 5.1 kΩ, 20 kΩ, 39 kΩ, 82 kΩ, 160 kΩ. Capacitor: monolithic, 0.01 µF. Switch: 4 position DIP, 8 position DIP. Screwdriver, miniature: Xcelite R3323. Hookup wire, tinned copper (AWG 22). wire cutter. Digital multimeter (4-1/2 digit).

I.    Introduction

Digital electronics are ubiquitous, having revolutionized our lives from the way we communicate to the way we listen to music. Unlike an analog signal that may vary continuously in time, a digital signal varies in discrete steps called Bits. The resolution of a digital signal depends on the number of Bits, expressed as a binary number (a power of two). For example, an 8 Bit binary number contains 8 discrete steps. See Figure 1.


Figure 1. 8 Bit Binary Equivalent of 2.55 V

The number is determined by how each Bit (DB1-DB8) is set. By convention, a Bit is set HI (=1) or set LO (=0). See Figure 2.


Figure 2. 8 Bit Binary Equivalent of 2.55 V

Each successive Bit increases in value by its binary weight (a factor of 2). The sum of n-Bits is (2n –1), or 255 for an 8 Bit number. In this example, (2n –1)(0.10 V) = 2.55 V.

II.    Nomenclature

4 Bits are called a nibble, 8 Bits are called a byte, and 16 Bits are called a word. The Least Significant Bit (LSB) corresponds to the smallest change in voltage, and the Most Significant Bit (MSB) corresponds to the greatest change in voltage. The resolution of an analog voltage that has been converted to 8 Bits is 1 part in 28 (256). By comparison, a typical compact disk (CD) contains analog voltages that have been converted with a resolution of 1 part in 224.

III.    Logic Levels

Logic HI and logic LO correspond to a voltage level that is set by convention. The most common convention is called Transistor-Transistor Logic (TTL), where HI = + 5 V and LO = 0 V. In practice:

  1. •Logic LO = - 0.5 V to + 0.4 V.

  2. •Logic HI  =  + 2.4 V to + 5.5 V.

  3. •Decision threshold (LO or HI): ≈ +1.3 V.

IV.    Construct a Digital-to-Analog Converter

Consider a 4 Bit DAC made from a summing amplifier with 4 input resistors that increase in value by their binary weight. See Figure 3.


Figure 3. A 4 Bit DAC

The current through each resistor will add at the summing junction, A, as shown in Figure 4.



Figure 4. Current at the Summing Junction

The four switches labeled DB1-DB4 represent the four data Bits and control how each Bit is set. When a switch is open a Bit is set LO, and when a switch is closed a Bit is set HI. A 4 Bit DAC can be designed for Vout = 2.550 V using the closest resistance values available in commercial, 5% tolerance, resistors. See Figure 5.


Figure 5. A 4 Bit DAC Using Commercial Resistors


In this case, the output voltage is shown in Figure 6.


Figure 6. Output Voltage

Construct the DAC shown in Figure 4 using an OP97FP op-amp and 4 SPST (Single-Pole Single-Throw) switches. The op-amp and the switches are supplied in a Dual-In-line Package (DIP). Place each DIP on the breadboard so that it straddles the central recess. Consider using a wire loop as a test point for a voltage measurement. See Figure 7.


Figure 7. Breadboard a 4-Bit DAC

Although the OP97FP is relatively insensitive to destruction by static electricity, it is a good practice to handle all op-amps with care. Do not touch the pins!

Make all connections to the op-amp before power is applied. The power supply voltage (±12 VDC) should be applied or removed from the op-amp simultaneously by connecting or disconnecting the power supply transformer from the wall receptacle. Measure the applied voltages to confirm they are within ±1 V of their nominal values.

V.    Record Digital Data

The nominal output voltage of the DAC was calculated from the nominal power supply voltage and the nominal value of each resistor when the corresponding Bit was set HI. The power supply voltage and the resistors you are using have a tolerance of 5% which will affect the output voltage of the DAC. Measure the actual value of the power supply voltage and the actual value of each resistor. Then measure and record the actual output voltage of the DAC and enter your results in your lab notebook. See Figure 8.


Figure 8. Digital Data (4 Bit DAC)


VI.    Test a Digital-to-Analog Converter

Assemble a circuit to test a commercial 8 Bit DAC (AD557) with 8 SPST switches. See Figure 9.


Figure 9. AD557 Test Circuit

The DAC and the switches are both supplied in a 16 pin Dual-Inline-Package (DIP). A circular indent next to pin 1 on the DAC defines its pin assignments. Place each DIP on the breadboard so that it straddles the central recess. Consider using a wire loop as a test point for a voltage measurement. See Figure 10.


Figure 10. Breadboard an 8-Bit DAC

Test the DAC by setting specific Bit patterns and recording the output voltage of the DAC in your lab notebook. See Figure 9.


Figure 11. AD557 Test Results

Confirm that the binary Bit patterns shown in the table correspond to their decimal and hexadecimal (HEX) equivalents. Measure the power supply voltage and enter its value, V+ (to the nearest millivolt).

Set the Bit patterns with the DIP switch and record Vout ± 0.001 V. Are the output voltages consistent with the voltage recorded when the LSB is set high (0000 0001)? Explain.

VII.    Homework Assignment

Update your lab notebook to include this exercise and questions raised in class.

Read the next laboratory exercise.

Complete GTutorial Exercise 11.