CP/PC364 Data Communications & Networks Laboratory

Voltage References

Overview

In the processes of analog to digital (A/D) and digital to analog (D/A) conversion, analog quantities are always converted relative to an analog reference voltage. This is for a couple of reasons:

It allows easy scaling of values
It makes precise conversions possible despite fluctuations in supply voltages.

This lab will investigate the use of voltage references.

Objectives

The objectives for this lab are:

to adjust conversion range by adjusting the reference voltage
to create a reference voltage which is much more reliable than the supply voltage

Do not dismantle the D/A circuit. Store this circuit in your lockup area for use later.

Equipment

bench supply, oscilloscope, function generator
breadboard, debugger board, resistors, capacitors
Arduino board with MAX522 and LCD display
LM 336-2.5 2.5 V zener diode [PDF: 11 pages]

Task

To wire the DAC, there are 3 different sub-circuits to wire. They are:

Voltage reference
Supply voltages and inputs
Outputs

You will wire each of these sections in turn.

Wire up the zener diode with an appropriate resistor to produce a 2.5 V reference voltage.
Demonstrate your circuit to the lab supervisor.
Connect the voltage reference you wired above to the reference voltage pin on the DAC.
Run or modify an appropriate sketch to send values to the DAC.
If you have not already done so. If you have done everything correctly, you should have a circuit which can take in an 8-bit digital value and produce an appropriate analog output between 0 and 2.5 volts.
Demonstrate your circuit to the lab supervisor. Keep this circuit for future labs.

For the built-in ADC, there is only the voltage reference.

Connect the 2.5 V reference from the Zener diode to the AREF pin on the Arduino.
Run or modify an appropriate sketch to display the analog input value on the LCD.
Your sketch will have to ensure that the reference voltage is being taken from the external analog reference pin, rather than the supply voltage.
If you have done everything correctly, you should have a circuit which can take in an analog voltage between 0 and 2.5 volts and produce an appropriate 8-bit digital output.
Demonstrate your circuit to the lab supervisor. Keep this circuit for future labs.