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Slide 1: Digital to Analogue Conversion: the mcp4275

Lecture 10

Uli Raich

UCC semester 2017/2018

Slide 2: Analogue versus digital

Up to now we only treated digital signals:

on/off for the LEDs
on/off to read the LED state
Powering or not powering coils to generate magnetic fields in a stepping motor

However:
The world is mostly analogue:

Temperatures are changing continuously and not in steps
Pressure is an analogue value
Distance, time, current, resistance … are all analogue values

Slide 3: Converting from digital to analogue

Since our computer is a digital device we must

Convert digital values to analogue voltage levels
Digital to Analogue Conversion (DAC)
… and we must convert external analogue values to digital
Analogue to Digital Conversion (ADC)

Slide 4: Digital to analogue conversion

A digital to analogue converter does not really convert into a continuous waveform
Since we have digital values as a base, there will be steps in the output waveform
The size of these steps depends on the resolution of the DAC
What is the smallest step a 12 bit DAC can produce on a 0..5V scale?

Slide 5: Can we smooth the output signal?

Yes, it is possible to smooth out these steps.

We need a low-pass filter, which filters out high frequencies

(the abrupt steps we have in the output signal) and lets pass only slower transitions.

When looking carefully at the output of our sine generator you will

also see these steps. In this case however they come from the

limited number of sine values (100) we calculate. To improve the resolution

we would have to increase the number of samples and the frequency

with which we send these values to the DAC.

Slide 6: How does a DAC work?

If you want to know more about DAC technology (and you should!)

have a look at this excellent tutorial, from which I have copied the above illustration.

Slide 7: The MCP4725 12 bit DAC

As a demo device we bought the MCP4725 DAC
This is a 12 bit DAC which can be accesses by the I2C bus
Here is its data sheet

and here a photo of the device

-- Uli Raich - 2017-10-31

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Topic revision: r3 - 2017-11-01 - uli