Discussion in "General help Guidance and Discussion" started by    prajwalsnh    Oct 21, 2013.
Mon Oct 21 2013, 07:03 AM
Hi everyone,

I have few stupid doubts in ADC. I just tried a simple adc test in my 8051 using ADC 0804. Well, i got the expected output. But after which some peculiar question came to my mind. I used 10K Preset as the ANALOG INPUT for ADC 0804. When i rotated the knob the resistance changed and accordingly the LED's connected to a port also varied between its on and off condition. But my question is we provide DC voltage which is a constant voltage for preset. on varying the preset how come the output is ANALOG??? I mean in terms of waveform DC voltage will be a straight LINE. But how will it change to alternating waveform at resistor end? Can anybody explain the clear concept of AC & DC, ANALOG and DIGITAL. I'm sorry if my question sounds funny
Mon Oct 21 2013, 10:05 AM
Looks like you're confused with basic definition of Analog and Digital signal. When you say digital it means it can only have two levels either 0 or 1. where zero is represented by 0V and 1 represented by 5V in TTL. So Digital signal does not understand any other voltage levels other than 0 or 1. Where as when we say Analog, Its just a signal with a voltage level, which can be either +ve or -ve or zero.

The AC, DC are different concept, they are concept of signal theory. And AC/DC signals are functions of time.
DC signal (Fn) can be represented by
Fn = x;
x is a constant value
where as AC signal Fn is
Fn = x(t)
where x is function of time, i.e. it changes with time so we call it AC signal.
Now at a particular time interval t, x(t) will have a certain value with will remain constant at that time, for t+1 it will be different.

This same theory is used in ADC, to measure an analog signal you sample incoming signal at particular time t where its value is x Volts and you get ADC value, and at t+1 it has y Volts. you store all these values in your controller and when you want to reconstruct the original signal just send the timed values to DAC and you will get similar waveform on the output.

I tried to explain some concepts of signal theory hope you understood.
Tags signal theorysignal and systemanalog signal representationdigital signaldigital signal representationsignal time function
Mon Oct 21 2013, 04:26 PM
@Ajay Bhargav

Thanks for your reply sir I know the theory whatever u told. But 'm not able to understand still. what wil be the voltage at micro-controller pin?? No doubt it will be DC 5V. But its analog or digital?? can i simulate in proteus to see the waveforms out of a preset as i told in earlier post??? If you don't mind can you give some example circuits and explain Thanks in advance.
Tue Oct 22 2013, 07:34 AM
When we say digital its just a convention to imply that system can only understand two levels 0 or 1. Even analog source can out put 5V but we call it analog because of its nature. Its hard to explain things like this its like general knowledge of electronics. In proteus you can connect one end of potentiometer to VCC and other GND, connect the centre one to oscilloscope. Now when you run the simulation you will see DC signal only but still we call it analog input (and not digital) because it can have any value from 0 to 5V unlike digital where you can only have 0 and 5V Hope this gives a better clarification.
 dikshac like this.
Tue Oct 22 2013, 03:57 PM
@Ajay Bhargav

Yes now i got it Nice explanation. Thanks a TON sir

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