# Analog Circuit Outputs

Discussion in 'General Electronics Discussion' started by Akshatha Venkatesh, Mar 23, 2017.

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1. ### Akshatha Venkatesh

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Jan 14, 2017
an analog circuit's output is given to an ADC pin of the microcontroller , does the current flow to the pin too ? I have some very fundamental doubts, It would be a great help if anyone can clear it ?
Thank you.

2. ### Harald KappModeratorModerator

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Nov 17, 2011
Where there's voltage and a resistance, there's current, too: I=V/R.
An ADC's input has a finite resistance (less than infinity). Do the math.
It also has some capacitance which needs to be charged and discharged by the input voltage.
The direction of the current depends on many parameters like kind of input (single ended, differential), supply voltage (single, dual), direction of change in input voltage (to charge or discharge the input capacitance) etc.

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3. ### Herschel Peeler

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Feb 21, 2016
The input of the ADC is usually a very high resistance so very little current should flow. Mostly it just "feels the voltage".
I think Kapp is only an engineer and doesn't understand simple questions. He only sees complicated solutions. Not wrong. Just not simple.

Last edited: Mar 29, 2017
4. ### Audioguru

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Sep 24, 2016
Kapp simply explained the simple basics of electricity (Ohm's Law) that was not taught in India.

5. ### Herschel Peeler

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Feb 21, 2016
That he did but did it answer his question in a way he understood? Simple questions need simple answers,

6. ### chopnhack

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Apr 28, 2014
Kapp touched on an excellent point that most (if not all) μcu's have as a limitation of their ADC port. The input pin has a switched capacitor on it - thus if the input impedance is too high (i.e. - using high resistances to reduce current coming into the pin) there will be a delay in acquiring the voltage as the cap charges up. Too long of an RC delay and the timing maybe altered. Some pic's have a max impedance of 2.5k Ω's!!

More information is needed to discuss possible solutions. To give some food for thought, you may isolate the ADC's limited impedance with a log amp.[/QUOTE]

Last edited by a moderator: Mar 29, 2017
7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
I am not a fan of excessive formality (see Peeler, Hack, et al; posts #3-6) and I think of my fellow moderator as Harald rather than Kapp. Unless you're referring to some text he has authored?

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8. ### Harald KappModeratorModerator

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Nov 17, 2011
I think I did understand the question. I could have replied simply with 'yes'.
What help would that have been to the op in clarifying his doubts?

With reference to 'only an engineer': A good engineer imho not just solves a problem - this any tinkerer can do given enough time. A good one at least tries to understand the issue at hand (he may not always succeed) to avoid the same issue in the future.

And yes, call me Harald

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9. ### Herschel Peeler

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Feb 21, 2016
[/QUOTE]

No argument at all with the correctness of his answer. I only suggest that the answers fit the level of the question.

10. ### Herschel Peeler

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Feb 21, 2016
Yes you were quite correct in your answer. You could have answered in French or German and been quite correct. If a tinkerer asks a question a tinkerer's answer is all that is needed.

11. ### Harald KappModeratorModerator

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Nov 17, 2011
How do you know Akshata is a tinkerer? For all we know he could as well be a student eager to learn something.

Too bad Akshata himself/herself hasn't replied.

By the way: my French isn't that good. Chnaces are an answer in French would have been not too correct

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