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Impedance

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stan001:
I am a beginner in electronics and have a hard time trying to understand the concept of impedance..

The below statement :-
Arduino (Atmega) pins configured as INPUT with pinMode() are said to be in a high-impedance state.

Is high impedance state equal to high resistance ??

What is the relationship between impedance and resistance ?

How do I know if a sensor or load is high or low impedance ??

Hope someone can help to clarify this or give some examples...

tyblu:
impedance: Z ; admittance: Y = 1/Z
resistance: R ; conductance G = 1/R
reactance: X ; susceptance B = 1/X

Z = R + jX ; Y = G + jB

Impedance takes into account frequency-dependent (frequency reactant) nature of a load. It's probably a good idea to write the above on the ceiling above your bed, or somewhere common. A resistor has impedance of Z = R; a capacitor has impedance of Z = Inf. (open-circuit) at DC, Z = -j/wC with AC; an inductor has impedance of Z = 0 (short-circuit) at DC, Z = jwL with AC. It is more useful to define impedance instead of resistance, as it tells the reader how it behaves with different frequency electrical signals.

Time:
Impedance is how something resists change in voltage or current.  If its not changing than there is nothing to impede.  This is due to the field effects of changing voltages and currents.  Fundamentally its all based around a set of equations known as Maxwell's equations which turn out to be quite elegant and beautiful in their own right.  Its a kind of difficult concept at first but if you are a student you will eventually take electromagnetics for engineers and this will shed light onto the concept.  

Something that is purely a resistance will resist the flow of electrons at some constant regardless of how its varying with time.  If you want to physically conceptualize this think of electrons flowing through something and bumping into the lattice of the bonded atoms.  The more they bump into things the slower they will flow which produces heat as a direct result.

As a whole an impedance is both how much electrons bump into the structure (resistance) and how much the fields from the changing current effect the flow of the electrons themselves.

Now, nothing is purely resistive or purely reactive (reactive being the portion of the impedance that accounts for only the field effects).  In real life everything has some finite resistance, inductance, and capacitance.  

So, with that being said when talking about devices you can do a little analysis and find that you essentially want infinite impedance looking into the input of the device and 0 output impedance looking into the output of the device.  This is something you will also learn about in school.  The reasoning for this is related how much energy is transferred from the output of something into the input of another thing.

If you want to know about the impedance of the input/output of a sensor you can usually find it in the manual.  If it does not say in the manual its usually safe to assume the input is high-Z or high impedance.

williefleete:
impedance is indeed similar to resistance, in fact in a dictionary they would have basically the same meaning
however as tyblu said resistance is a DC property impedance is more an AC property
a high impedance would be open circuit to DC and AC signals eg almost nil capacitance (picofarads at worst) and infinite resistance, inputs on micros and CMOS logic have this property hence the need for pull down resistance as the pin, case and its ground/positive acts as a parasitic capacitor, charges can be stored and trigger a false detection, a pull down merely discharges the parasitic capacitor to ground relatively quickly, lower the value the faster it discharges after a pulse

stan001:
Hi,

Thanks for all the replies..

I think my question is related to DC circuit instead of AC circuit.

Here is my understanding of impedance using the voltmeter example...

Voltmeter have high impedance so that it would not alter the voltage or current of the circuit it is measuring ...

Is the above statement a good example to explain about "high impedance" ??

Thanks

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