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What Are Input and Output impedances in simple words?

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Jul 30, 2012
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What Are Input and Output impedances in simple words? Take this scenario into account when explaining....A gas sensing sensor has to be connected to an analog input pin of a PIC uC, According to microchip the maximum recommended input impedance is 2.5Kohm(what does that mean?) and how do I know about the output impedance of the sensor? please do not assume that i already know about output and input impedance..explain them please.

Thanking you all in anticipation for your time and help :)

Impedance is a measure of the opposition to current when a voltage is applied. For DC, we call this resistance. For AC, we call it impedance. For DC circuits, impedance and resistance are the same thing.

The output impedance of the sensor should be mentioned in the sensor's datasheet.

Hope this helps.
Input impedance is actually seen by the power source connected to the network if the voltage and current is known then you can calculate is by ohm's law where as Output impedance is usually the effect on small amplitude signal.
According to microchip the maximum recommended input impedance is 2.5Kohm(what does that mean?)
The datasheet is talking about source impedance, not input impedance. The value is related to specified ADC error. You can use a higher source impedance but should be aware of larger errors. In case of gas sensor, which doesn't promise a mind-blowing accuracy, this shouln't be a problem.

If you refer to a typical gas sensor circuit like in this thread , the load resistor will be the effective source impedance.
I am using this sensor MQ-4 Gas Sensor

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need to know the output impedance if a load resistance is connected to the sensor

I think it's O.K. with 20 kOhm load resistor. I would add a filter capacitor, e.g. 10 nF ANx to ground to buffer the sampling current.
Been working lately on opamp's cause of a PIR sensor, and learned that a voltage follower/unity gain stage is a good circuit if u want somehow connect diferente impendance circuits. But i think that the 20k load resistor sugested before is a good starting point. Nothing better than the usual trial/error headaches.
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