Why are high impedance circuits more sensitive to noise?

Why are high impedance circuits being more sensitive to the noise? They have less current flowing through them, but how is that related to noise, since external noise becomes voltage on the wires, and then current proportional to resistance?

• Because external noise can also become a current in a wire (inductive noise coupling), and the noise voltage is proportional to current times resistance. Noise is not just coupled into your circuit capacitively! – user36129 Oct 9 '13 at 8:50
• @user26129 "noise voltage is proportional to current times resistance" Why? Can you explain that please? – 1p2r3k4t Oct 9 '13 at 9:01
• It's called ohm's law. – user36129 Oct 9 '13 at 9:12
• @user26129 Isn't current the result and voltage the cause? Isn't induced noise voltage the same regardless of wire resistance? – 1p2r3k4t Oct 9 '13 at 9:23
• Think power: P = UI. The noise has a certain *power. If the resistance is high, the current (I) will be low. So to absorb the given power a higher noise voltage will result: U = P/I. In other words: High impedance lines carry less power for their signals. When adding a little noise power to a small signal power the signal will be more disturbed than when adding small noise power to a bigger signal power. Related: "Signal to noise ratio" (SNR). – JimmyB Oct 9 '13 at 10:12

You can model a capacitive coupling between a noise source and your circuit with 3 elements :

• a voltage source (the noise source)
• a capacitor (the capacitive coupling)
• a resistor (the input impedance of your circuit)

If the resistor has a small value, you won't get much voltage at the input of your circuit If the resistor has a big value (high impedance), the voltage will be much higher.

When you start speaking about noise the subject becomes quite complex. Because noise must be defined, which is not that simple. A signal (meaningful information) in an electronic system A could be considered as a noise in an electronic system B. I could give an example where a low impedance input is more sensitive to noise than a high impedance one, it depends what you consider being noise and what you consider being signal.

I see in your question that you might be a bit confused with DC and AC... Ohm's law is basically a DC law, and can be extended to AC when replacing the resistance by impedance, like below: When you use the term impedance you implicitly consider AC signals. I suggest you to read the wikipedia page about the electrical impedance.

• I was talking about both DC and AC data signals, that's why I mentioned both resistance and impedance. Can you give an example with a simple sinusiodal ac signal and noise coming from EMI? – 1p2r3k4t Oct 10 '13 at 8:03

A high impedance circuit is generally more sensitive to noise. This is because a small current induced on a high impedance circuit (I times Z) results in a higher noise voltage. In contrast, the induced noise on a low Z circuit is generally smaller (I times Z).