# 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! Oct 9, 2013 at 8:50
• @user26129 "noise voltage is proportional to current times resistance" Why? Can you explain that please? Oct 9, 2013 at 9:01
• It's called ohm's law. Oct 9, 2013 at 9:12
• No, because there is not just induced noise voltage, also induced noise current. This gets amplified as the impedance of your line increases. Also, your noise source doesn't have infinitely low impedance: you can model it as a voltage and/or current source with some series/parallel resistance, which is its characteristic impedance. Oct 9, 2013 at 9:25
• 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). Oct 9, 2013 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? Oct 10, 2013 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).