# When is noise a factor in basic analog circuits?

I have a good understanding of circuit theory, but only a few large projects under my belt. In practical applications, at what voltage level does noise really become an issue.

Is there any 'typical' white noise level in circuits (e.g. 100mVpp can be expected in most circuits).

I am building a current sensor circuit with just a basic series resistor. I am going to measure the voltage drop across it to determine the current. Let's say I was going to measure 150V supply current. If I use a 0.1 ohm resistor in-line, and expect 10 Amps, that would be a 1V drop and 10W dissipated power. If I use a 0.01 resistor, I get only 1W of dissipated power, but now my signal is at 100mV. Will noise become an issue at the 100mV range? Will a simple capacitor filter out enough noise for me to reliably measure current?

EDIT:

I am interested in noise for analog circuits specifically. I realize that in digital circuits, there are many noise generators due to switching. If I have a 100mV signal in a purely analog board in an aluminum box, is there typically enough noise to make the measurement unreliable (let's say +/- 15%). Are any filtering techniques required, and if so, what are some options?

What if that same signal was 0-100 uV?

• Noise figures depend more on the parts you use and the EMI environment than on some typical "threshold" value. – JimmyB Jul 19 '16 at 19:57
• Noise tends to increase with the square root of circuit bandwidth. So (among other reasons) there's no one "typical" noise power across all circuits. "Low noise" for a sampling circuit with 10 Hz bandwidth is very different from one with 10 GHz bandwidth. – The Photon Jul 19 '16 at 20:29
• Resistors have a physics-limited noise floor called Johnson noise, but it's usually in the nanovolt range. – pjc50 Jul 19 '16 at 20:47
• The significance of noise depends on the accuracy you're shooting for. The 'white noise' of a 0.1$\Omega$ resistor is probably insignificant for you (41pV/$\sqrt{\text {Hz}}$ at room temperature). – Spehro Pefhany Jul 19 '16 at 20:52
• I added more detail to the question. I'm specifically interested in analog circuits for this question. – jareddbh Jul 19 '16 at 21:07

When evaluating an circuit’s performance for a low noise application, both internal and external noise sources must be considered.

External noise includes any type of external influences, such as external components and electrical/electromagnetic interference. Interference is defined as any unwanted signals arriving as either voltage or current, at any of the circuit’s terminals. It can appear as spikes,steps, sine waves, or random noise. Interference can come from anywhere: machinery, nearby power lines, RF transmitters or receivers, computers, or even circuitry within the same equipment (that is, digital circuits or switching-type power supplies).If all interference is eliminated by careful design and/or layout of the board, there can still be random noise associated with the circuit components.

Also for example there are internal sources of noise, take the case of an amplifier.

Noise appearing at the amplifier’s output is usually measured as a voltage. However, it is generated by both voltage and current sources. All internal sources are generally referred to the input,that is, treated as uncorrelated or independent random noise generators in series or in parallel with the inputs of an ideal noise-free amplifier.Flicker,Popcorn,Input referred and Output referred noise are some common internal noise in amplifiers.

• Thanks for the thoughtful answer. I added more detail to my original question. It seems in an analog environment and in an enclosed metal box, the only real noise source I should have is the ripple on the power supply lines, correct? If I want to measure a 100uV signal, do I just need to make sure my ripple is much less than that? – jareddbh Jul 19 '16 at 21:08

The practical factor is the Signal-to-Noise Ratio (SNR). And how much SNR is acceptable or excessive depends entirely on the intended function and parameters of the circuit and the larger system. In some cases the system will tolerate a pretty lousy (low) SNR because it easy to detect/process the signal even in the presence of significant noise.

But SOME circuits have a very low tolerance for noise. For example microphone preamps used in high-quality audio gear approaches the practical limit of 130dB.

And SNR is also dependent in a significant way on the bandwidth of the signal. It is easier to make high SNR circuits for narrower bandwidths than for wide bandwidths.