Skip to main content

How Does Noise Affect Circuits? - Understanding Noise Part I

I am back from a little hiatus. I've been meaning to write something about noise in the circuits, because I know it's one of the least understood topics in electronics. It's something that one can easily attribute to when something goes odd in the circuit, yet so few have the courage to delve into it.

In audio applications, it's easy to describe what noise is. We know if the music is clear and pure, it is free of noise. Anything that is unwanted we can call noise, well, in a broad sense of the word.

Noise comes in different forms and from different sources, but in the hope of making some order out of chaos we can attempt to categorize them in the following:

1. Device Noise - noise coming from active and passive devices
2. Emitted Noise - those coming from power lines, motors, radio trasmission, etc.
3. Conducted Noise - those that are already in the circuit, either from the device or transmitted into

Electronics components are indeed sources of noise in the circuit. The most prominent is the noise coming from the resistor and operational amplifiers. The noise from the resistor is called thermal or Johnson noise. It can also be called white noise, because it is present in all frequencies. If you make a frequency plot of the noise power, it will look something like this.

Johnson Noise


Resistor Noise
When you put your resistor in a circuit, it becomes like an unwanted voltage source, sourcing noise. Even if you take a resistor, let it lie on a table and get an RMS voltmeter, you can expect to measure a voltage equivalent to the noise that the resistor produces. This noise is inherent in the device and is due to the particles in the resistor, whether you apply power or not. The noise in rms is given by:



We don't really have to always use the formula, just keep in mind that a 1KOhm resistor produces 4nV per root Hz of noise. Stating that in mathematical form,

This representation is called noise spectral density. Intuitively, it simply states that there is 4nV of rms noise in 1 Hz step for a given range of frequency. So in order to determine the total noise of in a system due to a 1KOhm resistor, you multiply that with the square root of your bandwidth and you come up with the noise in RMS. A 2KOhm resistor will not double the noise value, but rather will have 5.65nV/(root Hz). This is because we add the noise by RSS (Root of the Sum of Squares), that is, square the value of the individual noise due to particular resistance, add them, and get the square root.

In the next parts of this series we will be discussing the meaning of the terms spectral noise density, noise power, noise rms, and noise peak-to-peak.

References:
1. Tackle Noise From Three Perspectives, Bonnie Baker - EDN
2. Chapter 8, Noise: The Three Categories by B. Baker- Analog Circuits: World Class Designs, Robert A. Pease



Labels:

Comments

Post a Comment

Popular posts from this blog

What Can You Do With Two Transistors (BJT)? Part 3

This is a continuation of a running series on the kinds of simple circuits we can do with basically just  two transistors. In Part 1 we talked about Current source and Multi-vibrator, and in Part 2 we discussed Linear Regulators. We will continue with more practical circuits that can be very useful in many applications. There are instances that we would like to translate a digital logic voltage from one level to another, say from 5V to 3V, or vice-versa. The input perhaps coming from a circuit that has a higher logic level that needs to input to a different logic level. For a single direction of signal, a single transistor would be enough. Bidirectional functionality can be achieved using two transistors, Bidirectional Logic Translator , as shown below. R1 serves as a pull-up resistor to 5V in the circuit. The input typically comes from an open-drain or open-collector output, pulled up to supply by R1. The enable pin should be tied to the lower of the two supplies. Whe...
5 comments
Read more

Arduino Transistor DC Motor Control

Arduino boards have PWM (Pulse Width Modulation) outputs that can be used to control like the speed of a DC motor. On a Pro Micro, those outputs are encircled white on the board. You can program it such that it outputs on a scale of 1-255, 1 being the slowest and 255 being the fastest. PWM gives out pulses whose width is varied (modulated) while the period is constant. The longer it is high, the higher power it delivers to the circuit. PWM can be used to drive a transistor (switch) which in turn drives the motor ON and OFF. The longer the switch is ON, the higher the power delivered to the motor, and thus faster. The circuit consists of a transistor Q1 (TIP31) NPN transistor, driven by the PWM from Arduino. The resistor is to limit the current to the base, but enough to operate the transistor in saturation when the input is high. The diode is to protect the motor from any back emf that might come from the motor when the current is cut off. The circuit is supplied by a 9V battery. ...
2 comments
Read more

Resistance, Capacitance, and Inductance (R-C and R-L Circuit)

Our aim here is to discuss the very important concepts and give intuitive explanations of the subjects. We'll start with the resistor which is the most straightforward, then with the less understood and often misunderstood concepts of capacitance and inductance. We will discuss the R-C and R-L circuits as well. These along with Ohm's Law, Kirchoff's Voltage Law (KVL) and Kirchoff's Current Law (KCL) are the very foundations of understanding the complete behaviors of practically any circuit one might encounter. Resistance It is a general term that refers to the opposition to the free flow of current in a circuit. More accurately, it refers or applies to DC (direct current). Resistance doesn't apply only to resistors but also to a host of other components in the circuit. For example, a copper trace has a resistance according to its dimensions, as well as the leads of he components.When a current flows through a resistor a potential difference is generated acr...
3 comments
Read more