Noise Analysis for the 555 timer - How to not ruin your design.

Noise Analysis for the 555 timer

With modern day advancement in electronics resulting in low power and high speed designs noise becomes an ever increasing challenge for circuit designer. The concept of noise its effect and analysis is quite complex apart from noise which can come into the system from external sources via different forms of coupling via i/o ports, power rail etc certain electronic components also generate noise on their own. Every data sheet specifies those parameters. So dealing with noise is quite a challenging aspect. However in this blog I am not going to get to complex mathematical analysis of noise but show you a rather simple example of what noise can do to your circuit if proper precaution is not taken. 

Let us start with a simple circuit a 555 timer hocked in astable configuration to generate a 4% on time duty cycle. Before I go any further let me tell you if you have switching elements in your circuitry there is a high chance you have to deal with noise issue as switching elements have very high frequency components which can get coupled to other parts of the circuit. 555 timer is a switching element hence we have to deal with noise. This is the circuit we will be dealing with.

This is a simple circuit used to generate a PWM waveform to blink a LED at the output. It can also provide voltage control to other parts of the circuit. Generally the duty cycle you get from 555 timer in normal astable configuration is always greater than 50%. In order to get around this I used diode D1 which bypasses R2 hence duty cycle can go below 50% now. As usual the circuit works perfectly fine giving duty cycle of 4% @ 2.93Hz. Here is a grab from the oscilloscope.

The rise time for this square wave output is about 130nsec. This is from a cheap Chinese 555 timer IC. The CMOS version of 555 can have very low rise time. Lower the rise time steeper will be the slope and the wave form will contain more high energy stuff and likely to produce more noise. Let us look at the Rise time curve.

Notice the measurement window at bottom left the rise time is shown there. With 130nsec still this chip will inject considerable noise.

Look at the circuit diagram notice the power pin for the IC (pin 8) it lacks a decoupling capacitor. The only decoupling the +5V rail gets is from the decoupling capacitor at the voltage regulator output. As the V_reg was quite far away pin 8 requires its own decoupling cap.

Now let us look at the power rail with our channel 2. Channel 2 is ac coupled so we are only looking at the noise which sits over the +5V dc.

Ok before you panic let me tell you this that I have built the circuit on a bread board not the ideal case when you want to do noise analysis. In a printed circuit board this effect would be somewhat reduced but always be there to ruin your design unless you do something. Ok coming back to the diagram look at the delta y measurement I have done that gives the peak to peak noise its almost half a volt. That much of noise is being injected into the rail. Now if you feed your other parts of circuitry with this rail say your precision low level amplifier your design is done. Most probably if your amplifier does not have good supply rejection it will start to oscillate not a good thing for your design. Another thing I noticed when the output is high the amplitude of the noise is low compared to when the output is low. The following picture will clearly show it when I have speed up the scope.

Now just by pacing a single 1uF cap as a decoupling capacitor the noise reduced almost 1 orders of magnitude. I have tried different cap values 0.1uF, 10uF, 47uF etc 1uF works the best.

Let us look at the decoupled version of the noise.

Again look at the delta y measurement the noise now merely is 58mV peak to peak. Still 58mV is not enough for certain application but then we have to design our circuit on a pcb. However a  ferrite bead series with the power rail will prevent some of those high frequency components from going into the power rail and subsequently into other parts of the circuit. Ferrite beads being basically an inductor provides high impedance to high frequency signals thereby blocking some of them. I didn't have a ferrite bead so I use a 1mH inductor not ideal but the noise did decrease a bit.

Here is the final noise result.

Final peak to peak noise with inductor is about 44.8mV.

So here it is an apparently simple circuit can wreck havoc in your design if you don't know the effect of noise. Although I have just scratched the surface of a vast topic but I hope this will give you enough knowledge to be cautious when dealing with switching components in your circuit. Till next time...