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Practical Noise Analysis Problem and Solution in NI Multisim
Hello Circuit Designers,
NI Multisim includes many circuit simulation tools (all available under Simulate>Analyses in the Multisim menu). One of the most critical pramaters affecting a circuit's behavior is the noise.
Using Multisim noise analysis, designers can accurately simulate the effects of noise and accordingly make the right design decisions. Resistor thermal noise could be evaluated as well as also other noise effects caused by semiconductor components such as shot noise, flicker noise, and 1/f noise.
For a step-by-step tutorial on how to properly setup noise analysis in Multisim, refer to this page. What I will be highlighting in the rest of this post is a practical design use-case where noise analysis is beneficial.
Design Use-Case
Assume there is a slowly-varying (<5Hz) input signal with a voltage range of 60mV which needs to be amplified and digitized. The requirement is to determine whether or not the circuit shown below, in combination with a 16-bit A/D, achieves a Signal-to-Noise (SNR) ratio of 100dB after the amplified signal is digitized. If it does not achieve this specification, then a noise filter should be added such that it does.
With the amplifier gain of 100, the output signal has a range of 6V. The RMS voltage noise, Vn_Totalrms , corresponding to 100dB and that which must not be exceeded at the A/D input is calculated as follows:
Of course, the A/D itself has quantization noise. The quantization noise on a 16-bit A/D operating with the same +5V/-5V supplies is:
Since the A/D quantization noise and the noise from the amplifier circuit, Vn_Amprms, are uncorrelated, they add up as a sum of squares, namely:
Therefore, the maximum RMS voltage noise produced by the Amplification circuit is 41 uVrms.
At this point, we can use Multisim to determine the noise that this amplifer circuit produces on node Out and see if it is less than 41uV. There are few ways to do this. We can use the Calculate total noise values option in the Noise Analysis dialog. This would provide a scalar value representing the integrated noise across a specified bandwidth. However this information would not provide to us a good indication of how integrated noise is affected by bandwidth, in case we need to make adjustments to the circuit. Therefore, we shall use the Calculate power spectral density curves option and then use some post processing functions to get a better understanding of how noise is affected by bandwidth.
In the Analysis tab, let's use the following settings:
In the Frequency parameters tab, let's use the following settings:
The analysis is carried out across a very large bandwidth (0.1Hz to 10GHz) so that we have an entire picture of how the noise varies with frequency. The simulation will be a plot of noise density in units V2/Hz. However, we are looking for an integrated noise plot in units Vrms. To do this, we can create an expression using the integral() function. It will return a plot in units V2, so we simply apply the sqrt() function to get the desired result. The overall expression is sqrt(integral(onoise_spectrum)). The result is shown below.
The shapes of the curves match intuition. The noise density plot is high at very low frequencies as a result of the op-amp’s 1/f noise. At approximately the cut-off frequency of the circuit, the noise density starts to fall, as would all signals in the circuit. The Integrated Noise plot represents the running integral (starting at 1Hz) of the noise density plot. Obviously starting with zero noise for zero bandwidth, the integrated noise climbs quickly, leveling off shortly after the cut-off frequency when little noise is being contributed by each frequency.
Therefore, if no additional filtering is added to the circuit to limit its bandwidth, the Integrated Noise plot shows that there will be approximately 530uV of noise on the output – far above our of required limit 41uV. However the plot shows us that in order to limit the noise to 41uV, the bandwidth should be limited to approximately 335Hz. So let us add-in a simple, buffered R-C filter as follows:
Also note that adding the buffer and resistors would introduce noise which are picked up at “out” pin.
The above circuit produces the following noise curves:
Now, the Integrated Noise has leveled off at approximately 53uV – a lot closer to the required 41uV! Therefore we may want to limit the bandwidth slightly more and see how that takes effect in the Multisim simulation. The purpose of this example was to illustrate how NI Multisim’s Noise Analysis can be such a beneficial tool for the designer.
