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# THS4131 : Voltage Noise & SNR Calculation

Other Parts Discussed in Thread: THS4131, THS4551, THS4541

Hello Team,

Would you spport below queries from customer?

1. When I simulate the voltage noise of THS4131, the results much higher than 10nV/root Hz. I compared the results between datasheet figure 24 and simulation out. Would you let me know how to properly simulate the voltage noise vs. frequency?

2. How to calculate the SNR of THS4131? Just THS4131 output voltage divide by noise output voltage and then chage the results with log scale?

Best regards,

Alice

• Hello Alice,

Please see attached circuit. I have done a few things here:

1. I have changed the temperature of the resistors to -273C (double-click the resistor and look at the temperature settings). I do this so that I can extract just the noise of the amplifier since at -273C the resistors are noiseless.

2. I have added two differential measurement circuits at the input and output of the amplifier.

Now if you do a noise analysis for "Output Noise" you will notice that the flatband noise at the input is 1.28nV/rtHz (as per datasheet). The output noise is around 2x the input noise. This makes sense because while the amp is configured in a signal gain of 1, the noise gain is 1+(390/390) = 2. So the noise gets multiplied by 2x. The reason that the noise isn't a perfect 2x is because of the amplifier current noise contribution which results in a noise of 2*1pA/rtHz*390 =780pV/rtHz. Noise is added by the square root of sum of squares so the final answer is around 2.7nV/rtHz.

For SNR, there is an "SNR analysis" under the "Noise Analysis" simulation. There you can select your signal level and frequency. Another method is to do the "Total Noise" under "Noise Analysis" and manually calculate the SNR as your (Input RMS/Total Noise in the frequency range of interest). Hope this makes sense. If you look at the datasheets of the THS4541 and THS4551 there is a clear explanation on the noise analysis and the various factors contributing to total output noise.

-SamirTHS4131_TestCkt.TSC

• Thank you for kind support.