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Differential LPF Cutout Frequency Simulation

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While designing a thermocouple circuit, I saw that a 1nF capacitor was placed at the differential signal input as shown in the figure. This was added for both differential signals, so I can see from the simulation that it is a common mode LPF. We can say that it has a cut-off frequency of 15.87775k, which means approximately 16kHz. However, when I analyzed the Differential lowpass filter, we can say that it is 756 Hz, that is, approximately 0.8khz. However, in this image, the cutoff frequency is written as 1.3khz. What point am I missing?

(When I add 1nF and 1nF Caps to the simulation, the cut-off frequency becomes 756.08325 Hz. How was the 1.3KHz cut-off frequency reached?)

source analog devices Analog Dialogue 44-10, October (2010)



I can only conclude they left out a factor of 2, yours and the calculation I did are exactly 1/2 of what they say. The article: https://www.ti.com/lit/an/slwa053b/slwa053b.pdf?ts=1708725804987&ref_url=https%253A%252F%252Fwww.google.com.au%252F
nicely describes converting between single ended and differential filters.

Thanks for this article ,its very informative.According to the documentation you posted, the single ended conversion is done like this. However, I still did not get the same cutoff frequency. I think I missed some points.

Your single ended conversion is correct except the cap should be 21nF, it looks like they made a mistake, very easily done.

A quick calc gives me 2.653 kHz, which seems to fit with your second sim (the first one is unreadable. PLEASE post zoomed cutouts of the relevant screen areas.


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