How to find the free running oscillation frequency of a Sigma Delta Circuit?

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I've been trying to find the equation that would tell me the free running oscillation frequency of this circuit. if Vin = 0

I know it has to do with this, but I'm not sure on how the derivation of the equation came to be.

Is there any type of reference anyone know of that can help me derive this.

Major Update:

here are 2 references I found:

Let me know if you think it should be different

Posted May 16, 2019, 7:20 PM UTC · edited