noise simulation in cadence

[liusupeng]

I have a current source which charges a capacitor to generate a slop voltage. I want to find out the noise at the capacitor output. Since the capacitor voltage keep rising, there is no steady state. How can i perform the noise analysis?

 

[keith1200rs]

My suggestion would be to add a resistor across the capacitor to create a steady state with the current source still operating correctly. Then measure the output noise and the noise contribution from the extra resistor and subtract it (using sum of squares).

Keith.

 

[liusupeng]

My suggestion would be to add a resistor across the capacitor to create a steady state with the current source still operating correctly. Then measure the output noise and the noise contribution from the extra resistor and subtract it (using sum of squares).

Keith.

Hi Keith,
thanks for your reply. I am not very confident in your suggestion.
I think adding a parallel resistor will modify the input current(from current source) to output voltage transfer function. just imaging a very very small resistor is added in parallel, the noise current from the current mirror will flow into the resistor instead of the capacitor. if the resistor approach 0, there will be no noise output voltage.

 

[keith1200rs]

I am assuming the resistor will be large, not small but then I don't know what your circuit looks like or the value of your current source or the operating voltage across your capacitor.

Bear in mind with any noise simulation it is the equivalent impedance that matters. Often adding a large value resistor in a circuit somewhere is not actually detrimental to the circuit noise because its impedance is in parallel with something else which reduces the noise resistance. For example, in your case you have a capacitor which will have a significant effect on the noise.

Maybe if you post your circuit it might help the discussion.

There are transient noise analyses which can be done which may help.

Keith.

 

[liusupeng]

I am assuming the resistor will be large, not small but then I don't know what your circuit looks like or the value of your current source or the operating voltage across your capacitor.

Bear in mind with any noise simulation it is the equivalent impedance that matters. Often adding a large value resistor in a circuit somewhere is not actually detrimental to the circuit noise because its impedance is in parallel with something else which reduces the noise resistance. For example, in your case you have a capacitor which will have a significant effect on the noise.

Maybe if you post your circuit it might help the discussion.

There are transient noise analyses which can be done which may help.

Keith.

Thanks keith. I agree that if the added resistor has much greater impedance the simulation should be able to give the correct results.
Thanks for you kind help

 

[keith1200rs]

Also, try a few different resistor values (making sure it keeps the circuit in the working range). Then if it does affect the noise you could extrapolate the noise to a value having no resistor.

Keith.