# Noise Simulation in a fully differential Circuit

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#### AlexanderTheGreat

##### Newbie level 4
mit noise simulation

Hello,

I need to do a noise simulation with a fully differential OP-AMP with CMFB.
The first stage is a Folded Cascode Amp, the second a Class AB Stage.
The CMFB Amp is a simple OTA.

My question: I use SpectreS, the noise simulation need as parameters the positive and negative output node and the input source. Well, do I have to simulate every input separate or both together with a voltage source between the two input nodes?
If i have to do the simulaten separate for both input nodes, which nods do I choose for the negative and positive output nodes in the simulation options?

Thanks!
Alex

#### kwkam

##### Full Member level 5
spectre noise simulation

first you have to understand what is noise of a opamp. simulation is same as measurement, you need the source of noise and the node you are interested the noise from the source.

#### AlexanderTheGreat

##### Newbie level 4
spectre fully differential

Hi,

I want to know the input refered noise for calculating the SNR.
I know how to use the noise simulation for a single ended OP-Amp but I don't know how to make a noise simulation for a fully differential OP Amp.

#### erikl

##### Super Moderator
Staff member
noise of fully differential opamp

Hi Alex,
just put your expected input control impedance (equiv. resistor + (evt.) capacitor) between your input nodes; the output nodes are your (single ended) output and GND (or both nodes of a symmetric (double ended) output).
The Spectre simulation result tells you the noise contribution of your input source and those of all devices of your circuit, either in absolute values, or related to the input or to the output noise. I hope (for you), SpectreS can do the same! ;-)
Viel Spass bei (oder mit) dem "Laerm"! ;-) erikl

### AlexanderTheGreat

Points: 2

#### sutapanaki

simulation,differential op

If you specify you input stimuli as shown on the picture, you can refer the output differential noise back to Vi without a problem.

a.g.electronic96

Points: 2

### a.g.electronic96

Points: 2

#### AlexanderTheGreat

##### Newbie level 4
noise simulation

Thanks!
I will soon try it!

#### coffeelox

##### Junior Member level 3
simulating differential noise

don't know if you have noise simulation in SpectreS?

#### liuyj610

##### Newbie level 1
Re: mit noise simulation

Dear AlexanderTheGreat,
Do you have solved this problem?

AlexanderTheGreat said:
Hello,

I need to do a noise simulation with a fully differential OP-AMP with CMFB.
The first stage is a Folded Cascode Amp, the second a Class AB Stage.
The CMFB Amp is a simple OTA.

My question: I use SpectreS, the noise simulation need as parameters the positive and negative output node and the input source. Well, do I have to simulate every input separate or both together with a voltage source between the two input nodes?
If i have to do the simulaten separate for both input nodes, which nods do I choose for the negative and positive output nodes in the simulation options?

Thanks!
Alex

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