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Simulators >> Circuit Simulators >> total input reffered noise is computed in SPICE
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Message started by john_xu on Jul 31^st, 2006, 1:08am

Title: total input reffered noise is computed in SPICE
Post by john_xu on Jul 31^st, 2006, 1:08am

I know how the total output reffered noise is caculated in noise simulation. But I have some puzzles on how the total input reffered noise is computed in SPICE.

Anyone help to clarify it?

Title: Re: total input reffered noise is computed in SPIC
Post by Geoffrey_Coram on Jul 31^st, 2006, 7:22am

I believe you specify an input source, and the simulator computes the gain from input to output, and uses this to scale the output noise, resulting in input-referred noise.

Title: Re: total input reffered noise is computed in SPIC
Post by john_xu on Jul 31^st, 2006, 5:58pm

Geoffrey_Coram wrote on Jul 31^st, 2006, 7:22am:

I believe you specify an input source, and the simulator computes the gain from input to output, and uses this to scale the output noise, resulting in input-referred noise.

Hi Geoffrey_Coram,
Thanks for th ereply.

The gain is frequency dependent. How the spice select the gain?its dc gain?

thanks

Title: Re: total input reffered noise is computed in SPIC
Post by Andrew Beckett on Jul 31^st, 2006, 9:25pm

It calculates the small-signal gain over the same frequency range that you did the noise sweep. So the gain is frequency dependent, as you say. Noise analyis computes the magnitude of each noise source, and then multiplies that by the transfer function from that noise source to the output, The same method of calculating transfer function is used for the signal input.

So you end up with a sum of all the noise powers at the output (over frequency). You can then divide this sum by the signal gain (again over frequency) to get the input referred noise (over frequency). Often the output-referred and input-referred noise are then integrated over a particular bandwidth to give you the total noise power in the bandwidth of interest.

Regards,

Andrew.

Title: Re: total input reffered noise is computed in SPIC
Post by john_xu on Aug 1^st, 2006, 6:14pm

Andrew Beckett wrote on Jul 31^st, 2006, 9:25pm:

Hi Andrew,
Thanks for the helpful explaination.

But some papers said the input reffered noise is the total output reffered noise divided the gain(mid-band gain). According it, the gain should not be frequency dependent, i.e., the input reffered noise is not divided by the transfer function from input to output(it is freq dependent), but divided buy a single gain.

Would you pls. help to clarify it?

Thanks

Title: Re: total input reffered noise is computed in SPIC
Post by Andrew Beckett on Aug 1^st, 2006, 11:03pm

Well, it's certainly not a single gain in spectre, nor in other simulators that I've used. If computed that way, the input-referred noise would not be very useful. I can't really see any computational benefit in doing it that way either, since you're already computing frequency-swept transfer functions for all the noise sources, so adding one more for the input source is not exactly going to cost you much.

Can you post a reference to the papers that talk about the input-referred noise being computed using a mid-band gain?

Regards,

Andrew.

Title: Re: total input reffered noise is computed in SPIC
Post by Geoffrey_Coram on Aug 2^nd, 2006, 4:56am

I could imagine that those papers were describing a measurement technique for computing IRN...

Title: Re: total input reffered noise is computed in SPIC
Post by john_xu on Aug 2^nd, 2006, 11:33pm

Geoffrey_Coram wrote on Aug 2^nd, 2006, 4:56am:

I could imagine that those papers were describing a measurement technique for computing IRN...

Yes, it is indeeded to guide the designer to measure SNR, i.e.,in computing the IRN, using single mid-gain or DC gain.

Thanks Gerffrey, Andrew, the replies are very helpful to me.