# How does Spice calculate Current Noise?

• posted

How does Spice calculate Current Noise?

I know it takes V(ONOISE) and refers it to the input _current_ noise, but _how_ is that calculation done?

Thanks! ...Jim Thompson

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| James E.Thompson                                 |    mens     |
• posted

Well, in LTspice I often plot things like 'V(Q1)*inoise/v(onoise)', which g ives Q1's contribution to the input-referred noise, which in my stuff is no rmally a current.

In a noise simulation, everything is linearized ahead of time, so Zin is a known function of frequency. You can express the simulated noise as either a current or as a voltage, but to separate the contributions of device nois e voltages and currents, you have to use actual algebra. ;)

Cheers

Phil Hobbs

(Back from a jaunt to Coney Island and preparing for another to the Catskil ls tomorrow)

• posted

I can do Algebra in PSpice ;-)

What's the (set of) equation(s)?

If I can measure Zin (I can), is it magnitude, or how is it handled? ...Jim Thompson

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| James E.Thompson                                 |    mens     |
• posted

I rarely leave the house on holidays... I despise crowds. ...Jim Thompson

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| James E.Thompson                                 |    mens     |
• posted

It wasn't crowded, just enough people for a buzz. The only wait we had was ~10 minutes for a rooftop table in a beach bar. I really like Coney Island

--I'd like it more if it didn't have certain zones whose music is way too l oud, but you can easily avoid those.

Cheers

Phil Hobbs

• posted

Figured it out:

Do a normal .NOISE, but step in VAC and IAC ...

.ac DEC 101 10 100Meg .noise V([OUTL],[OUTR]) V_VAC .STEP PARAM PASS LIST 1 2

Step 1 selects source as VAC, Step 2 selects source as IAC

V([OUTL],[OUTR]) because it's a differential stage

In Probe get V(INOISE) from V(INOISE)@1

then INOISE:

I(INOISE) = V(ONOISE)@2/(V(OUTL,OUTR)@2/I(IMZ)@2/I(IMZ)@2)

IMZ is measuring the input current thus providing the transimpedance gain needed to calculate I(INOISE).

Thus I can get both V(INOISE) and I(INOISE) on a single display, and watch current noise go up as voltage noise goes down, thus can quickly find an optimum for the problem at hand. ...Jim Thompson

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| James E.Thompson                                 |    mens     |
• posted

Introverts Unite!

Quietly.

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Tim Wescott
Wescott Design Services ```
• posted

Trypo, should be:

I(INOISE) = V(ONOISE)@2/V(OUTL,OUTR)@2*I(IMZ)@2 ...Jim Thompson

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| James E.Thompson                                 |    mens     |
• posted

For PSpice Macro users...

.ac DEC 101 10 100Meg .noise V(A,B) V_VAC .STEP PARAM PASS LIST 1 2

(Step 1 selects source as VAC, Step 2 selects source as IAC)

VNOISE = V(INOISE)@1

INOISE(A,B) = V(ONOISE)@2/V(A,B)@2*I(IMZIN)@2

Where V(A,B) is the reference output ...Jim Thompson

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| James E.Thompson                                 |    mens     |