# Presentation: 922819

## Models are Always Wrong, Right?What’s the Difference Between an$85 Model and a $2,000,000 Model

**Colin McAndrew**
- April 4, 2002

## Australia, not Austria

## Australia, not Austria

## Taz

## Tassie, not Taz

## Laws of Simulation and Modeling

**A simulation is only as accurate as the models it is based on**
**A model is (mostly) useless unless it is embedded in a simulator**
**Models are, by definition, inaccurate; it’s just a matter of degree**
**Models generally trade off complexity (simulation time) for accuracy**

## Zeroeth Law of Modeling

**Physics (reality)**
**trumps**
**Mathematics**

## Zeroeth Law of Modeling

**Measurements**
**trump**
**Models**

## Zeroeth Law of Modeling

**Measurements**
**trump**
**Models**

**Especially when Engineers Use Polynomials **

## Numerical Tricks May not Work

**cubic spline**

## Zeroeth Law of Modeling

**Measurements**
**trump**
**Models**

**Just be careful you know what you are measuring**

## Negative Capacitance Blooper

## Negative Capacitance Blooper

## Simple way to Get Capacitance

**Terminal currents from AC analysis**
**Sequentially excite terminals *****k***** with *****1+j0***** volts at ***********=1*****, i.e. *****f=1/(2************)***
**Change sign if**

## Oz is Acceptable

## Aussie, not Ozzie

## Victoria

## Charge or Capacitance?

**They’re really the same, right?**

## Charge or Capacitance?

- C

- v

## Charge or Capacitance?

- C

- v

## Will the IC Fail?

**Real circuitry**

**Protection circuitry**

## Metal Routing Missing

**Real circuitry**

**Protection circuitry**

## Simulation Results

**Capacitance based model did not conserve charge, isolated protection circuitry voltage ramped to infinity!**
**Charge based model showed the missing metal would ****NOT**** cause the protection circuitry to trigger**
**$2,000,000**** of already manufactured parts were shipped and not scrapped**

## Victoria

## Victoria

## Melb’n, not Mell-born

## Milling Coal for the SECV

- stress

- speed

## Milling Coal for the SECV

- stress

- speed

## Milling Coal for the SECV

- stress

- speed

## Melb’n, not Mell-born

## Melb’n, not Mell-born

## Eastern Suburbs

## Asymptotic Correctness

**Any model should give physically correct and reasonable behavior as any controlling parameter approaches an extreme**
**Temperature going very cold or hot**
**Geometry getting very large**
**It is amazing how infrequently this simple principle is applied**
**1/W and 1/L preferred to L and W**

## Universal SPICE 1/f Noise Model

## Universal SPICE 1/f Noise Model

**(at a fixed total current)**

## Universal SPICE 1/f Noise Model

## Universal SPICE 1/f Noise Model

## Symmetry

**Most MOSFET and resistor models are cast in the form I****ds****=GV****ds**
**G(V****sb****,V****db****,V****gb****) is the effective conductance of the MOSFET or resistor**
**Consider symmetric biasing****V****db****=V****db0****+V****x****, V****sb****=V****sb0****-V****x****, V****ds****=2V****x**
**I****ds**** ****MUST**** be an odd function of V****x**
**G ****MUST**** be an even function of V****x**
**Limiting of V****ds**** to V****sat**** ****MUST**** be odd**

## Eastern Suburbs

## Eastern Suburbs

## Eastern Suburbs

## Eastern Suburbs

## Kew

## Kew

## Kew

## Kew

## Kew

**$85**