SPICE simulation vs. Hand Calculation

[hyc]

simulation by hand

My understanding of typical analog design practice is that once the topology is chosen, one will do hand calculation to estimate the devices W/L before running SPICE simulation, however, as the technology progresses into submicron regime and the model becomes so complicated, the results from hand calculation and the results from SPICE simulation have so much discrepancy, I wonder if it still make sense (or necessary) to do hand calculation first or if there is better methodology.

Thanks,
Hau

 

[kishore2k4]

mos model for hand calculation

You can do hand calculations with MOS level 1 equations with approximate values. You can also do some characterisation for different W/L transistors and use them correspondingly. For example channel length modulation is very difficult to model in modern CMOS process so you can characterise it and read off the values from the graph and use them for that particular W/L and repeat.

Ultimately, you still have to do fine tuning until you meet all the specs. That is the reason analog designers always tell you to get a "feeling" of the circuit. Once you have done a couple of circuits you'll be comfortable with approximations that are an order of magintude away from the actual values and work your way through it.

Intuitive analysis is extremeley important in analog design and it will take some time to get there.

If you are feeling adventerous you can use the actual BSIM or EKV MOS equations for your hand calculations with some approximations, ofcourse.

 

[gabor_csipkes]

russian hand calculations

Hand calculations will prevent you shooting in the dark with your geometries and operaing points. Most designers consider it a good practice and starting point before fine tuning your circuit in the simulator.

Of course, modern technologies lead to larger discrepancies but you get a first idea on the values and your parameters will not be completely wrong.

 

[psmon]

how to perform simulation by hand

For beginners, start with hand calculation if not you have to be very lucky to know the right W/L.

 

[Mr.Cool]

simulation by hand

i still do hand calcs first because i need to know if my simualtion result is lying to me or not.

Mr.Cool

 

[Sadegh.j]

modeling of analog circuits vs actual models

In sub micron, the models become more complicated and you have to use the short channel model for the transistor. Based on this, estimate Ws and Ls. This will help you REALLY understand the operation of the circuit and get a feeling for trade offs. then go from there

 

[EKV_web]

fine tuning of the circuit by spice simulation

My understanding of typical analog design practice is that once the topology is chosen, one will do hand calculation to estimate the devices W/L before running SPICE simulation, however, as the technology progresses into submicron regime and the model becomes so complicated, the results from hand calculation and the results from SPICE simulation have so much discrepancy, I wonder if it still make sense (or necessary) to do hand calculation first or if there is better methodology.

Thanks,
Hau

let me recommend following "Hand Calculation" reading:

# Danica Stefanovic and Maher Kayal
Structured Analog CMOS Design Series: Analog Circuits and Signal Processing ISBN: 978-1-4020-8572-7

# Paul Jespers
The gm/ID Design Methodology for CMOS Analog Low Power Integrated Circuits ISBN: 978-0-387-47100-6

# D. M. Binkley
'Tradeoffs and Optimization in Analog CMOS Design', ISBN: 978-0-470-03136-0