When I visited a Chinese university, a PhD candidate asked a similar question:
How much margin is needed that the circuit really works in reality??
They made a IR remote control IC having a 38kHz bandpass filter internally. It was a gmC filter, and they assumed to treat RC tolerances a trim range of +-50% is enough. However the chips come back with even much larger variations in center frequency. So they switched away from the Chinese foundry, but also the 2nd fab gave similar variations.
In another French design for a temp sensor the accuracy was something like +-2°C vs PVT corners, but the real devices varied by +-50°C!!
In my own 1st RF PA design the Pout was too low. I we turned on the RF, the bandgap voltage goes down by 30%, resulting in too low bias current. The reason was that the big Miller cap bottom side plate picked up the substrate noise, and was rectified internally.
Both shows that PVT analysis is by far not enough, also do this:
- Look also at mixed corners, like SF, FS or bigCsmallRslowMOS!
- Check each factor separately for best understanding
- Do MC run with process variations
- Do MC run with mismatch variations (that was missing for the both mentioned design examples!!)
- Run parasitic extraction carefully (i.e. having decoupled mode is often much too ideal). RF designs need often also L extraction.
- Include package, maybe even a little substrate model (a simple model is better than nothing)
- Combine all the worst-cases
- Plan for debugging!!!
And do not pamper your design
![Cool Cool](https://designers-guide.org/forum/Templates/Forum/default/cool.gif)
: If you want T=-25...100°C, better do a much larger sweep and try to
understand why the circuit fails in certain situations, then improve further and implement backup solutions, such as trimmings.
Bye Stephan