August 22, 2012
Anyone who has read a few of my blogs has seen that I simulate circuits in SPICE. You may know that Bob Pease was pretty opinionated about SPICE and once wrote, “SPICE takes away your ability to get good insights on what is happening. Spice actually HURTS your understanding of how circuits function.” In honor of Bob’s birthday today, let’s consider some merits and pitfalls of SPICE.
Bob was a colorful character and often resorted to hyperbole to make a point. I understand his concern. Half-baked hacking around in SPICE can be counterproductive and may stunt your analog growth. I’m sure he witnessed this on occasion.
The fact is we may not be as skilled or experienced as Bob. We may not have the analog mentors to guide and teach us as he once did. Our design world moves at a quick pace and we may need help.
I believe that SPICE, when used judiciously, can improve your understanding of circuits and make you a better analog designer. It requires discipline. What does that mean?
When you perform a circuit simulation, have clear expectations. Think carefully first. From knowledge, experience and calculations, anticipate the result. If you get what you expected, great. If not, you have an opportunity to learn. Don’t immediately try something different. First, do your best to answer the question of why the results didn’t match your expectation.
Maybe it is due to a simple oversight and you can quickly move ahead. But if not, it may be an indication of some faulty thinking or a miscalculation on your part. Better to learn from it and gain new insight. Or maybe your models are not perfect (Bob would remind us that they never are). Could this account for the difference? Or maybe you have misused SPICE and gotten some quirky result (it happens). Whatever the reason, ignoring it and moving on is analog-hacking.
Figure 1. A small-signal transient simulation in SPICE can reveal potential instability. It’s used here to step through three values of CF to check compensation for input capacitance. Bob would remind us that the tiny capacitance values of CF would demand verification and optimization on actual circuit board layout!
Now I won’t say I always follow my own advice. Sometimes, I will suspect a cause and try something different. I will circle back later to resolve the discrepancy. There are plenty of ways to use or misuse SPICE but I think this is a basic philosophy that can guide your simulation efforts.
I would love to have a friendly discussion with Bob about SPICE. I bet there could be some “give” in his seeming never-use-SPICE position. In fact, here is another side of the story indicating that he indeed had some flex on the issue: Bob Pease didn’t hate SPICE.
In future posts I’ll provide some tricks and suggestions on how to get the most from SPICE simulations. And if you’re not already using SPICE, I recommend TI’s free SPICE program, TINA-TI. It’s very capable and quite user-friendly.
A Pedantic fact: SPICE = Simulation Program (with) Integrated Circuit Emphasis
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