How to define FOM(figure of merit) formula for opamp?

[superleaf]

I read some paper and found that there are different kinds of formulas to define the FOM of opamp?and i am confused that how to choose the right formula to define it! for example,there is one formula like this:FOM=GBW*C(load)/I(total),i have no idea it is correct or not,could someone can provide me some opinion?thank you in advanced!

 

[LvW]

For my opinion, it makes no sense to create such a FOM.
There are many, many different applications for opamp with different requirements.
On the other hand, there are many performance parameters describing the opamp properties like:
GBW, stability margins, slew rate, offset, offset drift, port impedances, power consumption, rail-to-rail properties, supply voltage range (lower bounds), sensitivity upon capacitive loads, ...

For most applications only one or only some of these parameters are important.
Therefore, I consider it as not appropriate to define such an artificial "figure of merit".

 

[superleaf]

your opinion sounds good and i also agree with your opinions!but some paper enjoy using the FOM to compare with other papers to explain its opamp much better~~~

For my opinion, it makes no sense to create such a FOM.
There are many, many different applications for opamp with different requirements.
On the other hand, there are many performance parameters describing the opamp properties like:
GBW, stability margins, slew rate, offset, offset drift, port impedances, power consumption, rail-to-rail properties, supply voltage range (lower bounds), sensitivity upon capacitive loads, ...

For most applications only one or only some of these parameters are important.
Therefore, I consider it as not appropriate to define such an artificial "figure of merit".

 

[LvW]

your opinion sounds good and i also agree with your opinions!but some paper enjoy using the FOM to compare with other papers to explain its opamp much better~~~

Please, can you give some references, which support such a FOM definition?

 

[erikl]

can you give some references, which support such a FOM definition?

E.g. David M. Binkley "Tradeoffs and Optimization in Analog CMOS Design" makes extensively use of several FOM definitions, as

• thermal-noise efficiency figure of merit - p. 199
• bandwidth–power figure of merit - p. 260
• bandwidth–power–accuracy figure of merit - p. 261

He also says: "In practice, the designer will rarely maximize or minimize a single figure of merit. Instead, the designer will usually balance multiple design requirements using intuition gained from an understanding of tradeoffs in performance and tradeoffs in figures of merit that combine important aspects of performance." - p. 346

 

[LvW]

Erik, thank you.

I think, it could make sense to define some specific FOM's involving only two or three particular parameters in case of contrarotating properties.
For example, I believe that also the gain-bandwidth-product GBW could be seen as such a quality factor because very often larger gain values are
available only at the expense of a smaller bandwidth and vice versa.

But a general FOM to qualify opamps is not meaningful resp. impossible.

 

[dick_freebird]

... some paper enjoy using the FOM to compare with other papers to explain its opamp much better~~~

It's not uncommon for vendors to select the figure of merit
that puts their product in the best light. Same is true of
academics, they're just selling a different "product".

Now as a user, you would rather figure out what matters
to your application, prioritize and weight the attributes of
interest, make your formula and let the chips fall where
they may.

Universal figure of merit, there is none. That's why there
are a bazillion different op amps out there when one ideal
one would do. Heh.

 

[superleaf]

Thank erikl and dick_freebird very much,your opinions and explanation is very reasonable and i benefits a great deal!