ahdlib opamp specifications for bandgap

[susmitha@99]

Hi,

i am trying to implement bandgap reference circuit using opamp from ahdlib in cadence virtuoso. But I am facing difficulty in giving the parameters for opamp.

What should be the values of the parameters gain, freq_unitygain, rin, vin_offset, slewrate ? Please let me know

 

[dick_freebird]

If you are after realism, make yourself a testbench and
put a "library" op amp in it from the technology's cell
libraries (if such can be found). Or find a char report for
something at least in a common "node". Realizing that
a full-featured rail-rail op amp may be gross overkill in
fact, as your bandgap-core application has very limited
input and output common mode range of interest,
negligible current throw (tens of uA?) and has to pretty
much just deal with DC, and not make a mess of PSRR.

Now as an output buffer, more op-amp-y goodness may
well be desirable.

You can "idealize" the op amp to see what outside of that
impacts performance and consistency. You can worsen it
to examine "how cheap and crappy can it be and still
make spec"? But for a "typical" report-of-performance
you ought to find a "typical" op amp and that means
picking a platform, first.

 

[susmitha@99]

If you are after realism, make yourself a testbench and
put a "library" op amp in it from the technology's cell
libraries (if such can be found). Or find a char report for
something at least in a common "node". Realizing that
a full-featured rail-rail op amp may be gross overkill in
fact, as your bandgap-core application has very limited
input and output common mode range of interest,
negligible current throw (tens of uA?) and has to pretty
much just deal with DC, and not make a mess of PSRR.

Now as an output buffer, more op-amp-y goodness may
well be desirable.

You can "idealize" the op amp to see what outside of that
impacts performance and consistency. You can worsen it
to examine "how cheap and crappy can it be and still
make spec"? But for a "typical" report-of-performance
you ought to find a "typical" op amp and that means
picking a platform, first.

I'm sorry, I didn't understand. Can you please explain a little clearer way. I am a beginner in cadence analog design.

 

[dick_freebird]

I think we would both be better off if you picked one thing you
do not understand, and attempt to understand it. A global
!clue is not something I can undertake to fix. A well formed
question might get a differently formed answer.

 

[nitishn5]

I would rather suggest that you use the vcvs from the analogLib library.
That is effectively an Ideal OPAMP and you just have to enter the gain.

This should be enough for your Bandgap Reference.


BTW, I am assuming that you would have attended a class/course on Linear Circuits and OpAmps which would have explained the parameters like "gain, freq_unitygain, rin, vin_offset, slewrate". Otherwise you are jumping into the deep end of the pool without even knowing to swim.

 

[susmitha@99]

If you are after realism, make yourself a testbench and
put a "library" op amp in it from the technology's cell
libraries (if such can be found). Or find a char report for
something at least in a common "node". Realizing that
a full-featured rail-rail op amp may be gross overkill in
fact, as your bandgap-core application has very limited
input and output common mode range of interest,
negligible current throw (tens of uA?) and has to pretty
much just deal with DC, and not make a mess of PSRR.

Now as an output buffer, more op-amp-y goodness may
well be desirable.

You can "idealize" the op amp to see what outside of that
impacts performance and consistency. You can worsen it
to examine "how cheap and crappy can it be and still
make spec"? But for a "typical" report-of-performance
you ought to find a "typical" op amp and that means
picking a platform, first.

Thank you for the reply. I wanted to know what would be the values for an ideal opamp. Neverthless, i figured out what values to give. But one more question I got is can I change the vin_offset to some value instead of 0.

Thanks for your time.

 

[susmitha@99]

I would rather suggest that you use the vcvs from the analogLib library.
That is effectively an Ideal OPAMP and you just have to enter the gain.

This should be enough for your Bandgap Reference.


BTW, I am assuming that you would have attended a class/course on Linear Circuits and OpAmps which would have explained the parameters like "gain, freq_unitygain, rin, vin_offset, slewrate". Otherwise you are jumping into the deep end of the pool without even knowing to swim.

Thank you for the reply and suggestion. Maybe I will try using the analogLib one as well.

 

[dick_freebird]

vcvs as an op amp lacks the phase lag which destabilizes closed loop
circuits. You may need to crutch that depending on the loop response,
outside of DC analysis which doesn't care.

You can emulate input offset with a placed voltage source at the
inverting input, or you can use the behavioral amp's vin_offset param.
I'd prefer the series source because you can sweep it if you choose
but the behavioral can only be stepped. Which may or may not be
an interest.