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Re: OpAmp design - from theory to practice

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On 2017-03-29 07:55, Francesco Zappon wrote:
> Hello everyone,
>
> first post here. I hope I do not break any rule; let me know if this
> is the case.
>

There are not many rules out here anymore :-)

<ducking for cover now>


> Disclaimer: not native English speaker, forgive any mistake.
>

Your English looks very good (I am non-native myself).


> My background: master degree in physics (with a few courses in
> electronics). PhD in electronics/physics, doing 99% of digital
> design. My last encounter with analog design: 10 years ago (ie: my
> knowledge is a bit "rusty").
>
> The problem: I work in high energy physics. Currently, my new project
> is to design a new version (in a new technology: from 250nm to 130nm)


130nm isn't considered very new these days but, of course, for radhard 
your selection will be limited. New designs are still done in the larger 
geometries. The most recent one I was involved in was 300nm.


> of a chip which has to perform current-to-frequency (CFC from now on)
> conversion. (for background information, if interested, you can read
> the previous designer work here:
> http://dx.doi.org/10.1016/j.mejo.2013.08.020 if you have access).
>

Do you have a link where this paper can be accessed without charge? From 
non-subscribers they want about $40 just to take a look.


> Basically, I need to design a CFC with input currents ranging from
> 1nA to 1mA, with a 40 us integration window and low input leakage
> (target: 100fA). Other details can be provided if needed.
>
> The main problem is the design of the amplifier. I am studying the
> common designs and the usual architectures; BUT I have a difficult
> time in connecting the high-level specification of the device (CFC,
> min/max input current, etc..) with the low level specs of the
> amplifier (a fully differential OTA, if we keep the previous
> architecture) such as GBP, phase margin, slew rate, etc...
>

1nA gets you into noise floor issues with the first stage if the 
integration window means >20kHz bandwidth.


> I understand that there is no such thing as a "magical" formula to
> design such circuits, but I assume that there must be, somewhere, a
> point from where to start, which is what I am missing here.
>
> All the examples and designs I see (in tutorials or articles) start
> from the wanted specifications of the OpAmp, which would be good, if
> I had an idea on how to obtain them!
>
> I hope that the question make sense to you.
>
> Any help you can give is greatly appreciated!
>

Usually the first stage needed would be a transimpedance amplifier or 
TIA, in other words a glorified current to voltage converter, followed 
by a V/F conversion or another more modern method. Phil Hobbs here in 
this group is the guru on low noise TIAs, maybe he could chime in.

It might also help to talk to these guys:

http://ams.aeroflex.com/pagesproduct/datasheets/MilAero_ASIC_Brochure.pdf

-- 
Regards, Joerg

http://www.analogconsultants.com/

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Thread

OpAmp design - from theory to practice Francesco Zappon <cescozep@gmail.com> - 2017-03-29 07:55 -0700
  Re: OpAmp design - from theory to practice Tim Wescott <tim@seemywebsite.com> - 2017-03-29 11:51 -0500
  Re: OpAmp design - from theory to practice Tim Wescott <tim@seemywebsite.com> - 2017-03-29 11:53 -0500
    Re: OpAmp design - from theory to practice Francesco Zappon <cescozep@gmail.com> - 2017-03-30 01:02 -0700
      Re: OpAmp design - from theory to practice Martin Brown <'''newspam'''@nezumi.demon.co.uk> - 2017-03-30 10:54 +0100
        Re: OpAmp design - from theory to practice sroberts6328@gmail.com - 2017-03-30 05:25 -0700
      Re: OpAmp design - from theory to practice Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2017-03-30 14:37 +0200
        Re: OpAmp design - from theory to practice Francesco Zappon <cescozep@gmail.com> - 2017-03-30 07:16 -0700
          Re: OpAmp design - from theory to practice Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2017-03-30 20:25 +0200
            Re: OpAmp design - from theory to practice bitrex <bitrex@de.lete.earthlink.net> - 2017-03-30 14:51 -0400
            Re: OpAmp design - from theory to practice Joerg <news@analogconsultants.com> - 2017-03-30 12:58 -0700
              Re: OpAmp design - from theory to practice pcdhobbs@gmail.com - 2017-03-30 13:25 -0700
              Re: OpAmp design - from theory to practice Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2017-03-30 23:55 +0200
                Re: OpAmp design - from theory to practice Joerg <news@analogconsultants.com> - 2017-03-30 16:43 -0700
          Re: OpAmp design - from theory to practice George Herold <gherold@teachspin.com> - 2017-03-30 11:43 -0700
            Re: OpAmp design - from theory to practice Francesco Zappon <cescozep@gmail.com> - 2017-03-30 15:02 -0700
            Re: OpAmp design - from theory to practice Jeroen Belleman <jeroen@nospam.please> - 2017-03-31 10:20 +0200
      Re: OpAmp design - from theory to practice Tim Wescott <tim@seemywebsite.com> - 2017-03-30 12:14 -0500
        Re: OpAmp design - from theory to practice Francesco Zappon <cescozep@gmail.com> - 2017-03-30 15:05 -0700
  Re: OpAmp design - from theory to practice Joerg <news@analogconsultants.com> - 2017-03-30 10:45 -0700
    Re: OpAmp design - from theory to practice JM <dontreplytothis173@gmail.com> - 2017-03-30 19:32 +0100
      Re: OpAmp design - from theory to practice Joerg <news@analogconsultants.com> - 2017-03-30 12:47 -0700
        Re: OpAmp design - from theory to practice Francesco Zappon <cescozep@gmail.com> - 2017-03-30 15:16 -0700
          Re: OpAmp design - from theory to practice George Herold <gherold@teachspin.com> - 2017-03-30 16:11 -0700
          Re: OpAmp design - from theory to practice Joerg <news@analogconsultants.com> - 2017-03-30 16:57 -0700

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