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Re: OpAmp design - from theory to practice

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On 2017-03-30 11:32, JM wrote:
> On 30/03/2017 18:45, Joerg wrote:
>> 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
>>
>
> https://www.dropbox.com/s/52nqfror1jhkqm0/venturini2013.pdf?dl=0


Thanks! That helps a great deal. The SNR numbers in there do not look 
very stellar but they claim to have achieved all objectives stated 
(using sub-ranging though).

Francesco, I am wondering what you'd like to improve above and beyond 
what these guys did? Is it the sub-ranging and subsequent SNR trade-off 
that you don't like?

If you really want to use a I/F concept (which as others have hinted may 
not be the optimum here) there are V/F converters where the manufacturer 
claims 6 decades which corresponds to 120dB:

http://www.ti.com/lit/ds/symlink/vfc320.pdf

This would have to be preceded by a low noise TIA for a current to 
voltage conversion and those architectures are described in the field of 
pre-biased photodiodes. Like here:

http://electrooptical.net/www/frontends/frontends.pdf

-- 
Regards, Joerg

http://www.analogconsultants.com/

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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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