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OpAmp design - from theory to practice

Started by Francesco Zappon March 29, 2017
On Thursday, 30 March 2017 21:47:29 UTC+2, Joerg  wrote:


> > 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).
Thanks JM for posting it before me! I was not directly involved with the project, but from the notes I found around it seems that the main "problems" they had with the first version (the one in the article) were: - too much leakage (degrading the low current measurements) - the need of an FPGA to operate the chip (to control it and readout the data) - some linearity issues
> > 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?
I don't know if I can really say I want to "improve" it. The chip itself was working quite nice, but for several reasons (I can explain, but it would take a long post and it is not super-relevant to the discussion here) we need a new version with a new technology. Now, I can keep the same approach (no problem on my side) but changing technology node means doing a redesign and re-dimension of the OTA, for starters. And this is where I struggle, because I am a bit in the dark about how to "obtain" the specs of the amplifier. But I think I might be starting from the wrong point (after reading the posts here): I probably should start with the OpAmp as a black box and consider it ideal, and from there calculate the behavior of the circuit in ideal conditions; and after this, probably the OpAmp becomes a bit more straightforward.
> > 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
Thanks for all the info. I have some reading to do! Francesco
On Thursday, March 30, 2017 at 6:17:04 PM UTC-4, Francesco Zappon wrote:
> On Thursday, 30 March 2017 21:47:29 UTC+2, Joerg wrote: > > > > > 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). > > Thanks JM for posting it before me! > > I was not directly involved with the project, but from the notes I found around it seems that the main "problems" they had with the first version (the one in the article) were: > - too much leakage (degrading the low current measurements) > - the need of an FPGA to operate the chip (to control it and readout the data) > - some linearity issues > > > > > > 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? > > I don't know if I can really say I want to "improve" it. The chip itself was working quite nice, but for several reasons (I can explain, but it would take a long post and it is not super-relevant to the discussion here) we need a new version with a new technology. > > Now, I can keep the same approach (no problem on my side) but changing technology node means doing a redesign and re-dimension of the OTA, for starters. And this is where I struggle, because I am a bit in the dark about how to "obtain" the specs of the amplifier. > > But I think I might be starting from the wrong point (after reading the posts here): I probably should start with the OpAmp as a black box and consider it ideal, and from there calculate the behavior of the circuit in ideal conditions; and after this, probably the OpAmp becomes a bit more straightforward.
Hmm, I guess a lot depends on where the problems are. Are the caps in the silicon? (could they be external?) Radiation damage seems like it would cause leakage everywhere.. (~proportional to area.) George H.
> > > > > > 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 > > Thanks for all the info. I have some reading to do! > > Francesco
On 2017-03-30 14:55, Gerhard Hoffmann wrote:
> Am 30.03.2017 um 21:58 schrieb Joerg: >> On 2017-03-30 11:25, Gerhard Hoffmann wrote: > >>> But it surprised me that even robust things like voltage >>> regulators are affected. >>> >> >> Even space-rated ones? > >> ... >> http://www.st.com/content/st_com/en/products/aerospace-and-defense-products/space-products/rad-hard-pwm-controllers.html?querycriteria=productId=SC1548 >> >> ... > > The documentation of the ST RHFL4913 & friends mentions exactly this, > including the workaround with the large output capacitors. >
Can't find the P/N RHFL4913 but yes, of course you need sufficient output caps in such an application. Then with many LDOs there is often inadequate stability and those I would never use in hi-rel. -- Regards, Joerg http://www.analogconsultants.com/
On 2017-03-30 15:16, Francesco Zappon wrote:
> On Thursday, 30 March 2017 21:47:29 UTC+2, Joerg wrote: > > >>> 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). > > Thanks JM for posting it before me! > > I was not directly involved with the project, but from the notes I > found around it seems that the main "problems" they had with the > first version (the one in the article) were: - too much leakage
Maybe you should then look into SOI like Phil suggested. Then guard rings and all the usual low-current tricks.
> (degrading the low current measurements) - the need of an FPGA to > operate the chip (to control it and readout the data) - some > linearity issues >
Linearity will always be tough because of your 120dB. FPGA could be done in space. Work together with Gerhard maybe? He is located close to you and is self-employed as far as I know.
> >> >> 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? > > I don't know if I can really say I want to "improve" it. The chip > itself was working quite nice, but for several reasons (I can > explain, but it would take a long post and it is not super-relevant > to the discussion here) we need a new version with a new technology. >
If it is more or less a transfer from an obsolete technology to a new one for foundry reasons and such I'd talk to an analog IC design house.
> Now, I can keep the same approach (no problem on my side) but > changing technology node means doing a redesign and re-dimension of > the OTA, for starters. And this is where I struggle, because I am a > bit in the dark about how to "obtain" the specs of the amplifier. >
I can't imagine going to another node being a large obstacle. You can always use larger geometries as you did before, it's just the minimum that is now lower. Plus of course process differences but that is where a good IC design house helps. We always used those for IC designs, never went it alone.
> But I think I might be starting from the wrong point (after reading > the posts here): I probably should start with the OpAmp as a black > box and consider it ideal, and from there calculate the behavior of > the circuit in ideal conditions; and after this, probably the OpAmp > becomes a bit more straightforward. >
Just keep in mind that in the real world nothing is ideal and you will always have offset, offset drift, offset tempco and such effects.
>> >> 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 > > Thanks for all the info. I have some reading to do! >
It sure is a nice engineering challenge. Those are the things that keep us young :-) -- Regards, Joerg http://www.analogconsultants.com/
On 2017-03-30 20:43, George Herold wrote:
> On Thursday, March 30, 2017 at 10:16:47 AM UTC-4, Francesco Zappon > wrote: >> On Thursday, 30 March 2017 14:37:21 UTC+2, Gerhard Hoffmann >> wrote: >>> Am 30.03.2017 um 10:02 schrieb Francesco Zappon: >> >> [cut] >> >>> regards, Gerhard >> >> Hi Gerhard, >> >> thanks for the answer. >> >> I will give a general answer valid also for the others that >> commented on radiation hardness: we are operating with a 7 TeV beam >> of protons (you might guess where I am working, at this point :D) >> and the radiation areas are *very* harsh. This is not a problem >> that can be solved just by shielding or some smart tricks (but we >> are diverging from my question). >> >> People way smarter than me tell me that no FPGA can withstand that >> kind of radiation for a reasonably long amount of time. >> >> The comment on the FPGA was just meant to say that I cannot mount >> in the proximity of the chip I am designing an FPGA to control it. >> Nothing more than that; you are right when you say that a CFC and >> an FPGA have nothing to do with each other :) >> >> thanks >> >> Francesco > > If you are at Cern, I would think there are many people there who > know more about your problem, than we do here... where's Jeroen > Belleman? > > George H. >
I'm right here. Francesco and I are even in the same beam instrumentation group (although I've never met him, I think). I have my own little problems at the moment. We're about to start up the accelerators and I have lots of work that can't wait. Jeroen Belleman