Reply by Brian Howie August 7, 20162016-08-07
In message <3a0ab788-cefb-4e79-a294-3c7b647ab8d3@googlegroups.com>, 
Lasse Langwadt Christensen <langwadt@fonz.dk> writes
> >apparently it has a name: http://xyproblem.info/ > >-Lasse
There's a parallel thread going in the LtSpice Yahoo group with the same OP and topic. It's interesting to compare and contrast. I designed a Laser Rangefinder Receiver with an RF Switcher PSU* on the same board, but I had the luxury of being able to switch it off and rely on a FGC to hold the charge during the ranging gate. When that was over all hell broke loose. Brian * Based loosely on this note :- http://cds.linear.com/docs/en/design-note/dn273f.pdf The first paragraph is what the OP is experiencing ..... -- Brian Howie
Reply by Phil Hobbs August 4, 20162016-08-04
On 08/04/2016 03:34 PM, John Larkin wrote:
> On Thu, 4 Aug 2016 09:18:32 -0700 (PDT), George Herold > <gherold@teachspin.com> wrote: > >> On Thursday, August 4, 2016 at 11:37:37 AM UTC-4, John Larkin wrote: >>> On Thu, 4 Aug 2016 11:04:59 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> On 08/04/2016 09:44 AM, George Herold wrote: >>>>> On Thursday, August 4, 2016 at 8:16:51 AM UTC-4, Phil Hobbs wrote: >>>>>> On 08/04/2016 02:31 AM, T Obulesu wrote: >>>>>>> On Tuesday, 2 August 2016 08:58:42 UTC+5:30, T Obulesu wrote: >>>>>>>> Hello guys.. >>>>>>>> >>>>>>>> I want to design a trans-impedance amplifier to detect currents of >>>>>>>> the APD (in the range of 200 nA to 100 micro A). >>>>>>>> >>>>>>>> Could some one suggest me the best IC with less bias current, >>>>>>>> single supply (+5 V only) and how to design a high sensitive TIA..? >>>>>>>> (typically, for every 100 nA change in the current, 100 mV voltage >>>>>>>> should be displayed). >>>>>>> >>>>>>> Let me say why am looking for new design.. We are currently using the >>>>>>> receiver module which has three stages: 1. Trans Impedance stage 2. >>>>>>> High Pass filter (HPF) stage 3. Unity gain amplifier (just for >>>>>>> inverting the output of the HPF) >>>>>>> >>>>>>> Well...we used LM2662 as a -5V supply.. Here on we could see the hell >>>>>>> noise below 100kHz.. I couldn't get rid of this noise by using bypass >>>>>>> caps..but I could just reduced it.. Yet there is a lot of noise all >>>>>>> across the circuit ranging from few kHz to hundreds of MHz... >>>>>> > >>>>>> > https://drive.google.com/open?id=0B_khpIBuCMVdb18ySmZubGsyRUU >>>>>> > >>>>>> > https://drive.google.com/open?id=0B_khpIBuCMVdbUVRWW85MHFmSXM >>>>>> > >>>>>> > https://drive.google.com/drive/u/0/folders/0B_khpIBuCMVdMmplSUc3U2FNdjA >>>>>> > >>>>>> > https://drive.google.com/open?id=0B_khpIBuCMVdeVMxZkRGR1NvM2M >>>>>> > >>>>>> > are the links where I have uploaded couple of documents.. >>>>>> > >>>>>> > I really don't know what sort of noise it is and from where it is >>>>>> > coming... >>>>>> > >>>>>> >>>>>> Well, if you're connecting a photodiode directly to a SMPS, what did you >>>>>> expect? >>>>>> >>>>>> Hint: Taking a spiky supply rail, differentiating it with a small >>>>>> capacitor, and sticking the resulting current into a high-gain current >>>>>> amplifier may not be what you want to do. >>>>>> >>>>>> Of course, three lines of algebra would have told you what the results >>>>>> were going to be, and you'd have learned something instead of just >>>>>> throwing spitballs. This theory stuff really works. ;) >>>>>> >>>>>> We've had several threads about capacitance multipliers recently, >>>>>> including one where I posted an LTspice schematic of a 2-stage cap >>>>>> multiplier that will knock that stuff down by about 180 dB, assuming >>>>>> that you have adequate RF shielding. >>>>> >>>>> He could start with a simple RC filter on the bias supply... that might be enough >>>>> to show him where the problem is. (My hint is to look at the noise on the >>>>> bias voltage and see if it looks the same as your signal noise.) >>>>> >>>>> It is weird how this started as a thread to find an opamp. >>>> >>>> The OP doesn't know what he's doing, and wants a quick fix. There is >>>> one, of course--your RC or my cap multiplier--but understanding it does >>>> sort of require doing those three lines of algebra to see just how >>>> vulnerable a TIA is to bias supply noise. It never ceases to amaze me >>>> how many people chicken out of doing that, even when their livelihood >>>> (or their thesis) is on the line. C'mon, T. Obulesu, you can do it. >>>> All you need is the maximum slope of the ripple waveform, the diode >>>> capacitance, and I=C dV/dt. >>>> >>>>> >>>>> To the OP, do you have any thermal regulation on the APD? >>>>> I've read that the gain is a strong function of the bias voltage and >>>>> temperature. >>>> >>>> Yup. It's really easy to turn them to lava. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> >>> Sometimes the noise path is not visible on the schematic; it's >>> capacitive coupling through the air, or ground loops, or magic. >>> Switchers should be far away from low-noise amps. >> >> Hah, I meant to comment up-thread on putting a (gain/R) switch in a TIA. >> The first (or maybe second) time I did a PD TIA I put the rotary >> switch in backwards. Too much capacitance. You need to put the >> pole on the inverting input. > > The gain-range switch is in the wrong place no matter where you put > it. > > I did one box that had two TIAS, with 100:1 gain difference, and > switched the photodiode between them with the cascode transistors, a > spin on Phil's bootstrap cascode. That wasn't as awful as some other > ideas. > > It was a brilliant product, but the customer decided to "design" their > own, after I showed them how to do it. > > https://dl.dropboxusercontent.com/u/53724080/Optos/PT7.JPG > > https://dl.dropboxusercontent.com/u/53724080/Optos/PT9.JPG > > https://dl.dropboxusercontent.com/u/53724080/Optos/PT10.JPG > > Sometimes that happens. When a customer has an in-house design team, > they usually manage to win. > >
Oh, _those_ guys. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by John Larkin August 4, 20162016-08-04
On Thu, 4 Aug 2016 09:18:32 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>On Thursday, August 4, 2016 at 11:37:37 AM UTC-4, John Larkin wrote: >> On Thu, 4 Aug 2016 11:04:59 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> >On 08/04/2016 09:44 AM, George Herold wrote: >> >> On Thursday, August 4, 2016 at 8:16:51 AM UTC-4, Phil Hobbs wrote: >> >>> On 08/04/2016 02:31 AM, T Obulesu wrote: >> >>>> On Tuesday, 2 August 2016 08:58:42 UTC+5:30, T Obulesu wrote: >> >>>>> Hello guys.. >> >>>>> >> >>>>> I want to design a trans-impedance amplifier to detect currents of >> >>>>> the APD (in the range of 200 nA to 100 micro A). >> >>>>> >> >>>>> Could some one suggest me the best IC with less bias current, >> >>>>> single supply (+5 V only) and how to design a high sensitive TIA..? >> >>>>> (typically, for every 100 nA change in the current, 100 mV voltage >> >>>>> should be displayed). >> >>>> >> >>>> Let me say why am looking for new design.. We are currently using the >> >>>> receiver module which has three stages: 1. Trans Impedance stage 2. >> >>>> High Pass filter (HPF) stage 3. Unity gain amplifier (just for >> >>>> inverting the output of the HPF) >> >>>> >> >>>> Well...we used LM2662 as a -5V supply.. Here on we could see the hell >> >>>> noise below 100kHz.. I couldn't get rid of this noise by using bypass >> >>>> caps..but I could just reduced it.. Yet there is a lot of noise all >> >>>> across the circuit ranging from few kHz to hundreds of MHz... >> >>> > >> >>> > https://drive.google.com/open?id=0B_khpIBuCMVdb18ySmZubGsyRUU >> >>> > >> >>> > https://drive.google.com/open?id=0B_khpIBuCMVdbUVRWW85MHFmSXM >> >>> > >> >>> > https://drive.google.com/drive/u/0/folders/0B_khpIBuCMVdMmplSUc3U2FNdjA >> >>> > >> >>> > https://drive.google.com/open?id=0B_khpIBuCMVdeVMxZkRGR1NvM2M >> >>> > >> >>> > are the links where I have uploaded couple of documents.. >> >>> > >> >>> > I really don't know what sort of noise it is and from where it is >> >>> > coming... >> >>> > >> >>> >> >>> Well, if you're connecting a photodiode directly to a SMPS, what did you >> >>> expect? >> >>> >> >>> Hint: Taking a spiky supply rail, differentiating it with a small >> >>> capacitor, and sticking the resulting current into a high-gain current >> >>> amplifier may not be what you want to do. >> >>> >> >>> Of course, three lines of algebra would have told you what the results >> >>> were going to be, and you'd have learned something instead of just >> >>> throwing spitballs. This theory stuff really works. ;) >> >>> >> >>> We've had several threads about capacitance multipliers recently, >> >>> including one where I posted an LTspice schematic of a 2-stage cap >> >>> multiplier that will knock that stuff down by about 180 dB, assuming >> >>> that you have adequate RF shielding. >> >> >> >> He could start with a simple RC filter on the bias supply... that might be enough >> >> to show him where the problem is. (My hint is to look at the noise on the >> >> bias voltage and see if it looks the same as your signal noise.) >> >> >> >> It is weird how this started as a thread to find an opamp. >> > >> >The OP doesn't know what he's doing, and wants a quick fix. There is >> >one, of course--your RC or my cap multiplier--but understanding it does >> >sort of require doing those three lines of algebra to see just how >> >vulnerable a TIA is to bias supply noise. It never ceases to amaze me >> >how many people chicken out of doing that, even when their livelihood >> >(or their thesis) is on the line. C'mon, T. Obulesu, you can do it. >> >All you need is the maximum slope of the ripple waveform, the diode >> >capacitance, and I=C dV/dt. >> > >> >> >> >> To the OP, do you have any thermal regulation on the APD? >> >> I've read that the gain is a strong function of the bias voltage and >> >> temperature. >> > >> >Yup. It's really easy to turn them to lava. >> > >> >Cheers >> > >> >Phil Hobbs >> >> Sometimes the noise path is not visible on the schematic; it's >> capacitive coupling through the air, or ground loops, or magic. >> Switchers should be far away from low-noise amps. > >Hah, I meant to comment up-thread on putting a (gain/R) switch in a TIA. >The first (or maybe second) time I did a PD TIA I put the rotary >switch in backwards. Too much capacitance. You need to put the >pole on the inverting input.
The gain-range switch is in the wrong place no matter where you put it. I did one box that had two TIAS, with 100:1 gain difference, and switched the photodiode between them with the cascode transistors, a spin on Phil's bootstrap cascode. That wasn't as awful as some other ideas. It was a brilliant product, but the customer decided to "design" their own, after I showed them how to do it. https://dl.dropboxusercontent.com/u/53724080/Optos/PT7.JPG https://dl.dropboxusercontent.com/u/53724080/Optos/PT9.JPG https://dl.dropboxusercontent.com/u/53724080/Optos/PT10.JPG Sometimes that happens. When a customer has an in-house design team, they usually manage to win. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by Lasse Langwadt Christensen August 4, 20162016-08-04
Den torsdag den 4. august 2016 kl. 18.12.30 UTC+2 skrev George Herold:
> On Thursday, August 4, 2016 at 11:05:03 AM UTC-4, Phil Hobbs wrote: > > On 08/04/2016 09:44 AM, George Herold wrote: > > > On Thursday, August 4, 2016 at 8:16:51 AM UTC-4, Phil Hobbs wrote: > > >> On 08/04/2016 02:31 AM, T Obulesu wrote: > > >>> On Tuesday, 2 August 2016 08:58:42 UTC+5:30, T Obulesu wrote: > > >>>> Hello guys.. > > >>>> > > >>>> I want to design a trans-impedance amplifier to detect currents of > > >>>> the APD (in the range of 200 nA to 100 micro A). > > >>>> > > >>>> Could some one suggest me the best IC with less bias current, > > >>>> single supply (+5 V only) and how to design a high sensitive TIA..? > > >>>> (typically, for every 100 nA change in the current, 100 mV voltage > > >>>> should be displayed). > > >>> > > >>> Let me say why am looking for new design.. We are currently using the > > >>> receiver module which has three stages: 1. Trans Impedance stage 2. > > >>> High Pass filter (HPF) stage 3. Unity gain amplifier (just for > > >>> inverting the output of the HPF) > > >>> > > >>> Well...we used LM2662 as a -5V supply.. Here on we could see the hell > > >>> noise below 100kHz.. I couldn't get rid of this noise by using bypass > > >>> caps..but I could just reduced it.. Yet there is a lot of noise all > > >>> across the circuit ranging from few kHz to hundreds of MHz... > > >> > > > >> > https://drive.google.com/open?id=0B_khpIBuCMVdb18ySmZubGsyRUU > > >> > > > >> > https://drive.google.com/open?id=0B_khpIBuCMVdbUVRWW85MHFmSXM > > >> > > > >> > https://drive.google.com/drive/u/0/folders/0B_khpIBuCMVdMmplSUc3U2FNdjA > > >> > > > >> > https://drive.google.com/open?id=0B_khpIBuCMVdeVMxZkRGR1NvM2M > > >> > > > >> > are the links where I have uploaded couple of documents.. > > >> > > > >> > I really don't know what sort of noise it is and from where it is > > >> > coming... > > >> > > > >> > > >> Well, if you're connecting a photodiode directly to a SMPS, what did you > > >> expect? > > >> > > >> Hint: Taking a spiky supply rail, differentiating it with a small > > >> capacitor, and sticking the resulting current into a high-gain current > > >> amplifier may not be what you want to do. > > >> > > >> Of course, three lines of algebra would have told you what the results > > >> were going to be, and you'd have learned something instead of just > > >> throwing spitballs. This theory stuff really works. ;) > > >> > > >> We've had several threads about capacitance multipliers recently, > > >> including one where I posted an LTspice schematic of a 2-stage cap > > >> multiplier that will knock that stuff down by about 180 dB, assuming > > >> that you have adequate RF shielding. > > > > > > He could start with a simple RC filter on the bias supply... that might be enough > > > to show him where the problem is. (My hint is to look at the noise on the > > > bias voltage and see if it looks the same as your signal noise.) > > > > > > It is weird how this started as a thread to find an opamp. > > > > The OP doesn't know what he's doing, and wants a quick fix. There is > > one, of course--your RC or my cap multiplier--but understanding it does > > sort of require doing those three lines of algebra to see just how > > vulnerable a TIA is to bias supply noise. It never ceases to amaze me > > how many people chicken out of doing that, even when their livelihood > > (or their thesis) is on the line. C'mon, T. Obulesu, you can do it. > > All you need is the maximum slope of the ripple waveform, the diode > > capacitance, and I=C dV/dt. > > Not knowing what you are doing is fine by me. (It's how we all start.) > The common mistake (I think we've probably all made at some time.) is > to assume you know what the problem is. (wrong opamp) And then ask for > that. Rather than laying out the whole problem. > I guess it can be hard for some people to say, "I don't know what's wrong here." >
apparently it has a name: http://xyproblem.info/ -Lasse
Reply by Phil Hobbs August 4, 20162016-08-04
On 08/04/2016 12:18 PM, George Herold wrote:
> On Thursday, August 4, 2016 at 11:37:37 AM UTC-4, John Larkin wrote: >> On Thu, 4 Aug 2016 11:04:59 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 08/04/2016 09:44 AM, George Herold wrote: >>>> On Thursday, August 4, 2016 at 8:16:51 AM UTC-4, Phil Hobbs wrote: >>>>> On 08/04/2016 02:31 AM, T Obulesu wrote: >>>>>> On Tuesday, 2 August 2016 08:58:42 UTC+5:30, T Obulesu wrote: >>>>>>> Hello guys.. >>>>>>> >>>>>>> I want to design a trans-impedance amplifier to detect currents of >>>>>>> the APD (in the range of 200 nA to 100 micro A). >>>>>>> >>>>>>> Could some one suggest me the best IC with less bias current, >>>>>>> single supply (+5 V only) and how to design a high sensitive TIA..? >>>>>>> (typically, for every 100 nA change in the current, 100 mV voltage >>>>>>> should be displayed). >>>>>> >>>>>> Let me say why am looking for new design.. We are currently using the >>>>>> receiver module which has three stages: 1. Trans Impedance stage 2. >>>>>> High Pass filter (HPF) stage 3. Unity gain amplifier (just for >>>>>> inverting the output of the HPF) >>>>>> >>>>>> Well...we used LM2662 as a -5V supply.. Here on we could see the hell >>>>>> noise below 100kHz.. I couldn't get rid of this noise by using bypass >>>>>> caps..but I could just reduced it.. Yet there is a lot of noise all >>>>>> across the circuit ranging from few kHz to hundreds of MHz... >>>>> > >>>>> > https://drive.google.com/open?id=0B_khpIBuCMVdb18ySmZubGsyRUU >>>>> > >>>>> > https://drive.google.com/open?id=0B_khpIBuCMVdbUVRWW85MHFmSXM >>>>> > >>>>> > https://drive.google.com/drive/u/0/folders/0B_khpIBuCMVdMmplSUc3U2FNdjA >>>>> > >>>>> > https://drive.google.com/open?id=0B_khpIBuCMVdeVMxZkRGR1NvM2M >>>>> > >>>>> > are the links where I have uploaded couple of documents.. >>>>> > >>>>> > I really don't know what sort of noise it is and from where it is >>>>> > coming... >>>>> > >>>>> >>>>> Well, if you're connecting a photodiode directly to a SMPS, what did you >>>>> expect? >>>>> >>>>> Hint: Taking a spiky supply rail, differentiating it with a small >>>>> capacitor, and sticking the resulting current into a high-gain current >>>>> amplifier may not be what you want to do. >>>>> >>>>> Of course, three lines of algebra would have told you what the results >>>>> were going to be, and you'd have learned something instead of just >>>>> throwing spitballs. This theory stuff really works. ;) >>>>> >>>>> We've had several threads about capacitance multipliers recently, >>>>> including one where I posted an LTspice schematic of a 2-stage cap >>>>> multiplier that will knock that stuff down by about 180 dB, assuming >>>>> that you have adequate RF shielding. >>>> >>>> He could start with a simple RC filter on the bias supply... that might be enough >>>> to show him where the problem is. (My hint is to look at the noise on the >>>> bias voltage and see if it looks the same as your signal noise.) >>>> >>>> It is weird how this started as a thread to find an opamp. >>> >>> The OP doesn't know what he's doing, and wants a quick fix. There is >>> one, of course--your RC or my cap multiplier--but understanding it does >>> sort of require doing those three lines of algebra to see just how >>> vulnerable a TIA is to bias supply noise. It never ceases to amaze me >>> how many people chicken out of doing that, even when their livelihood >>> (or their thesis) is on the line. C'mon, T. Obulesu, you can do it. >>> All you need is the maximum slope of the ripple waveform, the diode >>> capacitance, and I=C dV/dt. >>> >>>> >>>> To the OP, do you have any thermal regulation on the APD? >>>> I've read that the gain is a strong function of the bias voltage and >>>> temperature. >>> >>> Yup. It's really easy to turn them to lava. >>> >>> Cheers >>> >>> Phil Hobbs >> >> Sometimes the noise path is not visible on the schematic; it's >> capacitive coupling through the air, or ground loops, or magic. >> Switchers should be far away from low-noise amps. > > Hah, I meant to comment up-thread on putting a (gain/R) switch in a TIA. > The first (or maybe second) time I did a PD TIA I put the rotary > switch in backwards. Too much capacitance. You need to put the > pole on the inverting input.
Yup. For really high-Z stuff I've used relays with bootstrapped coils to get rid of that last 0.4 pF. Tough to get many ranges that way, though. For my first few products I'm going to use a single range. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by Phil Hobbs August 4, 20162016-08-04
On 08/04/2016 12:12 PM, George Herold wrote:
> On Thursday, August 4, 2016 at 11:05:03 AM UTC-4, Phil Hobbs wrote: >> On 08/04/2016 09:44 AM, George Herold wrote: >>> On Thursday, August 4, 2016 at 8:16:51 AM UTC-4, Phil Hobbs >>> wrote: >>>> On 08/04/2016 02:31 AM, T Obulesu wrote: >>>>> On Tuesday, 2 August 2016 08:58:42 UTC+5:30, T Obulesu >>>>> wrote: >>>>>> Hello guys.. >>>>>> >>>>>> I want to design a trans-impedance amplifier to detect >>>>>> currents of the APD (in the range of 200 nA to 100 micro >>>>>> A). >>>>>> >>>>>> Could some one suggest me the best IC with less bias >>>>>> current, single supply (+5 V only) and how to design a high >>>>>> sensitive TIA..? (typically, for every 100 nA change in the >>>>>> current, 100 mV voltage should be displayed). >>>>> >>>>> Let me say why am looking for new design.. We are currently >>>>> using the receiver module which has three stages: 1. Trans >>>>> Impedance stage 2. High Pass filter (HPF) stage 3. Unity gain >>>>> amplifier (just for inverting the output of the HPF) >>>>> >>>>> Well...we used LM2662 as a -5V supply.. Here on we could see >>>>> the hell noise below 100kHz.. I couldn't get rid of this >>>>> noise by using bypass caps..but I could just reduced it.. Yet >>>>> there is a lot of noise all across the circuit ranging from >>>>> few kHz to hundreds of MHz... >>>>> >>>>> https://drive.google.com/open?id=0B_khpIBuCMVdb18ySmZubGsyRUU >>>> >>>>> > >>>>> https://drive.google.com/open?id=0B_khpIBuCMVdbUVRWW85MHFmSXM >>>> >>>>> > >>>>> https://drive.google.com/drive/u/0/folders/0B_khpIBuCMVdMmplSUc3U2FNdjA >>>> >>>>> > >>>>> https://drive.google.com/open?id=0B_khpIBuCMVdeVMxZkRGR1NvM2M >>>> >>>>> > >>>>> are the links where I have uploaded couple of documents.. >>>>> >>>>> I really don't know what sort of noise it is and from where >>>>> it is coming... >>>>> >>>> >>>> Well, if you're connecting a photodiode directly to a SMPS, >>>> what did you expect? >>>> >>>> Hint: Taking a spiky supply rail, differentiating it with a >>>> small capacitor, and sticking the resulting current into a >>>> high-gain current amplifier may not be what you want to do. >>>> >>>> Of course, three lines of algebra would have told you what the >>>> results were going to be, and you'd have learned something >>>> instead of just throwing spitballs. This theory stuff really >>>> works. ;) >>>> >>>> We've had several threads about capacitance multipliers >>>> recently, including one where I posted an LTspice schematic of >>>> a 2-stage cap multiplier that will knock that stuff down by >>>> about 180 dB, assuming that you have adequate RF shielding. >>> >>> He could start with a simple RC filter on the bias supply... that >>> might be enough to show him where the problem is. (My hint is to >>> look at the noise on the bias voltage and see if it looks the >>> same as your signal noise.) >>> >>> It is weird how this started as a thread to find an opamp. >> >> The OP doesn't know what he's doing, and wants a quick fix. There >> is one, of course--your RC or my cap multiplier--but understanding >> it does sort of require doing those three lines of algebra to see >> just how vulnerable a TIA is to bias supply noise. It never ceases >> to amaze me how many people chicken out of doing that, even when >> their livelihood (or their thesis) is on the line. C'mon, T. >> Obulesu, you can do it. All you need is the maximum slope of the >> ripple waveform, the diode capacitance, and I=C dV/dt. > > Not knowing what you are doing is fine by me. (It's how we all > start.)
Sure, I did too, and judging from the IIT Madras logo block, he's probably a grad student. What I don't understand is getting all the way through schematic capture, prototyping, testing, and making Powerpoint slides without ever estimating anything. I'd have expected a bit of professorial input there at the very least.
> The common mistake (I think we've probably all made at some time.) is > to assume you know what the problem is. (wrong opamp) And then ask > for that. Rather than laying out the whole problem.
Well, T. Obulesu did lay it out very well in the PP deck. No problem there, except for a lot of wasted effort. (Not wasted if he learns to estimate stuff ahead of time, of course.)
> I guess it can be hard for some people to say, "I don't know what's > wrong here."
He eventually did that, too, so he deserves full points there as well. Doing the estimate is the hat trick. ;) I'm considering doing a small book on the apparently lost art of doing photon budgets. I can mostly string together stuff I've done over the years, so it wouldn't be a huge schlepp. Maybe next year after I get my third edition finished. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by George Herold August 4, 20162016-08-04
On Thursday, August 4, 2016 at 11:37:37 AM UTC-4, John Larkin wrote:
> On Thu, 4 Aug 2016 11:04:59 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > > >On 08/04/2016 09:44 AM, George Herold wrote: > >> On Thursday, August 4, 2016 at 8:16:51 AM UTC-4, Phil Hobbs wrote: > >>> On 08/04/2016 02:31 AM, T Obulesu wrote: > >>>> On Tuesday, 2 August 2016 08:58:42 UTC+5:30, T Obulesu wrote: > >>>>> Hello guys.. > >>>>> > >>>>> I want to design a trans-impedance amplifier to detect currents of > >>>>> the APD (in the range of 200 nA to 100 micro A). > >>>>> > >>>>> Could some one suggest me the best IC with less bias current, > >>>>> single supply (+5 V only) and how to design a high sensitive TIA..? > >>>>> (typically, for every 100 nA change in the current, 100 mV voltage > >>>>> should be displayed). > >>>> > >>>> Let me say why am looking for new design.. We are currently using the > >>>> receiver module which has three stages: 1. Trans Impedance stage 2. > >>>> High Pass filter (HPF) stage 3. Unity gain amplifier (just for > >>>> inverting the output of the HPF) > >>>> > >>>> Well...we used LM2662 as a -5V supply.. Here on we could see the hell > >>>> noise below 100kHz.. I couldn't get rid of this noise by using bypass > >>>> caps..but I could just reduced it.. Yet there is a lot of noise all > >>>> across the circuit ranging from few kHz to hundreds of MHz... > >>> > > >>> > https://drive.google.com/open?id=0B_khpIBuCMVdb18ySmZubGsyRUU > >>> > > >>> > https://drive.google.com/open?id=0B_khpIBuCMVdbUVRWW85MHFmSXM > >>> > > >>> > https://drive.google.com/drive/u/0/folders/0B_khpIBuCMVdMmplSUc3U2FNdjA > >>> > > >>> > https://drive.google.com/open?id=0B_khpIBuCMVdeVMxZkRGR1NvM2M > >>> > > >>> > are the links where I have uploaded couple of documents.. > >>> > > >>> > I really don't know what sort of noise it is and from where it is > >>> > coming... > >>> > > >>> > >>> Well, if you're connecting a photodiode directly to a SMPS, what did you > >>> expect? > >>> > >>> Hint: Taking a spiky supply rail, differentiating it with a small > >>> capacitor, and sticking the resulting current into a high-gain current > >>> amplifier may not be what you want to do. > >>> > >>> Of course, three lines of algebra would have told you what the results > >>> were going to be, and you'd have learned something instead of just > >>> throwing spitballs. This theory stuff really works. ;) > >>> > >>> We've had several threads about capacitance multipliers recently, > >>> including one where I posted an LTspice schematic of a 2-stage cap > >>> multiplier that will knock that stuff down by about 180 dB, assuming > >>> that you have adequate RF shielding. > >> > >> He could start with a simple RC filter on the bias supply... that might be enough > >> to show him where the problem is. (My hint is to look at the noise on the > >> bias voltage and see if it looks the same as your signal noise.) > >> > >> It is weird how this started as a thread to find an opamp. > > > >The OP doesn't know what he's doing, and wants a quick fix. There is > >one, of course--your RC or my cap multiplier--but understanding it does > >sort of require doing those three lines of algebra to see just how > >vulnerable a TIA is to bias supply noise. It never ceases to amaze me > >how many people chicken out of doing that, even when their livelihood > >(or their thesis) is on the line. C'mon, T. Obulesu, you can do it. > >All you need is the maximum slope of the ripple waveform, the diode > >capacitance, and I=C dV/dt. > > > >> > >> To the OP, do you have any thermal regulation on the APD? > >> I've read that the gain is a strong function of the bias voltage and > >> temperature. > > > >Yup. It's really easy to turn them to lava. > > > >Cheers > > > >Phil Hobbs > > Sometimes the noise path is not visible on the schematic; it's > capacitive coupling through the air, or ground loops, or magic. > Switchers should be far away from low-noise amps.
Hah, I meant to comment up-thread on putting a (gain/R) switch in a TIA. The first (or maybe second) time I did a PD TIA I put the rotary switch in backwards. Too much capacitance. You need to put the pole on the inverting input. George H.
> > We recently tested a fiber link intended for clock distribution with > femtosecond jitter. The wideband noise floor was below -170 dBc, but > it has half a dozen birdies in the 5-300KHz range, most of which we > can't account for. If we productize this one, the most serious part of > the design is going to be the metal boxes. > > Of course we can't measure 180 fS RMS jitter. We have to inferr it > from a spectral phase-noise analysis. > > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronics
Reply by George Herold August 4, 20162016-08-04
On Thursday, August 4, 2016 at 11:05:03 AM UTC-4, Phil Hobbs wrote:
> On 08/04/2016 09:44 AM, George Herold wrote: > > On Thursday, August 4, 2016 at 8:16:51 AM UTC-4, Phil Hobbs wrote: > >> On 08/04/2016 02:31 AM, T Obulesu wrote: > >>> On Tuesday, 2 August 2016 08:58:42 UTC+5:30, T Obulesu wrote: > >>>> Hello guys.. > >>>> > >>>> I want to design a trans-impedance amplifier to detect currents of > >>>> the APD (in the range of 200 nA to 100 micro A). > >>>> > >>>> Could some one suggest me the best IC with less bias current, > >>>> single supply (+5 V only) and how to design a high sensitive TIA..? > >>>> (typically, for every 100 nA change in the current, 100 mV voltage > >>>> should be displayed). > >>> > >>> Let me say why am looking for new design.. We are currently using the > >>> receiver module which has three stages: 1. Trans Impedance stage 2. > >>> High Pass filter (HPF) stage 3. Unity gain amplifier (just for > >>> inverting the output of the HPF) > >>> > >>> Well...we used LM2662 as a -5V supply.. Here on we could see the hell > >>> noise below 100kHz.. I couldn't get rid of this noise by using bypass > >>> caps..but I could just reduced it.. Yet there is a lot of noise all > >>> across the circuit ranging from few kHz to hundreds of MHz... > >> > > >> > https://drive.google.com/open?id=0B_khpIBuCMVdb18ySmZubGsyRUU > >> > > >> > https://drive.google.com/open?id=0B_khpIBuCMVdbUVRWW85MHFmSXM > >> > > >> > https://drive.google.com/drive/u/0/folders/0B_khpIBuCMVdMmplSUc3U2FNdjA > >> > > >> > https://drive.google.com/open?id=0B_khpIBuCMVdeVMxZkRGR1NvM2M > >> > > >> > are the links where I have uploaded couple of documents.. > >> > > >> > I really don't know what sort of noise it is and from where it is > >> > coming... > >> > > >> > >> Well, if you're connecting a photodiode directly to a SMPS, what did you > >> expect? > >> > >> Hint: Taking a spiky supply rail, differentiating it with a small > >> capacitor, and sticking the resulting current into a high-gain current > >> amplifier may not be what you want to do. > >> > >> Of course, three lines of algebra would have told you what the results > >> were going to be, and you'd have learned something instead of just > >> throwing spitballs. This theory stuff really works. ;) > >> > >> We've had several threads about capacitance multipliers recently, > >> including one where I posted an LTspice schematic of a 2-stage cap > >> multiplier that will knock that stuff down by about 180 dB, assuming > >> that you have adequate RF shielding. > > > > He could start with a simple RC filter on the bias supply... that might be enough > > to show him where the problem is. (My hint is to look at the noise on the > > bias voltage and see if it looks the same as your signal noise.) > > > > It is weird how this started as a thread to find an opamp. > > The OP doesn't know what he's doing, and wants a quick fix. There is > one, of course--your RC or my cap multiplier--but understanding it does > sort of require doing those three lines of algebra to see just how > vulnerable a TIA is to bias supply noise. It never ceases to amaze me > how many people chicken out of doing that, even when their livelihood > (or their thesis) is on the line. C'mon, T. Obulesu, you can do it. > All you need is the maximum slope of the ripple waveform, the diode > capacitance, and I=C dV/dt.
Not knowing what you are doing is fine by me. (It's how we all start.) The common mistake (I think we've probably all made at some time.) is to assume you know what the problem is. (wrong opamp) And then ask for that. Rather than laying out the whole problem. I guess it can be hard for some people to say, "I don't know what's wrong here." George (not shamed by my ignorance) Herold
> > > > > To the OP, do you have any thermal regulation on the APD? > > I've read that the gain is a strong function of the bias voltage and > > temperature. > > Yup. It's really easy to turn them to lava. > > Cheers > > Phil Hobbs > -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC > Optics, Electro-optics, Photonics, Analog Electronics > > 160 North State Road #203 > Briarcliff Manor NY 10510 > > hobbs at electrooptical dot net > http://electrooptical.net
Reply by Phil Hobbs August 4, 20162016-08-04
On 08/04/2016 11:37 AM, John Larkin wrote:
> On Thu, 4 Aug 2016 11:04:59 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 08/04/2016 09:44 AM, George Herold wrote: >>> On Thursday, August 4, 2016 at 8:16:51 AM UTC-4, Phil Hobbs wrote: >>>> On 08/04/2016 02:31 AM, T Obulesu wrote: >>>>> On Tuesday, 2 August 2016 08:58:42 UTC+5:30, T Obulesu wrote: >>>>>> Hello guys.. >>>>>> >>>>>> I want to design a trans-impedance amplifier to detect currents of >>>>>> the APD (in the range of 200 nA to 100 micro A). >>>>>> >>>>>> Could some one suggest me the best IC with less bias current, >>>>>> single supply (+5 V only) and how to design a high sensitive TIA..? >>>>>> (typically, for every 100 nA change in the current, 100 mV voltage >>>>>> should be displayed). >>>>> >>>>> Let me say why am looking for new design.. We are currently using the >>>>> receiver module which has three stages: 1. Trans Impedance stage 2. >>>>> High Pass filter (HPF) stage 3. Unity gain amplifier (just for >>>>> inverting the output of the HPF) >>>>> >>>>> Well...we used LM2662 as a -5V supply.. Here on we could see the hell >>>>> noise below 100kHz.. I couldn't get rid of this noise by using bypass >>>>> caps..but I could just reduced it.. Yet there is a lot of noise all >>>>> across the circuit ranging from few kHz to hundreds of MHz... >>>> > >>>> > https://drive.google.com/open?id=0B_khpIBuCMVdb18ySmZubGsyRUU >>>> > >>>> > https://drive.google.com/open?id=0B_khpIBuCMVdbUVRWW85MHFmSXM >>>> > >>>> > https://drive.google.com/drive/u/0/folders/0B_khpIBuCMVdMmplSUc3U2FNdjA >>>> > >>>> > https://drive.google.com/open?id=0B_khpIBuCMVdeVMxZkRGR1NvM2M >>>> > >>>> > are the links where I have uploaded couple of documents.. >>>> > >>>> > I really don't know what sort of noise it is and from where it is >>>> > coming... >>>> > >>>> >>>> Well, if you're connecting a photodiode directly to a SMPS, what did you >>>> expect? >>>> >>>> Hint: Taking a spiky supply rail, differentiating it with a small >>>> capacitor, and sticking the resulting current into a high-gain current >>>> amplifier may not be what you want to do. >>>> >>>> Of course, three lines of algebra would have told you what the results >>>> were going to be, and you'd have learned something instead of just >>>> throwing spitballs. This theory stuff really works. ;) >>>> >>>> We've had several threads about capacitance multipliers recently, >>>> including one where I posted an LTspice schematic of a 2-stage cap >>>> multiplier that will knock that stuff down by about 180 dB, assuming >>>> that you have adequate RF shielding. >>> >>> He could start with a simple RC filter on the bias supply... that might be enough >>> to show him where the problem is. (My hint is to look at the noise on the >>> bias voltage and see if it looks the same as your signal noise.) >>> >>> It is weird how this started as a thread to find an opamp. >> >> The OP doesn't know what he's doing, and wants a quick fix. There is >> one, of course--your RC or my cap multiplier--but understanding it does >> sort of require doing those three lines of algebra to see just how >> vulnerable a TIA is to bias supply noise. It never ceases to amaze me >> how many people chicken out of doing that, even when their livelihood >> (or their thesis) is on the line. C'mon, T. Obulesu, you can do it. >> All you need is the maximum slope of the ripple waveform, the diode >> capacitance, and I=C dV/dt. >> >>> >>> To the OP, do you have any thermal regulation on the APD? >>> I've read that the gain is a strong function of the bias voltage and >>> temperature. >> >> Yup. It's really easy to turn them to lava. >> >> Cheers >> >> Phil Hobbs > > Sometimes the noise path is not visible on the schematic; it's > capacitive coupling through the air, or ground loops, or magic. > Switchers should be far away from low-noise amps.
Yup, and they should have nice smooth slow edges, a tight layout, generously-sized toroidal inductors, and electrostatic shields. (Magnetic coupling is tougher to get rid of, but I haven't had a problem with 150-kHz switchers and toroids, as long as nothing saturates.) One thing that really helps is making sure the board is grounded to the box in several places, especially near the SMPS. I'm using some little L brackets in a one-piece Hammond box with die-cast end plates and a conductive gasket. Works nicely, though it's a bit on the fiddly side.
> > We recently tested a fiber link intended for clock distribution with > femtosecond jitter. The wideband noise floor was below -170 dBc, but > it has half a dozen birdies in the 5-300KHz range, most of which we > can't account for. If we productize this one, the most serious part of > the design is going to be the metal boxes.
It's the hardest to simulate, for sure.
> > Of course we can't measure 180 fS RMS jitter. We have to infer it > from a spectral phase-noise analysis.
Fortunately that's pretty easy to do! Electro-optic sampling can measure numbers like that standing on its head. Besides cost, the big drawback is that you have to sync to the laser, it can't sync to you. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by John Larkin August 4, 20162016-08-04
On Thu, 4 Aug 2016 11:04:59 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 08/04/2016 09:44 AM, George Herold wrote: >> On Thursday, August 4, 2016 at 8:16:51 AM UTC-4, Phil Hobbs wrote: >>> On 08/04/2016 02:31 AM, T Obulesu wrote: >>>> On Tuesday, 2 August 2016 08:58:42 UTC+5:30, T Obulesu wrote: >>>>> Hello guys.. >>>>> >>>>> I want to design a trans-impedance amplifier to detect currents of >>>>> the APD (in the range of 200 nA to 100 micro A). >>>>> >>>>> Could some one suggest me the best IC with less bias current, >>>>> single supply (+5 V only) and how to design a high sensitive TIA..? >>>>> (typically, for every 100 nA change in the current, 100 mV voltage >>>>> should be displayed). >>>> >>>> Let me say why am looking for new design.. We are currently using the >>>> receiver module which has three stages: 1. Trans Impedance stage 2. >>>> High Pass filter (HPF) stage 3. Unity gain amplifier (just for >>>> inverting the output of the HPF) >>>> >>>> Well...we used LM2662 as a -5V supply.. Here on we could see the hell >>>> noise below 100kHz.. I couldn't get rid of this noise by using bypass >>>> caps..but I could just reduced it.. Yet there is a lot of noise all >>>> across the circuit ranging from few kHz to hundreds of MHz... >>> > >>> > https://drive.google.com/open?id=0B_khpIBuCMVdb18ySmZubGsyRUU >>> > >>> > https://drive.google.com/open?id=0B_khpIBuCMVdbUVRWW85MHFmSXM >>> > >>> > https://drive.google.com/drive/u/0/folders/0B_khpIBuCMVdMmplSUc3U2FNdjA >>> > >>> > https://drive.google.com/open?id=0B_khpIBuCMVdeVMxZkRGR1NvM2M >>> > >>> > are the links where I have uploaded couple of documents.. >>> > >>> > I really don't know what sort of noise it is and from where it is >>> > coming... >>> > >>> >>> Well, if you're connecting a photodiode directly to a SMPS, what did you >>> expect? >>> >>> Hint: Taking a spiky supply rail, differentiating it with a small >>> capacitor, and sticking the resulting current into a high-gain current >>> amplifier may not be what you want to do. >>> >>> Of course, three lines of algebra would have told you what the results >>> were going to be, and you'd have learned something instead of just >>> throwing spitballs. This theory stuff really works. ;) >>> >>> We've had several threads about capacitance multipliers recently, >>> including one where I posted an LTspice schematic of a 2-stage cap >>> multiplier that will knock that stuff down by about 180 dB, assuming >>> that you have adequate RF shielding. >> >> He could start with a simple RC filter on the bias supply... that might be enough >> to show him where the problem is. (My hint is to look at the noise on the >> bias voltage and see if it looks the same as your signal noise.) >> >> It is weird how this started as a thread to find an opamp. > >The OP doesn't know what he's doing, and wants a quick fix. There is >one, of course--your RC or my cap multiplier--but understanding it does >sort of require doing those three lines of algebra to see just how >vulnerable a TIA is to bias supply noise. It never ceases to amaze me >how many people chicken out of doing that, even when their livelihood >(or their thesis) is on the line. C'mon, T. Obulesu, you can do it. >All you need is the maximum slope of the ripple waveform, the diode >capacitance, and I=C dV/dt. > >> >> To the OP, do you have any thermal regulation on the APD? >> I've read that the gain is a strong function of the bias voltage and >> temperature. > >Yup. It's really easy to turn them to lava. > >Cheers > >Phil Hobbs
Sometimes the noise path is not visible on the schematic; it's capacitive coupling through the air, or ground loops, or magic. Switchers should be far away from low-noise amps. We recently tested a fiber link intended for clock distribution with femtosecond jitter. The wideband noise floor was below -170 dBc, but it has half a dozen birdies in the 5-300KHz range, most of which we can't account for. If we productize this one, the most serious part of the design is going to be the metal boxes. Of course we can't measure 180 fS RMS jitter. We have to inferr it from a spectral phase-noise analysis. -- John Larkin Highland Technology, Inc lunatic fringe electronics