Separating high pass and low pass TIA output bands.

Started by Artist July 11, 2017
I have an assignment to design a transimpedance amplifier for a 
photodiode. The bandwidth is to be 40MHz, and there are to be two 
outputs, a high pass output above 20kHz and a low pass output below 
20kHz all the way to DC. It is permissible that the two bands overlap a 
little bit.

The noise performance of an amplifier is primarily determined by the 
noise in the first stage where it is desirable to have its gain as high 
as possible. I realize the first stage's transimpedance will be limited 
by the input current's DC output level, and if this were absent the 
transimpedance value could be much higher to take full advantage of the 
op amp's output range, and thereby have much better noise performance.

So I am wondering if it is practical to have two first stage 
transimpdance amplifiers, one for each output band, and between these 
two, and the photodiode, separate the two output bands by means of a 
passive LC network.

At this time I know very little about the photodiode's source impedance, 
and what current levels to expect.

I am aware of inductor and capacitor self resonances, and would select 
these passives accordingly. I have studied high frequency board layout 
techniques. And I know about opamp gain bandwidth constants and how to 
figure gain and bandwidth compromises in a op amp voltage gain circuit.

-- 
To email me directly remove sj. from my email address's domain name. 
This is a spam jammer.
On 07/11/2017 03:13 PM, Artist wrote:
> I have an assignment to design a transimpedance amplifier for a > photodiode. The bandwidth is to be 40MHz, and there are to be two > outputs, a high pass output above 20kHz and a low pass output below > 20kHz all the way to DC. It is permissible that the two bands overlap a > little bit. > > The noise performance of an amplifier is primarily determined by the > noise in the first stage where it is desirable to have its gain as high > as possible. I realize the first stage's transimpedance will be limited > by the input current's DC output level, and if this were absent the > transimpedance value could be much higher to take full advantage of the > op amp's output range, and thereby have much better noise performance. > > So I am wondering if it is practical to have two first stage > transimpdance amplifiers, one for each output band, and between these > two, and the photodiode, separate the two output bands by means of a > passive LC network. > > At this time I know very little about the photodiode's source impedance, > and what current levels to expect. > > I am aware of inductor and capacitor self resonances, and would select > these passives accordingly. I have studied high frequency board layout > techniques. And I know about opamp gain bandwidth constants and how to > figure gain and bandwidth compromises in a op amp voltage gain circuit. >
The frequencies don't seem that high in the grand scheme of things, once the signal is thru the first stage it seems like the SNR would be good enough to deploy a gyrator-based second order filter rather than use real inductors, you can control the response better
Good TIAs at that bandwidth are not that easy to design. I suggest spending most of
your time on a fast, quiet DC-coupled TIA and filter afterwards. 

You might find this article helpful. 
<http://electrooptical.net/www/frontends/frontends.pdf>

Cheers

Phil Hobbs
On Tue, 11 Jul 2017 13:09:29 -0700 (PDT), pcdhobbs@gmail.com wrote:

>Good TIAs at that bandwidth are not that easy to design. I suggest spending most of
your time on a fast, quiet DC-coupled TIA and filter afterwards.
> >You might find this article helpful. ><http://electrooptical.net/www/frontends/frontends.pdf> > >Cheers > >Phil Hobbs
I have played with the idea of putting a TIA on both ends of a photodiode. It's probably dumb. The only use might be to do precise DC zeroing of a fast but DC-inaccurate amp. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On 2017-07-11 12:13, Artist wrote:
> I have an assignment to design a transimpedance amplifier for a > photodiode. The bandwidth is to be 40MHz, and there are to be two > outputs, a high pass output above 20kHz and a low pass output below > 20kHz all the way to DC. It is permissible that the two bands overlap a > little bit. > > The noise performance of an amplifier is primarily determined by the > noise in the first stage where it is desirable to have its gain as high > as possible. I realize the first stage's transimpedance will be limited > by the input current's DC output level, and if this were absent the > transimpedance value could be much higher to take full advantage of the > op amp's output range, and thereby have much better noise performance. > > So I am wondering if it is practical to have two first stage > transimpdance amplifiers, one for each output band, and between these > two, and the photodiode, separate the two output bands by means of a > passive LC network. > > At this time I know very little about the photodiode's source impedance, > and what current levels to expect. > > I am aware of inductor and capacitor self resonances, and would select > these passives accordingly. I have studied high frequency board layout > techniques. And I know about opamp gain bandwidth constants and how to > figure gain and bandwidth compromises in a op amp voltage gain circuit. >
I have done this once. The bands did not overlap (probably could have) and there was one TIA optimized for RF, low noise and such but that had high offset and drift. The other was optimized for low drift and low offset errors because that was used to servo something. I can't go into details but it all worked fine. RF guys call this sort of splitter a diplexer. As Phil said in your case this might not make much sense because 20kHz is already audio range and 40MHz is RF but not very high. In my case it was different, an audio range servo loop and then the RF was a certain band in the low VHF range. If you need professional support on this Phil is the guru in that field. -- Regards, Joerg http://www.analogconsultants.com/
On Tuesday, July 11, 2017 at 3:14:20 PM UTC-4, Artist wrote:
> I have an assignment to design a transimpedance amplifier for a > photodiode. The bandwidth is to be 40MHz, and there are to be two > outputs, a high pass output above 20kHz and a low pass output below > 20kHz all the way to DC. It is permissible that the two bands overlap a > little bit. > > The noise performance of an amplifier is primarily determined by the > noise in the first stage where it is desirable to have its gain as high > as possible. I realize the first stage's transimpedance will be limited > by the input current's DC output level, and if this were absent the > transimpedance value could be much higher to take full advantage of the > op amp's output range, and thereby have much better noise performance. > > So I am wondering if it is practical to have two first stage > transimpdance amplifiers, one for each output band, and between these > two, and the photodiode, separate the two output bands by means of a > passive LC network. > > At this time I know very little about the photodiode's source impedance, > and what current levels to expect. > > I am aware of inductor and capacitor self resonances, and would select > these passives accordingly. I have studied high frequency board layout > techniques. And I know about opamp gain bandwidth constants and how to > figure gain and bandwidth compromises in a op amp voltage gain circuit. > > -- > To email me directly remove sj. from my email address's domain name. > This is a spam jammer.
Look at figure 12 in this document. http://www.ti.com/lit/ds/sbbs001/sbbs001.pdf It's the DC restoration circuit. There will be noise from both A1 and R3 in the feedback loop, so there are trade offs here you need to make.
On 7/11/2017 4:37 PM, Wanderer wrote:
> On Tuesday, July 11, 2017 at 3:14:20 PM UTC-4, Artist wrote: >> I have an assignment to design a transimpedance amplifier for a >> photodiode. The bandwidth is to be 40MHz, and there are to be two >> outputs, a high pass output above 20kHz and a low pass output below >> 20kHz all the way to DC. It is permissible that the two bands overlap a >> little bit. >> >> The noise performance of an amplifier is primarily determined by the >> noise in the first stage where it is desirable to have its gain as high >> as possible. I realize the first stage's transimpedance will be limited >> by the input current's DC output level, and if this were absent the >> transimpedance value could be much higher to take full advantage of the >> op amp's output range, and thereby have much better noise performance. >> >> So I am wondering if it is practical to have two first stage >> transimpdance amplifiers, one for each output band, and between these >> two, and the photodiode, separate the two output bands by means of a >> passive LC network. >> >> At this time I know very little about the photodiode's source impedance, >> and what current levels to expect. >> >> I am aware of inductor and capacitor self resonances, and would select >> these passives accordingly. I have studied high frequency board layout >> techniques. And I know about opamp gain bandwidth constants and how to >> figure gain and bandwidth compromises in a op amp voltage gain circuit. >> >> -- >> To email me directly remove sj. from my email address's domain name. >> This is a spam jammer. > > Look at figure 12 in this document. > > http://www.ti.com/lit/ds/sbbs001/sbbs001.pdf > > It's the DC restoration circuit. There will be noise from both A1 and R3 in the
feedback loop, so there are trade offs here you need to make.
> > >
I do not have choice regarding the photodiode. That will be chosen by someone else. An integrated photodiode tia package is not an option. -- To respond to me directly remove sj. from the my email address's domain name. This is a spam jammer.
On Tuesday, July 11, 2017 at 7:46:36 PM UTC-4, Pflanze, Stephen wrote:
> On 7/11/2017 4:37 PM, Wanderer wrote: > > On Tuesday, July 11, 2017 at 3:14:20 PM UTC-4, Artist wrote: > >> I have an assignment to design a transimpedance amplifier for a > >> photodiode. The bandwidth is to be 40MHz, and there are to be two > >> outputs, a high pass output above 20kHz and a low pass output below > >> 20kHz all the way to DC. It is permissible that the two bands overlap a > >> little bit. > >> > >> The noise performance of an amplifier is primarily determined by the > >> noise in the first stage where it is desirable to have its gain as high > >> as possible. I realize the first stage's transimpedance will be limited > >> by the input current's DC output level, and if this were absent the > >> transimpedance value could be much higher to take full advantage of the > >> op amp's output range, and thereby have much better noise performance. > >> > >> So I am wondering if it is practical to have two first stage > >> transimpdance amplifiers, one for each output band, and between these > >> two, and the photodiode, separate the two output bands by means of a > >> passive LC network. > >> > >> At this time I know very little about the photodiode's source impedance, > >> and what current levels to expect. > >> > >> I am aware of inductor and capacitor self resonances, and would select > >> these passives accordingly. I have studied high frequency board layout > >> techniques. And I know about opamp gain bandwidth constants and how to > >> figure gain and bandwidth compromises in a op amp voltage gain circuit. > >> > >> -- > >> To email me directly remove sj. from my email address's domain name. > >> This is a spam jammer. > > > > Look at figure 12 in this document. > > > > http://www.ti.com/lit/ds/sbbs001/sbbs001.pdf > > > > It's the DC restoration circuit. There will be noise from both A1 and R3 in the
feedback loop, so there are trade offs here you need to make.
> > > > > > > I do not have choice regarding the photodiode. That will be chosen by > someone else. An integrated photodiode tia package is not an option. > > -- > To respond to me directly remove sj. from the my email address's domain > name. This is a spam jammer.
I'm not suggesting that you use the OPT301, just the circuit. It's just a TIA with another opamp in the feedback. The OpAmp in the feedback is the low frequency stuff. The output of the forward TIA is the high frequency stuff.
>I have played with the idea of putting a TIA on both ends of a >photodiode. It's probably dumb. The only use might be to do precise DC >zeroing of a fast but DC-inaccurate amp.
One good reason to do that is to avoid range switching. You bias the noninverting inputs far apart, to get some bias on the PD, and use diode switching to select between the high- and low-gain paths by shorting out the one you don't want to use. Of course you have to invert and level-shift one side, but it's sometimes useful. Cheers Phil Hobbs
On 7/11/2017 4:52 PM, Wanderer wrote:
> On Tuesday, July 11, 2017 at 7:46:36 PM UTC-4, Pflanze, Stephen wrote: >> On 7/11/2017 4:37 PM, Wanderer wrote: >>> On Tuesday, July 11, 2017 at 3:14:20 PM UTC-4, Artist wrote: >>>> I have an assignment to design a transimpedance amplifier for a >>>> photodiode. The bandwidth is to be 40MHz, and there are to be two >>>> outputs, a high pass output above 20kHz and a low pass output below >>>> 20kHz all the way to DC. It is permissible that the two bands overlap a >>>> little bit. >>>> >>>> The noise performance of an amplifier is primarily determined by the >>>> noise in the first stage where it is desirable to have its gain as high >>>> as possible. I realize the first stage's transimpedance will be limited >>>> by the input current's DC output level, and if this were absent the >>>> transimpedance value could be much higher to take full advantage of the >>>> op amp's output range, and thereby have much better noise performance. >>>> >>>> So I am wondering if it is practical to have two first stage >>>> transimpdance amplifiers, one for each output band, and between these >>>> two, and the photodiode, separate the two output bands by means of a >>>> passive LC network. >>>> >>>> At this time I know very little about the photodiode's source impedance, >>>> and what current levels to expect. >>>> >>>> I am aware of inductor and capacitor self resonances, and would select >>>> these passives accordingly. I have studied high frequency board layout >>>> techniques. And I know about opamp gain bandwidth constants and how to >>>> figure gain and bandwidth compromises in a op amp voltage gain circuit. >>>> >>>> -- >>>> To email me directly remove sj. from my email address's domain name. >>>> This is a spam jammer. >>> >>> Look at figure 12 in this document. >>> >>> http://www.ti.com/lit/ds/sbbs001/sbbs001.pdf >>> >>> It's the DC restoration circuit. There will be noise from both A1 and R3 in the
feedback loop, so there are trade offs here you need to make.
>>> >>> >>> >> I do not have choice regarding the photodiode. That will be chosen by >> someone else. An integrated photodiode tia package is not an option. >> >> -- >> To respond to me directly remove sj. from the my email address's domain >> name. This is a spam jammer. > > I'm not suggesting that you use the OPT301, just the circuit. It's just a TIA with
another opamp in the feedback. The OpAmp in the feedback is the low frequency stuff. The output of the forward TIA is the high frequency stuff.
>
I see that schematic now, and it does look to me to be a better alternative to an LC network between the photodiode and two TIAs. This has an error integrator in the feedback loop to adjust the TIA's output to zero offset. That method is compatible with the circuit in the article Phil Hobbs linked to. I will try it. Thanks for this. -- To respond to me directly remove sj. from the my email address's domain name. This is a spam jammer.