Separating high pass and low pass TIA output bands.

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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.

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Re: Separating high pass and low pass TIA output bands.
On 07/11/2017 03:13 PM, Artist wrote:
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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

Separating high pass and low pass TIA output bands.
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

Re: Separating high pass and low pass TIA output bands.
On Tue, 11 Jul 2017 13:09:29 -0700 (PDT), snipped-for-privacy@gmail.com wrote:

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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  


Re: Separating high pass and low pass TIA output bands.
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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

Re: Separating high pass and low pass TIA output bands.
On 7/11/2017 1:09 PM, snipped-for-privacy@gmail.com wrote:
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Thanks for your suggestion. I have your book, and I am studying that  
circuit in its chapter 18. The candidate opamp is:

Analog Devices ADA4817:  
http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4817-1_4817-2.pdf

It has a very high GBW and very low differential and common mode input  
capacitances. It also has a very convenient pin out for the feedback  
resistor and guard ring traces. Its offset error is a bit high, and it  
is supply voltage is limited to +/-5V.

There is a version that has two opamps in one package. I am going to  
avoid using it because I do not have a way to predict if cross talk  
within the package will be a problem.

Also considered are:
TI THS4631: http://www.ti.com/product/THS4631/technicaldocuments
TI OPA656: http://www.ti.com/lit/ds/symlink/opa656.pdf

I would use your capacitance buffering circuit in conjunction with the  
error integrator in the feedback loop in Fig 12, page 11, of this PDF:  
http://www.ti.com/lit/ds/sbbs001/sbbs001.pdf to eliminate DC while at  
the same time making the transimpedance value as high as possible. For  
this integrator I plan to use the same opamp I use for the TIA.

I am looking using metal foil resistors for the noise ctrical parts.

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Re: Separating high pass and low pass TIA output bands.
wrote:

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I don't think they help. The Johnson noise is the same for all
resistors, and reasonable-value cermets don't seem to have shot noise.
I think I have seen shot noise in high-value cermets, like 100Mohms,
but it's a difficult measurement, and I may have just been seeing
tempco.




--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Separating high pass and low pass TIA output bands.
On 07/19/2017 09:51 PM, John Larkin wrote:
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The main issue with thick film resistors is 1/f noise, due apparently to  
conductance fluctuations at  boundaries between conducting grains.

Cheers

Phil Hobbs

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Principal Consultant
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Re: Separating high pass and low pass TIA output bands.
On Fri, 21 Jul 2017 09:41:15 -0400, Phil Hobbs

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Yep.  Back when I made hybrids, we used thin-film resistor chips for
low-noise applications.
        
                                        ...Jim Thompson
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| Analog Innovations                               |     et      |
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Re: Separating high pass and low pass TIA output bands.
On Friday, July 21, 2017 at 8:37:16 AM UTC-7, Jim Thompson wrote:
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...

They can take up a lot of space on-chip if you need high resistance though.

A mult-ichannel high-voltage amplifier chip I architected with Supertex had 16 12 Megohm SiCr resistors to meet noise requirements - they dominated the chip area.

kevin

Re: Separating high pass and low pass TIA output bands.
On Fri, 21 Jul 2017 10:16:41 -0700 (PDT), kevin93

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I was referring to hybrids, Alumina substrates (1" x 2"), separate
chips for resistors.

On today's microchips, 1K/?sq is commonplace, and several of the
processes I use have 10K/sq.

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Yep.  But that's a bit extreme.  Usually I have only a few meg-Ohms
total... mostly in the bandgaps and critical mirrors.
        
                                        ...Jim Thompson
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| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Separating high pass and low pass TIA output bands.
On Friday, July 21, 2017 at 10:27:27 AM UTC-7, Jim Thompson wrote:
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I realize that - sorry for being unclear what I meant by 'chip'.

I also did a hybrid implementation but we only had thick-film and we didn't go into production with that version.

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I think we used ~5k/sq.

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This was for a system where I had ~3,000 HV amplifiers for driving MEMS mirrors at up to 170v in an optical switch.

I had to keep the power down - hence the high value resistors.  The stability and noise requirements forced us to thin-film rather than polysilicon.

A competitors amplifier had 100's of millivolts of noise when the output was at 150v - noise went up with output voltage - I guess they used poly.

kevin

Re: Separating high pass and low pass TIA output bands.
On Fri, 21 Jul 2017 11:26:51 -0700 (PDT), kevin93

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Sounds like a project I worked at, IIRC, SpatiaLight.

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                                        ...Jim Thompson
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Re: Separating high pass and low pass TIA output bands.
On Wednesday, July 19, 2017 at 8:45:53 PM UTC-4, Artist wrote:
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ding most of your time on a fast, quiet DC-coupled TIA and filter afterward
s.
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7-1_4817-2.pdf
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If you're doing what I think you're doing you are going to be shot noise li
mited by the high dc current in the photodiode but you need to do the noise
 analysis. You should be able to find how to do that in a number of books o
r app notes.  

I've used the opa656 for similar circuits. Be careful of that split resisto
r feedback circuit shown in the datasheet. It doesn't give the best roll of
f of the noise bandwidth. I ended up sticking a 0.5 pF cap across the feedb
ack resistor without a cap in order to meet my requirements.

Re: Separating high pass and low pass TIA output bands.
On 07/19/2017 08:45 PM, Artist wrote:
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The point of the bootstrap is that it needs to be quieter than the op  
amp, so that the eN*C mechanism multiplies the 0.8 nV noise of the FET  
rather than the 4 nV noise of the op amp.  That's 14 dB right there,  
assuming the FET doesn't dominate the PD's capacitance and that the PD's  
series resistance isn't too large.

What's your photodiode capacitance?

Cheers

Phil Hobbs


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Principal Consultant
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Re: Separating high pass and low pass TIA output bands.
On 7/20/2017 5:42 AM, Phil Hobbs wrote:
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It will be one of the FDXXX Series photodiodes from Fermionics. Within  
these series a photodiode with the largest area whose capacitance will  
permit a bandwidth of 40MHz will be chosen.

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Re: Separating high pass and low pass TIA output bands.
On 07/20/2017 05:42 AM, Phil Hobbs wrote:
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For this design I intend to use 0402 (imperial) size parts to minimize  
stray capacitance and inductances at 40MHz, and if I can, up to 100MHz.  
will this make a difference at 100MHz? Or will 0603, or even 0805 work  
as well at 100MHz?


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Re: Separating high pass and low pass TIA output bands.
On Wed, 16 Aug 2017 11:55:05 -0700, Artist wrote:

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With care Tektronix used 1/4 watt through-hole parts up to 200MHz overall BW.
Sure, higher-impedance nodes have capacitance issues; low-impedance branches
have inductance issues.  Most places - it's the device reactances, or
block-interconnect reactances that cause problems.

  -F

Re: Separating high pass and low pass TIA output bands.
On Wednesday, August 16, 2017 at 2:55:14 PM UTC-4, Artist wrote:
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4817-1_4817-2.pdf  
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s  
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Maybe. The larger packages will have a higher parasitic capacitance. Whethe
r that matters depends on how high the resistance is. I have used a larger  
package to use just the parasitic capacitance across the feedback resistor.
 You may need to cut out the planes below the high frequency parts to preve
nt parasitic capacitance from that also. But if you are adding a capacitor  
of a few picofarads across the resistor, the fractional picofarad addition  
from the part won't matter much. You can test the effect of different paras
itic capacitances with spice simulations.

Re: Separating high pass and low pass TIA output bands.
On Wed, 16 Aug 2017 16:19:39 -0700 (PDT), Wanderer

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Does the parasitic capacitance to the PCB have a significant effect,
or is it all end-to-end?
        
                                        ...Jim Thompson
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| Analog Innovations                               |     et      |
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