Transimpedance amplifier (TIA) oscillating at above 1 GHz. Need Help

[Jamie]

For anyone following along at home, here's an LTspice file (with
LT1127 opamp)
If you change C1 from 10 pF to 1 pF it will stop with the
oscillations. I don't know what to make of the 10k ohm * 10 pF = 1e-7
second time constant.

George H.

Version 4
SHEET 1 896 680
WIRE 192 80 176 80
WIRE 304 80 272 80
WIRE 176 144 176 80
WIRE -256 208 -256 176
WIRE -128 208 -128 176
WIRE 304 208 304 80
WIRE 304 208 112 208
WIRE 400 208 304 208
WIRE 304 224 304 208
WIRE 400 256 400 208
WIRE 176 288 176 272
WIRE 112 304 112 208
WIRE 144 304 112 304
WIRE 304 320 304 304
WIRE 304 320 208 320
WIRE 400 320 304 320
WIRE 480 320 400 320
WIRE 576 320 560 320
WIRE -256 336 -256 288
WIRE -128 336 -128 288
WIRE 80 336 -32 336
WIRE 144 336 80 336
WIRE -32 352 -32 336
WIRE 176 368 176 352
WIRE 576 400 576 320
WIRE -32 448 -32 432
FLAG -128 336 0
FLAG 176 272 +V
FLAG -128 176 +V
FLAG 176 368 -V
FLAG -256 336 0
FLAG -256 176 -V
FLAG -32 448 0
FLAG 176 144 0
FLAG 80 336 IN+
FLAG 576 400 0
SYMBOL voltage -128 192 R0
SYMATTR InstName V1
SYMATTR Value 15
SYMBOL voltage -256 192 R0
SYMATTR InstName V2
SYMATTR Value -15
SYMBOL opamps\\LT1127 176 256 R0
SYMATTR InstName U1
SYMBOL res 288 208 R0
SYMATTR InstName R3
SYMATTR Value 10k
SYMBOL res 288 64 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R6
SYMATTR Value 2k
SYMBOL res 576 304 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R7
SYMATTR Value 10K
SYMBOL voltage -32 336 R0
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V4
SYMATTR Value SINE(0 10m 1meg)
SYMATTR Value2 AC 10m
SYMBOL cap 384 256 R0
SYMATTR InstName C1
SYMATTR Value 10pf
TEXT 408 424 Left 2 !.tran 30u

2k in series with the 10p..

Jamie

 

[George Herold]

 " 2k in series with the 10p.."

Huh? The 'thing' oscillates at about 23MHz, that's like a 7-8 ns time
constant, 2k and 10pF is only 20ns. You can change the 2k to 4k ohm
and it doens't change much. I guess it's some other pole in the opamp
transfer function, but I'm not sure how to think about it.

George H.

 

[Jamie]

Huh? The 'thing' oscillates at about 23MHz, that's like a 7-8 ns time
constant, 2k and 10pF is only 20ns. You can change the 2k to 4k ohm
and it doens't change much. I guess it's some other pole in the opamp
transfer function, but I'm not sure how to think about it.

Look, I gave you an alternate to your sim to remove the obvious mess
it was generating. what I got as a results was nothing more than the
normal phase margin error between input and output..

I didn't see any oscillations after that with the sim you supplied ?

Maybe we're using version XYZ from planet mars!

Jamie

 

[George Herold]

  Look, I gave you an alternate to your sim to remove the obvious mess
it was generating. what I got as a results was nothing more than the
normal phase margin error between input and output..

   I didn't see any oscillations after that with the sim you supplied?

  Maybe we're using version XYZ from planet mars!

Jamie- Hide quoted text -

- Show quoted text -

Oh sorry, you should have said that the file didn't work.
It was just an lt1127 with gain of 6, non-inverting, and 10pF in
parallel with the 10k ohm feedback resistor.

George H.

 

[josephkk]

Am Dienstag, 14. Mai 2013 10:25:29 UTC+2 schrieb Julian Arnold:

Sorry for the questions about basic stuff.
I am still studying and have not heard so much about OpAmps so far,
but should not the gain always be a function of frequency?

Furthermore Tim said:
"The open-loop gain is determined by the amplifier characteristics and the
feedback network you put onto it"
but is not the open-loop gain only determined by the amplifier and can be approximated by a single pole response?

Not really. Normally there are two or three dominant poles and often
enough one zero in the open loop response.

?-)

 

[Julian Arnold]

Am Dienstag, 14. Mai 2013 10:25:29 UTC+2 schrieb Julian Arnold:

Hey Folks,



I need to design a broadband TIA for photo current amplification.

I am using a LMH6629.

I simulated the circuit in Tina-TI an it is working pretty well for a feedback resistor of 330Ohm and a feedback cap of 4pf and a signal of 10MHz.

However the simulation is showing oscillation if I increase the cap to something above 20pf. In my opinion it should be stable for higher caps and only start oscillating if the cap is to small.



However, I developed the board and now it is oscillating at 1.2 GHz with a perfect sine, even without a signal applied!

This oscillation is nearly independent of the feedback cap. I tried form 1pf up to 10pf.

Only if the cap is missing totally (still around 2pf from resistor) the oscillation is getting worth.

So, I think it has nothing to do with the feedback cap but some other issue.



I really have no ideas left..



Schematic: http://imageshack.us/a/img211/617/schematic.png

Board-top: http://imageshack.us/a/img585/4955/boardtop.png

Board-bottom: http://imageshack.us/a/img822/2739/boardbottom.png



Cheers,

Julian Arnold

Hey everyone,

sorry for the late reply but I had to first gain some more theoretical knowledge
and proceed with my project.

I do now understand, that my TIA is running out of gain but as I am not able to equip the very small capacitor needed, I simply live with the resulting oscillation.
This oscillation is very small in amplitude and thus, the applied signal is easily dominating.
I am now able to transmit data at rates above 100 Mbps.
However, a new problem has shown up.
It seems, that for some particular input power a second, even worse, oscillation arises.
I am not quiet sure if it is some saturation problem or something completely different like reflections in front of the receiver diode.
The following images show the received signal at different input power levels.

With very low power there is only the 'normal' high frequency oscillation.
http://imageshack.us/a/img62/1231/increasingoneszerososc1.png

When increasing the input power (by moving the receiver optics deeper into the beam) the additional oscillation appears.
http://imageshack.us/a/img23/1231/increasingoneszerososc1.png

When further increasing the optical power, the 'down' parts of the signal calm down (Maybe some saturation ?).
http://imageshack.us/a/img208/1231/increasingoneszerososc1.png

I do not think, that it is an transient effect because the following image shows that the oscillation appears nearly randomly when operating near the 'critical' power.
http://img692.imageshack.us/img692/1231/increasingoneszerososc1.png

The yellow curve is the signal applied to the transmitter.

I am thankful for any suggestion!

Cheers,
Julian

 

[George Herold]

Am Dienstag, 14. Mai 2013 10:25:29 UTC+2 schrieb Julian Arnold:















Hey everyone,

sorry for the late reply but I had to first gain some more theoretical knowledge
and proceed with my project.

I do now understand, that my TIA is running out of gain but as I am not able to equip the very small capacitor needed, I simply live with the resulting oscillation.
This oscillation is very small in amplitude and thus, the applied signal is easily dominating.
I am now able to transmit data at rates above 100 Mbps.
However, a new problem has shown up.
It seems, that for some particular input power a second, even worse, oscillation arises.
I am not quiet sure if it is some saturation problem or something completely different like reflections in front of the receiver diode.
The following images show the received signal at different input power levels.

With very low power there is only the 'normal' high frequency oscillation..http://imageshack.us/a/img62/1231/increasingoneszerososc1.png

When increasing the input power (by moving the receiver optics deeper into the beam) the additional oscillation appears.http://imageshack.us/a/img23/1231/increasingoneszerososc1.png

When further increasing the optical power, the 'down' parts of the signalcalm down (Maybe some saturation ?).http://imageshack.us/a/img208/1231/increasingoneszerososc1.png

I do not think, that it is an transient effect because the following image shows that the oscillation appears nearly randomly when operating near the 'critical' power.http://img692.imageshack.us/img692/1231/increasingoneszerososc1.png

The yellow curve is the signal applied to the transmitter.

I am thankful for any suggestion!

Cheers,
Julian- Hide quoted text -

- Show quoted text -

Maybe you could repost your schematic... as it is now.
Maybe it's a power supply 'thing'. (any sign of the oscillation on
the power line?) The original schematic showed the photodiode tied
directly to the power rail... maybe a little series resistor and
capacitor to ground as RC isolation.

I'd be worried about the continued HF oscillations. (If you build
more than one it's guaranteed that to be worse in some.)

George H.

 

[Julian Arnold]

Am Mittwoch, 12. Juni 2013 18:27:50 UTC+2 schrieb George Herold:

Maybe you could repost your schematic... as it is now.

Maybe it's a power supply 'thing'. (any sign of the oscillation on

the power line?) The original schematic showed the photodiode tied

directly to the power rail... maybe a little series resistor and

capacitor to ground as RC isolation.



I'd be worried about the continued HF oscillations. (If you build

more than one it's guaranteed that to be worse in some.)



George H.

Hey George,

here is the link to the actual schematic: http://img51.imageshack.us/img51/7702/schematicj.png

On the board, the diode is next to the power decoupling caps of the TIA. Isit OK? I have not measured the power supply yet but I will do it on Friday..
Do you think this could be related to the optical input power?

I only need to get that one somehow working for my project

 

[George Herold]

Am Mittwoch, 12. Juni 2013 18:27:50 UTC+2 schrieb George Herold:














Hey George,

here is the link to the actual schematic:http://img51.imageshack.us/img51/7702/schematicj.png

A much nicer schematic.
Is the 0.4pF Cf the estimated stray capacitance?

On the board, the diode is next to the power decoupling caps of the TIA. Is it OK?

I would think so, but I've rarely tread above 100MHz. So I'm not
much of a HF 'expert'

I have not measured the power supply yet but I will do it on Friday.

Do you think this could be related to the optical input power?

Well more power means more output signal.. more current.
Maybe a little decoupling would help, like this...

V(+)----RRR-+---|<|---->to opamp(-)
| (PD)
C
|
GND

So perhaps 10 ohms for R and 0.1uF for C.

I only need to get that one somehow working for my project

Well is the Professor going to see the oscillations?
And stopping the HF oscillations may also take care of this other
issue.

George H.

- Hide quoted text -

 

[Julian Arnold]

Am Mittwoch, 12. Juni 2013 19:50:37 UTC+2 schrieb George Herold:

Am Mittwoch, 12. Juni 2013 18:27:50 UTC+2 schrieb George Herold:








A much nicer schematic.

Is the 0.4pF Cf the estimated stray capacitance?



I would think so, but I've rarely tread above 100MHz. So I'm not

much of a HF 'expert'



I have not measured the power supply yet but I will do it on Friday.




Well more power means more output signal.. more current.

Maybe a little decoupling would help, like this...



V(+)----RRR-+---|<|---->to opamp(-)

| (PD)

C

|

GND



So perhaps 10 ohms for R and 0.1uF for C.







Well is the Professor going to see the oscillations?

And stopping the HF oscillations may also take care of this other

issue.



George H.



- Hide quoted text -

Hey,

yes, the professor is going to see them but I do not have the time to make new circuit boards.

Your suggestion sounds good!
Maybe I will have the time left to apply it.
But I think, I will first measure if there is any noise on the power supply..

Cheers,
Julian

 

[George Herold]

Am Mittwoch, 12. Juni 2013 19:50:37 UTC+2 schrieb George Herold:






















Hey,

yes, the professor is going to see them but I do not have the time to make new circuit boards.

Your suggestion sounds good!
Maybe I will have the time left to apply it.
But I think, I will first measure if there is any noise on the power supply.

Cheers,
Julian- Hide quoted text -

- Show quoted text -

Julian, I was walking around after work today. And I think I know
how to fix both your problems. You should attach the PD lead tied to
the supply volatge to 'some other' node in your circuit. This will
both increase the capacitance seen by the inverting input,
and eliminate the PS feedback path.
You get to figure out which node.

George H.

 

[[email protected]]

Am Mittwoch, 12. Juni 2013 18:27:50 UTC+2 schrieb George Herold:




Hey George,



here is the link to the actual schematic: http://img51.imageshack.us/img51/7702/schematicj.png

....


Hello Julian,
What is the status of this issue? Did you resolve it? I am finding a verysimilar issue on a transimpedance amplifier design I am making with LMH6629, and I believe that I have uncovered the solution which is that there must be a minimum source impedance seen by the LMH6629 input pins. For instance, try a 64.9 Ohm resistor (value what I found to work) between pin 3 and the bias network (C1 etc. on your schematic). Depending on wiring length to the photodiode another series resistor may be needed there, i.e. between the photodiode and the junction of inverting input pin and the feedback R-C.
I believe the issue (which these resistors seem to mitigate) is an internal oscillation of the LMH6629 input stage. I see the oscillation dependent significantly on the power supply voltage and even on the history of power supply voltage, and although I did not try it I believe it may well be dependent on signal amplitude too.
In any case, I do not think it is wise to have any oscillation in your final design, no oscillation is small enough to ignore. At the very least, the unknown phase of the oscillation amounts to extra noise, and nonlinearities must mix it into your frequency bands of interest.
I hope these comments help, and I would really appreciate to hear if anyone else has encountered similar issues with LMH6629 in any circuit. I don'tthink this issue is about transimpedance applications only. By the way, the datasheet mentions 25 Ohm minimum source impedance, but IMHO this is wrong... Also by the way, the oscillations I saw were typically at ~1.23 GHz, with other satellite peaks too.
Sincerely,

Gerard Visser
Indiana University

 

[Phil Hobbs]

When increasing the input power (by moving the receiver optics deeper
into the beam) the additional oscillation
appears.http://imageshack.us/a/img23/1231/increasingoneszerososc1.pngI do not think, that it is an transient effect because the following
image shows that the oscillation appears nearly randomly when operating
near the 'critical'
power.http://img692.imageshack.us/img692/1231/increasingoneszerososc1.png

...


Hello Julian, What is the status of this issue? Did you resolve it? I
am finding a very similar issue on a transimpedance amplifier design
I am making with LMH6629, and I believe that I have uncovered the
solution which is that there must be a minimum source impedance seen
by the LMH6629 input pins. For instance, try a 64.9 Ohm resistor
(value what I found to work) between pin 3 and the bias network (C1
etc. on your schematic). Depending on wiring length to the photodiode
another series resistor may be needed there, i.e. between the
photodiode and the junction of inverting input pin and the feedback
R-C.

I believe the issue (which these resistors seem to mitigate) is an
internal oscillation of the LMH6629 input stage. I see the
oscillation dependent significantly on the power supply voltage and
even on the history of power supply voltage, and although I did not
try it I believe it may well be dependent on signal amplitude too. In
any case, I do not think it is wise to have any oscillation in your
final design, no oscillation is small enough to ignore. At the very
least, the unknown phase of the oscillation amounts to extra noise,
and nonlinearities must mix it into your frequency bands of
interest.

I hope these comments help, and I would really appreciate to hear if
anyone else has encountered similar issues with LMH6629 in any
circuit. I don't think this issue is about transimpedance
applications only. By the way, the datasheet mentions 25 Ohm minimum
source impedance, but IMHO this is wrong... Also by the way, the
oscillations I saw were typically at ~1.23 GHz, with other satellite
peaks too.

Sincerely,

Gerard Visser Indiana University

I doubt that's the issue, particularly since almost that exact circuit
is on the first page of the datasheet. Because of the differential-mode
input capacitance, puttting a resistor in series with pin 3 applies a
feedback zero, reducing the high frequency loop gain.

I suspect that the problem is one of four things.

1. Layout problems

2. Too small a feedback cap (uncontrolled noise gain)

3. Too large a feedback cap (the op amp isn't unity gain stable)

4. Layout problems.

(Did I mention layout?) You can't put photodiodes on cables in this
sort of circuit, for instance. It has to be a very tight layout with
excellent grounds and as close as humanly possible to zero lead length.
Then you have to tune the feedback cap for stability.

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