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
Transimpedance amplifier (TIA) oscillating at above 1 GHz. Need Help
Started by ●May 14, 2013
Reply by ●May 14, 20132013-05-14
On 5/14/2013 4:25 AM, julian.arnold88@gmail.com wrote:> 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 >Wow, that's one ugly schematic. Conventionally you use a triangle for an op amp, and put the input on the left and the output on the right. With no photodiode and no feedback cap, you're trying to run the LMH6629 at a noise gain of 2, whereas it's only barely stable at a noise gain of 10. It's also a really hot amplifier, with a GBW of nearly 10 GHz, so the layout is going to be very critical. You overestimate the capacitance of the feedback resistor itself--it'll generally be between 0.05 and 0.15 pF, depending on the type. The photodiode capacitance is key to making this thing stable, because at high frequency the noise gain is set by the ratio of the input and feedback capacitances--you need a PD whose capacitance is at least 10 times the feedback cap to make it even marginally stable. What sort of photodiode are you planning to use? 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 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●May 14, 20132013-05-14
Am Dienstag, 14. Mai 2013 10:25:29 UTC+2 schrieb julian....@gmail.com:> 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 ArnoldHey, Actually it is so ugly because it was my first approach to add a custom part to my layout editor ;) How do you calculate the noise gain of 2? Without the feedback cap and the diode it should be 1, shouldn`t it? I am using a pin diode with 1.2pf of capacitance at 3.3V reverse bias. I thought the more input capacitance the more unstable the circuit is so I tried to keep it small.. Cheers, Julian
Reply by ●May 14, 20132013-05-14
On 05/14/2013 08:56 AM, julian.arnold88@gmail.com wrote:> Am Dienstag, 14. Mai 2013 10:25:29 UTC+2 schrieb > julian....@gmail.com: >> 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> Wow, that's one ugly schematic. Conventionally you use a triangle > for an op amp, and put the input on the left and the output on the > right. > > With no photodiode and no feedback cap, you're trying to run the > LMH6629 at a noise gain of 2, whereas it's only barely stable at a > noise gain of 10. It's also a really hot amplifier, with a GBW of > nearly 10 GHz, so the layout is going to be very critical. > > You overestimate the capacitance of the feedback resistor > itself--it'll generally be between 0.05 and 0.15 pF, depending on the > type. > > The photodiode capacitance is key to making this thing stable, > because at high frequency the noise gain is set by the ratio of the > input and feedback capacitances--you need a PD whose capacitance is > at least 10 times the feedback cap to make it even marginally > stable. > > What sort of photodiode are you planning to use? > > Cheers > > Phil Hobbs> > Hey, > > Actually it is so ugly because it was my first approach to add a > custom part to my layout editor ;) > > How do you calculate the noise gain of 2? Without the feedback cap > and the diode it should be 1, shouldn`t it? > > I am using a pin diode with 1.2pf of capacitance at 3.3V reverse > bias. I thought the more input capacitance the more unstable the > circuit is so I tried to keep it small.. > > Cheers, Julian >(Context restored) The noise gain is the closed-loop noninverting gain of the stage. With no PD and no cap, you've got 1.2k input and 1.2k feedback resistors, for a noninverting gain of 2, which is guaranteed to oscillate with this part. If your PD is only 1.2 pF (which is a bit hard to believe at zero bias), then if the pads are small and the traces short, you should have ~ 1 pF from the board, 1.2 pF from the PD, plus the differential input capacitance of the amp. This number is suspiciously absent from the datasheet, but I'm guessing that it's about 2 pF. Thus your total C_in is about 4 pF. To maintain stability, your feedback capacitance needs to be less than 0.4 pF. I'd try replacing your PD with a 1-pF capacitor and running with no feedback cap, first. If it still oscillates, try pressing your finger down so that R_f squashes right into the pad of your finger, and see if you can make it stable. If so, choose the right C_f and you're probably done. If not, try reducing R_f. With too high an R_f, the phase shift of R_f*C_in can destabilize the loop even if the amp were unity-gain stable. The corner frequency of 1.2k and 2 pF is only 66 MHz, so you could also have an oscillation in the low hundreds of megahertz if the 1.2 GHz one didn't take over. Do you really need such a hot amplifier? 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 ●May 14, 20132013-05-14
On Tue, 14 May 2013 01:25:29 -0700 (PDT), julian.arnold88@gmail.com wrote:>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 ArnoldIs the pd connected to pad1-pad2? Why is there a resistor across the photodiode? To Vcc? That will make troubles. What is Vcc? -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Reply by ●May 14, 20132013-05-14
Am Dienstag, 14. Mai 2013 10:25:29 UTC+2 schrieb Julian Arnold:> Hey Folks, >=20 >=20 >=20 > I need to design a broadband TIA for photo current amplification. >=20 > I am using a LMH6629. >=20 > I simulated the circuit in Tina-TI an it is working pretty well for a fee=dback resistor of 330Ohm and a feedback cap of 4pf and a signal of 10MHz.>=20 > However the simulation is showing oscillation if I increase the cap to so=mething above 20pf. In my opinion it should be stable for higher caps and o= nly start oscillating if the cap is to small.>=20 >=20 >=20 > However, I developed the board and now it is oscillating at 1.2 GHz with =a perfect sine, even without a signal applied!>=20 > This oscillation is nearly independent of the feedback cap. I tried form =1pf up to 10pf.>=20 > Only if the cap is missing totally (still around 2pf from resistor) the o=scillation is getting worth.>=20 > So, I think it has nothing to do with the feedback cap but some other iss=ue.>=20 >=20 >=20 > I really have no ideas left..=20 >=20 >=20 >=20 > Schematic: http://imageshack.us/a/img211/617/schematic.png >=20 > Board-top: http://imageshack.us/a/img585/4955/boardtop.png >=20 > Board-bottom: http://imageshack.us/a/img822/2739/boardbottom.png=20 >=20 >=20 >=20 > Cheers, >=20 > Julian ArnoldThanks a lot for the responses! Unfortunately I need to build up the circuit for a university project where= I need to transmit an OOK modulated signal at around 100 MHz... The resistor across the diode is not soldered to the board. I think I should remove it from the schematic. It was placed there to remove biasing voltage if the diode would not be con= nected to VCC, which is at 5V. So if the stability of the circuit depends on the noise gain, which needs t= o be above 10, than wouldn=B4t it be impossible to get it stable without a = signal applied to the diode? Because without the resistor across the diode = the noise gain should always be 1 without applying a signal. Cheers, Julian
Reply by ●May 14, 20132013-05-14
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 ArnoldJust made a few measurements with different caps and no optical signal applied: R_f = 1,2k C_d = 3pf => V_o: Sine wave with 440mVp-p at 418MHz R_f = 1,2k C_d = 3,9pf => 448mVp-p at 477MHz R_f = 1,2k C_d = 1pf => 190mVp-p at 1,2GHz
Reply by ●May 14, 20132013-05-14
On Tue, 14 May 2013 07:54:18 -0700 (PDT), Julian Arnold <julian.arnold88@gmail.com> wrote:>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 > >Thanks a lot for the responses! > >Unfortunately I need to build up the circuit for a university project where I need to transmit an OOK modulated signal at around 100 MHz... > >The resistor across the diode is not soldered to the board. >I think I should remove it from the schematic. >It was placed there to remove biasing voltage if the diode would not be connected to VCC, which is at 5V. > >So if the stability of the circuit depends on the noise gain, which needs to be above 10, than wouldn�t it be impossible to get it stable without a signal applied to the diode? Because without the resistor across the diode the noise gain should always be 1 without applying a signal. > >Cheers, >JulianPhil is the expert here, but I'd go for a parallel, 100 MHz tuned LC network driving the gate of a phemt. If s/n is not critical, just dump the photodiode current into a cheap MMIC. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Reply by ●May 14, 20132013-05-14
On May 14, 10:54=A0am, Julian Arnold <julian.arnol...@gmail.com> wrote:> 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 f=eedback 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 wit=h a perfect sine, even without a signal applied!> > > This oscillation is nearly independent of the feedback cap. I tried for=m 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 i=ssue.> > > 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 > > Thanks a lot for the responses! > > Unfortunately I need to build up the circuit for a university project whe=re I need to transmit an OOK modulated signal at around 100 MHz...> > The resistor across the diode is not soldered to the board. > I think I should remove it from the schematic.So the PD is across pad 1 and pad 2? And R5 1.2k is not in the circuit? (I was wondering what R5 was doing.) I must admit I'm also a bit confused by Phils advise that you need more C on the input. But I've never used a opamp in a TIA that wasn't unity gain stable. Your circuit looks very similar to one in the LMH6629 data sheet... but there they do have a 10 pF PD on the input. Finally I assume that you've got some nice bypass caps on the power supplies. Please let us know when (and how) you get this working. I might have learned something today :^) George H.> It was placed there to remove biasing voltage if the diode would not be c=onnected to VCC, which is at 5V.> > So if the stability of the circuit depends on the noise gain, which needs=to be above 10, than wouldn=B4t it be impossible to get it stable without = a signal applied to the diode? Because without the resistor across the diod= e the noise gain should always be 1 without applying a signal.> > Cheers, > Julian- Hide quoted text - > > - Show quoted text -
Reply by ●May 14, 20132013-05-14
Yes, the Resistor across the diode is not uses, sorry for that. The schematic actually is: http://imageshack.us/a/img211/2842/schematicx.png What I am confused about is, that the datasheet tells, that at least a gain= of 10 is required for a stable operation. What gain does this refer to? If= it is the noise gain, than a stable operation could never be possible with= out a signal applied because than the noise gain is 0. Furthermore, if I use no feedback cap, than the TIA is nearly oscillating f= rom the upper to the lower rail. With the feedback cap it is only oscillati= ng with an amplitude of around 70mV. These 70mV even stay the same if I red= uce the feedback resistor form 1,2kOhm to 330Ohm. When I press my finger onto the feedback network, the amplitude is only inc= reasing slightly. Maybe Phil can take away some of our confusion tomorrow;) Cheers, Julian Am Dienstag, 14. Mai 2013 10:25:29 UTC+2 schrieb Julian Arnold:> Hey Folks, >=20 >=20 >=20 > I need to design a broadband TIA for photo current amplification. >=20 > I am using a LMH6629. >=20 > I simulated the circuit in Tina-TI an it is working pretty well for a fee=dback resistor of 330Ohm and a feedback cap of 4pf and a signal of 10MHz.>=20 > However the simulation is showing oscillation if I increase the cap to so=mething above 20pf. In my opinion it should be stable for higher caps and o= nly start oscillating if the cap is to small.>=20 >=20 >=20 > However, I developed the board and now it is oscillating at 1.2 GHz with =a perfect sine, even without a signal applied!>=20 > This oscillation is nearly independent of the feedback cap. I tried form =1pf up to 10pf.>=20 > Only if the cap is missing totally (still around 2pf from resistor) the o=scillation is getting worth.>=20 > So, I think it has nothing to do with the feedback cap but some other iss=ue.>=20 >=20 >=20 > I really have no ideas left..=20 >=20 >=20 >=20 > Schematic: http://imageshack.us/a/img211/617/schematic.png >=20 > Board-top: http://imageshack.us/a/img585/4955/boardtop.png >=20 > Board-bottom: http://imageshack.us/a/img822/2739/boardbottom.png=20 >=20 >=20 >=20 > Cheers, >=20 > Julian Arnold
