Your feedback circuit doesn't make sense, although I don't think that is your problem.
I would suggest you look at the DC voltage levels before and after the laser is switched on. Look at both inputs and the opamp output. It should help to understand what is going wrong. I will check datasheets later.
Keith
I have had a chance to look at the data sheet and there may be other possibilities (I was thinking of a simple overload problem or connection problem, which may still be the case).
The opamp is way faster than I would have thought was necessary for 10MHz bandwidth and you need to be very careful in the layout and construction to ensure stability. Some 100pF capacitors for decoupling would be a good idea - normal 10nF capacitors will not be very effective at 1GHz. A 100MHz opamp would do the job. A FET input isn't required either - you have 1k or 2.2k to provide the bias current for a bipolar opamp.
Look at the output with the fastest oscilloscpe you have to make sure it is not oscillating.
In my simulations of your circuit with 1k it would be very peaky and hence potentially unstable. A transient analysis shows significant ringing.
Keith.
LvW said:Hi HoracioG,
I didn't check the data sheet in detail - however, I think the opamp chosen is NOT compensated. That means, the voltage gain must not be too small (probably larger than 10). In this context, I doubt if a feedback resistor of 1 k is appropriate.
What about trying a larger one?
LvW
dick_freebird said:Do you believe that op amp is suitable for driving a 50-ohm load
directly? If you simulated the lumped element load as a lossy
transmission line instead, would you get a more realistic simulation?
Hi!
I'm new in the forum. I was reading several post about Photodiode Amps. I'm working on a project trying to design on of them and I have a BW problem.
I'll appreciate if you can help me or give me some clue.
***
I'm designing a TIA to detect small amplitude sinusoidal signal up to 10MHz, this signal is mounted on a high intensity light source (laser, 3mW/632nm). So basically I have a modulated laser with low modulation index (1/500).
I've simulated and build the circuit below:
View attachment 49779
The transmittance obtained with TINA-TI (free software) app. was the following:
View attachment 49783
But when I've measured at the Lab, I found a different behavior. Take a look below:
View attachment 49780
View attachment 49782
As you can see, the behavior of the device is different from the simulated one. However if an optical attenuation is implemented, it seems works correctly. This is not due the photodiode, I checked it alone (without amplification) and worked in an acceptable way up to 10 MHz (it is specified for 7ns).
By the way, the set up is the following:
View attachment 49781
Do you know which could be the problem?
I'm suspecting the opamp goes off ideal condition due the high photocurrent (800uA max, measured with 50ohm Broadband load) but I'm not sure.
Please let me know if you need more detail.
Thanks in advance!
Horacio
ps: I cannot post link yet, so I attached the images. Sorry.
View attachment 49779View attachment 49780View attachment 49781View attachment 49782View attachment 49783
hi
put ur design and i'll solve ur poblem
yes i know but i want him to post the modelling coz i want to run it firts, coz really i am so busy to moddel it again
I mention overload problems because your performance is not consistent with amplitude. It could be just a slew rate limitation but you really ought to check the DC levels to make sure the circuit is operating correctly - it seems a logical first step in checking a circuit.
Please correct me if I'm wrong:Connection problems? Yes, PCB issues. The circuit doesn't work as expected to a connection error or bad connection is a possibility.
I don't see why you need 1.6GHz amp to recover a "small" 10MHz signal. A 1GHz amplifier won't make the 10MHz signal any larger than a 100MHz amp.
Why do you need low input bias current? You have 1k or 2.2k feedback so you could tolerate 500nA or 1uA of bias current.
Low noise? FET opamps don't actually have very low noise voltage. Where they excel is low current noise. However, with 1k transimpedance that is not likely to be a huge issue. The OPA657 has 4.8nV/rt(Hz) but there are plenty of opamps down to 1nV/rt(Hz). You do need to watch out for the current noise contribution and input bias current though.
I use SIMetrix for simulation and yes, I used the latest model from TI - revision E, which they cleverly put in a zip file called revision C.
You say you have applied RF PCB design concepts but only include one decoupling capacitor. I use between 2 and 4 depending on the circuit. You can buy "microwave" 100nF capacitors which work well up to GHz but they are not cheap, nor easy to get hold of so I still use multiple values starting at 47pF or 100pF. I have ample measured evidence to show that they ARE necessary.
I have had my fair share of oscillating amplifiers and without test equipment to see it, it can be difficult to diagnose. A couple of clues are: strange and unexplained DC operating points (they are not really DC - you just cannot see the oscillations) and performance changes with amplitude. Changes in performance when you probe the circuit can also be a clue, although that can often happen with a circuit that works properly.
I have had spurious oscillations up to several GHz. That shouldn't be the case with the OPA657, but it could still be >100MHz. If you have access to a spectrum analyser, that may help.
Keith.
hi
i installed the software and i told u i have no time to rebuild the circuit so plz if u r free build it and attach it. tnx and sorry
Just a short comment on the original circuit. I fear, it's a stupid idea to use a decompensated OP with a capacitive voltage divider instead of universal compensated type with a single integration capacitor. I'm rather sure, that it doesn't bring any benefit, but a lot of parasitic circuit effects. As the datasheet suggests, the OPA657 can be useful for a highspeed TIA, if the source (photodiode) capacitance already allows a gain > 10.
One other comment - a TIA essentially runs with the opamp in unit gain mode. This assumes everything else is perfect (no photodiode capacitance, no input capacitance, no leakage resistance) but it means that a 10MHz opamp in a perfect system would have a 10MHz bandwidth in a TIA. I have attached an example.
Also, here is the SIMetrix file with the OPA657. I have imported the model and corrected the pin order. You can see the model if you hit F11 when looking at the schematic in SIMetrix.
Keith.
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