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TIDA-060025: Follow up to lidar design question-comparator/preamplifier

Part Number: TIDA-060025
Other Parts Discussed in Thread: TINA-TI, OPA858, LM158, TLV3501, OPA836, LM7171, LM6171

Hello again,

My team is still working on this particular design and it has been suggested that we should add in an op amp amplifier to increase the overdrive voltage therefor decreasing the propagation delays and error in final measurement.  The issue that we are trying to get over is that our system specifications are to have a distance measurement accuracy plus or minus 5 cm.  Our understanding is that the propagation delay from the overdrive issue is not linear and we can't calibrate for that in the software later.  So adding in a pre-amplifier to increase the overdrive voltage will decrease error.  I'm trying to do simulations to model the error in TINA-TI and want to make sure I'm setting the circuit up correctly and interpreting the results right.  I made the following model based on suggestions from another engineer: 

Having some trouble trying to run the transient analysis, this is what I'm seeing:

I'm not really very familiar with SPICE and this is the first time I've ever used TINA-TI so I'm going through a learning curve here. 

Also not sure if the LM158 is the best op amp to use for this, maybe it would be more optimal to go with the OPA858 as used in the lidar reference design but not configured as a transimpedance amplifier?  It would be good if the op amp came in a packaging option that would be easy for prototyping on a breadboard. 

Thanks for any input you have!


  • Hello Sara!

      Would you be able to remind me what your max and min output signal? I believe I remember you were receiving a +25mV signal at your max distance. Is this still the case, and how about at the minimum distance? Would you be able to provide scope shots at these two distances? I noticed I wrote this in the post regarding the PDA100A2: It seems to have a gain bandwidth of around 600MHz and an output voltage of 0 to 10V (Hi-Z). Is this still the input to the preamplifier? I would suggest choosing an amplifier in the same range and bandwidth/slew rate as the transimpedance amplifier in the PDA100A2; so, the LM158 would not be a suitable amplifier for your application. 

      In regards to overdrive voltage, I pulled these figures from the TLV3501 datasheet:


     From these figures, it seems you should have atleast 40mV-50mV of overdrive voltage above the threshold you have set in the Tina-TI schematic of 25mV. After the confirmations of the answers above, I could help with choosing a suitable amplifier. Also, would you be able to provide your Tina-Ti schematic?

    Thank you!


  • Sima,

    the max output we saw from the detector was about 5V (I think, trying to confirm this with my team), the min was 25 mV in the previous tests.  Unfortunately I don't have oscilloscope shots, we need to get back to the lab for more testing soon.  The detector would directly input into the preamplifier. 

    I have tried to attach the schematic here but I get "unknown error" when I try to reply. 



  • Here is a simulation I tried to run with the OPA858 as the preamplifier before the comparator.  I know it's used in the original lidar reference design with the 3501 configured as a transimpedance there a reason this wouldn't work as the amplifier configured in normal noninverting fashion?

    Thanks again, sorry for the very basic questions, 


  • Hello Sara,

       After looking into the performance specs of the transimpedance amplifier you are using as well taking note of your max/min input signals, I have come up with a list of possible amplifiers: here. In regards to placing these parts on a breadboard, you would not be able to do this with any of the amplifiers from that link. There are two devices we have (High Speed Amplifiers) that would let you test it on a breadbaord (LM7171 and LM6171); however, it is highly recommended not to do so. Also, you would not be able to test the TLV3501 on a breadboard either. I would recommend purchasing a dem board (empty pcb board), and soldering on both these devices for prototyping. Here is one that would work for the OPA836 SOT-23 package: board.

        For the rough Tina-TI schematic I have attached below, I used the OPA836 as your preamplifier. I did not include the required decoupling capacitors that would be necessary for your application. For more information on decoupling capacitors here are some sources: link1 and link2, and there are very useful hints at the end of part's datasheet (for example OPA836's datasheet) in the power supply recommendations and layout guidelines as well. 

    Note: This might not be the most optimal design for your application, but hopefully it gives you a place to start. I would also run stability and noise simulations like how it's done in the reference design. Also, make sure having a propagation delay of 3-4ns rather than 7-10ns will have a significant effect on your application. If not, this preamplifier would not be very critical; it will only introduce more noise into your design due to the increase of components especially with a 25mV signal. If you can, you should also measure output noise of your transimpedance system to determine which stage will dominate in terms of noise.


    Thank you,


  • Just saw your additional replies; OPA858 would not work in this situation either. It is a decompensated amplfiier which means it is only stable at gains above 7 V/V. 

  • Thanks, very helpful, I'll start looking at the simulations with these other amplifiers! 


  • No problem! Feel free to let us know if you have any follow up questions/concerns.