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LMH5401: Measured BW is lower than Tina Simulation

Part Number: LMH5401
Other Parts Discussed in Thread: TINA-TI

Hello,

I used LMH5410 in one of my designs, while the Tina simulation shows the BW of more than 4 GHz, the measured BW is very below 4GHz. I will be thankful if you could provide some feedback; in the measurement results, the feedback cap of 0.2pF has been de-soldered. 

  • Hello Nasim,

       Would you be able to share your layout and setup?

    Thanks,

    Sima

  • Hi Nasim,

    have you forgotten the 50R source resistance?

    nasim_lmh5401.TSC

    Kai

  • Hello Kai,

    I had added the 50R to the Voltage Generator; please see the below figure. Here are some points that I would like to mention about our circuit:

         !) the circuit is driving an ADC that has a 0V common-mode, because of that coupling caps have been added to the output

         2) the input should be AC coupled as well 

         3) the CM has been generated by a resistor divider from the VCC

         4) some parasitic effects have been added to the Tina model like C1/C2/C3/C$

         5) I added 10nH inductor as the parasitic effect of the coaxial cable

    I have attached the Tina TSC file of our circuit; the BW is around 2GHz. But as I mentioned before, the measured BW is very below this value. 

    I will be thankful if you could have a look at the file and let me know what might be the  root cause of the BW limitation.

    Sincerely,

    Nasim

    0728.Nasim_LMH5401_05_21_2021.TSC

  • Hello Sima, thanks for the message. Could you share your email address with me? This front-end circuit is a small portion of a very large design and I cannot share it in the forum.

  • Hi Nasim,

    4GHz amplification is an application where each single millimeter of layout copper traces counts. And it's very difficult to obtain proper measurement results. Are you using these expensive semi-rigid coax cables?

    As Sima already asked for, one would need to see the layout and a photo of your measuring setup to give further help.

    Kai

  • Hello Nasim,

       Yes I can, feel free to send the layout to my email address: s-jalaleddine@ti.com. Also, If you and Kai do not mind, you can share this type of information privately over E2E by requesting friendship through the connect button on the upper right hand corner of the profile. Kai is very experienced and provides a lot of great inputs. 

       Is 10nH an estimated value of the cable + trace length in the Tina-TI simulation you attached in the last reply? It looks like the bandwidth drops to less than 1GHz in that case:

          LMH5401_Nasim.tsc

    Thank you,

    Sima

  • Hi Sima,

    I assumed a parasitic 10nH at the input to show how much impact such an unwanted inductance will have in the GHz frequency range. 10nH equals about 10mm copper trace which can often be seen in suboptimal layouts. 

    Kai