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THS3491RGTEVM: Output Matching Resistor

Renan Santos Adriano
Mastermind 24815 points

Part Number: THS3491RGTEVM
Other Parts Discussed in Thread: THS3491

Hello Experts,

According to datasheet on page 4 (2.2) :

 Desoldering the output matching resistor and replacing it with a single 50-Ω series output resistor that gives only 6-dB output voltage loss at the measuring instrument is easily done.

I dont find which one is the does output matching resistor means wheater it is 54.9Ohm or 54.9 + 287 + 287 Ohm or just 54.9 + 287 Ohm, and which one should be replaced for having bit more gain? 

Additional to above what will it happend if I will remove 287 + 54.9Ohm and replace 0 Ohm instead of removed 287 Ohm, will this amplified by 6-dB voltage loss or what can you please clearify please.

Regards,

Madan

  • 0
    TI__Mastermind 19320 points

    Hi Madan,

    If the goal is to setup a 6-dB drop, you could remove R9, R10, and R13 and leave them floating. You could then replace R11 with a 0ohm resistor and solder R12 with a 50ohm resistor. The values I am referencing are from the schematic on page 9. This will leave a 50ohm series resistor that would attenuate by 6dB if you are driving into a 50ohm load like Figure 2 shows.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 30 points

    Hi Thanks for the details, 

    The main consideration is the amplifier’s stability in the presence of signal reflections. With a 0 Ω source impedance feeding a 50 Ω transmission line that’s left open at the end, the source voltage could nearly double, causing ringing. For our setup, this isn’t a major issue since we always calibrate the actual signal at the sample. The input voltage will not exceed 1 V. As a reasonable compromise, introducing a small resistor (about 2–5 Ω) could help reduce ringing.

    Would it therefore be acceptable to replace R11 with 0 Ω and R12 with approximately 5 Ω to achieve a gain of about 5 V/V from the amplifier, and would this configuration be suitable for stable operation?

    Thanks 

    Regards

    Madan

  • 0 in reply to Madan Bhandari
    TI__Mastermind 19320 points

    Hi Madan,

    If the load side of your circuit is going to be open, we would recommend using a 50ohm series resistor instead of 5ohms. The reason is that there is still some level of matching with the path any reflections would take so having a 50ohm series resistor would be better than 5ohms. The concern wouldn't be so much of a stability issue from the device but to ensure you limit the reflections going back and forth, therefore having 50ohms would be better.

    Best regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 30 points

    Hi Ignacio,

    Thank you for your response — it’s been very helpful. I noticed that R9 and R10 are quite large in size; are they simply soldered or also glued in place? I’m wondering if there’s a recommended method for removing them safely.

    Additionally, what would be the impact if we were to leave R9 and R10 as they are, replace R11 with 0 Ω, R12 with 50 Ω, and remove R13?

    Thank you so much for your time and information. 

  • 0 in reply to Madan Bhandari
    TI__Mastermind 19320 points

    Hi Madan,

    These resistors are soldered on. They are difficult to remove, and I have had similar issues removing these. The easiest way to remove these is with a heat gun. I have not had much success simply using a soldering iron. Leaving them on and setting the other resistors how you suggested will work and should not be a concern. These are used to adjust the load seen by the THS3491, which in your configuration would still be ~100ohms so you should be ok.

    Best Regards,

    Ignacio