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OPA858: OPA858 Evaluation Board - gain issue

richard wiskin
Prodigy 60 points
Part Number: OPA858

Tool/software:

Apologies if I am being a bit stupid here. I have bought the OPA858 eval module to use with some high frequency projects I am working on. It is set by default to 7x gain with Rf being 453R and Rg being 75R.

I am needing more gain and have therefore replaced Rf with 10K aiming for a gain 134. I actually get 13.4 x gain and I have measured Rf and it is as expected. Am I missing something obvious. Many thanks in advance of some suggestions. BTW my input signal is around 500Khz currently for testing.

  • 0
    TI__Genius 16615 points

    Hi Richard,

    We can definitely assist you with figuring out what is happening with your circuit. There are a few things we can look into to see what it could be. By any chance is the scope you are using set on some kind attenuation setting? We often see these types of things especially when we see your measured signal is exactly 10x less than the expected gain. Just to highlight as well, if the EVM was not altered (besides the Rf resistor), there is around ~6.8V/V of attenuation due to the load configuration, which also needs to be taken into account. Although this does not seem to be the source of this error.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 60 points

    Hi Ignacio

    The probes are set to 0x, indeed the input signal was tested directly to the scope from the Sig Gen before connecting the opamp board. I am a bit perplexed as to why the 0402 resistors were used for Rf. Very unkind!. Anyway I managed to swap the resistor from 453R to 10K and the result was not as expected. Perhaps coincidence that it is close to 1/10th the expected gain.

  • 0 in reply to richard wiskin
    TI__Genius 16615 points

    Hi Richard,

    I agree the size of the resistors makes things difficult, however, the reason we used relatively small resistors is to limit any parasitic capacitance as this is a very fast device, any unwanted capacitance can lead to stability issues. There should not a bandwidth concern at this frequency. I was wondering if you could maybe adjust the Rf and Rg resistors to a known gain and resistor network that works as a way to rule out any setup or circuit concerns. We highlight the device in a gain of 20V/V using an Rf value of 453Ohms and an Rg value of ~23.8Ohms. It would be good to confirm we are seeing this result in this choice of Rf/Rg. One potential concern that could be a problem, excluding a setup issue, is the large choice of feedback resistor at 10kOhms. Using such a large feedback resistor could potentially lead to stability issues with high-speed devices like the OPA858.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 60 points

    OK, I will try these default values and see if the predicted output is achieved. I had no idea you could go so low with Rg BTW.