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VCA821: VCA821 Rg and Rf VS gain

Lin_Charles
Expert 4260 points
Part Number: VCA821
Other Parts Discussed in Thread: THS3491, VCA820

Hi team,

   My customer want to use this device as below, Vs=+/-5V, Vppmax=2V, frequency=30MHz, AVmax=40dB, we use the datasheet's equation, 

Rg>385 ohm, Rf<19.2k, so we use these two value in this application. But when I use Vg=2V, Vpp=1mV, sine frequency=30MHz, I cannot get waveform in the output of the amp? Could you please share the reason?

  • Can we use this device as above application?
  • How to get the relationship between Vg and gain? When max gain is 40dB, I believe when Vg=2V, gain=40dB, when Vg=0V, gain=0dB. Right?

  • 0
    TI__Expert 8720 points

    Hi Charles, 

    The VCA821 uses a current feedback style architecture, which means that the bandwidth is directly inversely related to the value of the feedback resistor. With a ~20k Ohm resistor, the circuit is likely reducing the bandwidth well below the required speed. I suggest for 40 dB of gain that you follow the suggested values of Rf = 402 and Rg = 18 in table 2 of the datasheet. 

    Regards, 

  • 0 in reply to Jacob Freet
    TI__Expert 4260 points

    Hi Jacob,

      If you choose Rf = 402 and Rg = 18, the max gain will be 40, not 40dB. Please help suggest the right Rg and Rf in this application, or if you think this device cannot support this case, please help find the root cause?

      My customer's main requirement are below, max sine frequency are 30MHz, max input Vpp=2V, and max gain is 100. Please help re-consider this device again, Thanks.

  • 0 in reply to Lin_Charles
    TI__Expert 4260 points

    Hi Jacob,

       Any feedback about this issue? Thanks.

  • 0 in reply to Lin_Charles
    Guru 48395 points

    you can't do it, 

    The maximum current in the Rg resistor is 2.6mA, at +/-1V input that requires a 384ohm Rg externally, then to get your 40dB gain that pushes the Rf value so high it will bandlimit the CFA output stage. 

    At one time I had sketched out a version of this product using a decomp VFA output stage that would have fixed this issue, never went anywhere

  • 0 in reply to Michael Steffes
    TI__Expert 4260 points

    Hi Michael,

        Thank you for your response here. If VCA821 can not support this case. Do you have any recommend solution can support customer's requirements?

    Max sine frequency are 30MHz, max input Vpp=2V, and max gain is 100. Thanks.

  • 0 in reply to Lin_Charles
    TI__Mastermind 23226 points

    Hi Lin,

    Do you need a programmable gain amplifier? A THS3491 will fit your voltage range and bandwidth requirements if this is a fixed gain application.

    Best regards,

    Sean

  • 0 in reply to Sean Cashin
    TI__Expert 4260 points

    Hi Sean,

       We need programmable gain amplifier here. But we only need to have 1/10/100 gain in this application. When input signal frequency is 30MHz, we need gain=100. Do you have any idea about how to realize this application? Thanks.

  • 0 in reply to Lin_Charles
    TI__Expert 8720 points

    Hi Charles, 

    You may be able to achieve this function using the THS3491 if you switch in two different gain resistors to get the different gains, but you would likely get some gain error from the switches. Otherwise a two stage design would be able to easily reach the required gain ranges. 

    Regards, 

  • 0 in reply to Jacob Freet
    TI__Expert 4260 points

    Hi Jacob,

       Thank you for your response here. I have tried VCA821/VCA820, these two device can not realize the function I mentioned here. Does TI do not have such VCA to realize below requirement?

    • Vs=+/-5V, Vppmax=2V, frequency=30MHz, AVmax=40dB;
    • When frequency is 30MHz, Av=100, the input signal should be about 1mV;
    • Av=1/10/100 is enough for this application;

    Thanks.

  • 0 in reply to Lin_Charles
    TI__Expert 8720 points

    Hi, 

    Since this discussion has been taken to email, I will close the thread. 

    Regards,