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THS3201EVM: THS3201DGN G=20 resistor combination recommend (manual don't have)

Xiansong Meng
Prodigy 10 points
Part Number: THS3201EVM
Other Parts Discussed in Thread: THS3491

Hi, Sir,

     I need to amplify a small signal ( 0.7V, 1GHz, PRBS signal, output from FPGA board) to a large signal, about 7V Vp-p, Based on surveys, I selected THS3201DGN as the amplifier. Because its supply voltage is 7.5V, our system is a 50ohm system(including FPGA and measurement tool, all is 50ohm). so when we use recommended Gain=10 resistor combination. We only get 3.3V output, because the 50ohm system will only share half output voltage with the load resistor(50ohm). So we need  Gain=20 and supply voltage >14V. The manual has not given Gain=20 recommend resistor combination. Does anyone know it? and if I could use power supply 14V?  When I try to use RG=50ohm, RF=1K ohm. the signal will have distortion.

The input signal looks like below.

The manual has recommended a resistor combination.

  • 0
    TI__Mastermind 18406 points

    Hi Xiansong,

    For a current feedback amplifier, you should keep the Rf relatively similar for higher gain. Therefore, I suggest that you try Rg= 25Ω and Rf= 500Ω. Higher resistors will make the amplifier more stable but lower bandwidth, and vice versa.

    This device is not recommended for new designs. I would recommend the THS3491 (https://www.ti.com/lit/ds/symlink/ths3491.pdf) instead, which has 20V/V well characterized in the datasheet.

    Also, if you are trying to buffer for a digital signal, you might want to consider a comparator instead. It will output a square wave with 90° corners.

    Best regards,

    Sean

  • 0 in reply to Sean Cashin
    Prodigy 10 points

    Hi, Sean,

        

            Thank you for your advice, since I have no resistor of 25ohm and 500ohm. So I tried Gain ~= 20 : Rg= 20Ω and Rf= 430Ω, and Vs+ is 15V, Vs- is near -7V. the signal looks like below(20dbm attenuation to oscilloscope ): 

    the signal looks not well. Do you know the reason? 

    Before, I used the manual recommended resistor combination, Gain = 10 : Rg= 54.9Ω and Rf= 487Ω,  Vs+ is 9V, Vs- is near -5V , The waveform is very good, see below. (When Vs+ is 7.5V, Vs- is -7.5V, the waveform is bad),

    2. I will buy THS3491 and try it. But the speed is 320MHz when Vpp = 10V, is that a problem?

    3. Sorry, I have not used comparator, maybe I will try it later. But I don't know if the output voltage could be larger than 10V? and the speed > 1GHz.

     

    Best regards,

    Xiansong

  • 0 in reply to Xiansong Meng
    TI__Mastermind 18406 points

    Maybe the gain and bandwidth is not sufficient for this signal and 20V/V. You might need to experiment to find the most gain you can use without distorting the signal.

    Keep Rf=487Ω, and slowly decrease Rg down from 54.9Ω. The gain will get higher and the bandwidth will get lower until you reach an optimized trade off for this amplifier.