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THS3202: 40-dB Cascaded Design, Second Stage Oscillation

Arthur Chang
Prodigy 30 points
Part Number: THS3202
Other Parts Discussed in Thread: OPA2694

We are cascading two identical THS3202 amplifiers (on the same chip) to achieve a 40-dB gain. This setup is shown in Fig. 3 below, with 200-Ohm R_f and 10.5-Ohm R_g.

www.ti.com/.../slyt102.pdf

When we probe the output of the first amplifier, we see the expected signal with ~20mVpp at 50MHz. This is then fed into the (+) input on the second amplifier. While the (-) input of the second amplifier also show the same waveform (suggesting that feedback is working correctly), the output of the second amplifier oscillates wildly to the full rail (+/- 5V) at >100MHz. 

Any idea what we can do to solve this issue? 

  • 0
    TI__Genius 12631 points
    Hello Arthur,
    The THS3202 does have some known issues with very high frequency response peaking which may lead to oscillations. The part is Not Recommended for New Designs (NRND). What you are describing however is quite strange. Are you using the 50 ohm resistors between the 1st and 2nd channel?
    Is there a 50-ohm at the output of the 2nd channel? and are you probing with a scope on the right hand side of the 50-ohm resistor? The reason I ask is that the probe capacitance will load the amplifier down and can cause it to oscillate. Probing on the right hand side of the resistor will provide the necessary oscillation from the capacitance.

    -Samir
  • 0 in reply to Samir Cherian
    Prodigy 30 points
    Hi Samir,

    Thank you for your response. We've tracked down the oscillation to the load capacitance -- though it's quite interesting that the 2nd stage is much more sensitive to load capacitance than the first.

    Since the THS3202 is NRND, do you have any recommendation for a replacement part (preferably a pin/package compatible replacement). We'd like to have a gain of >10 per stage, with a bandwidth of >100MHz.

    Thanks,
    Arthur
  • 0 in reply to Arthur Chang
    TI__Genius 15975 points

    Hi Arthur,

    The closest pin/package compatible replacement for what you are trying to do is the OPA2694. I think this device can achieve a gain of 10 per stage with bandwidth > 100MHz.

    Best Regards,

    Rohit