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LMH6702: LMH6702 oscillation problem

Part Number: LMH6702

Hi E2E,

1 The function of the circuit is to amplify the input pulse voltage signal. Each level's magnification is 5.5. The circuit schematic diagram and PCB  is shown below

2 Problems in debug.

Test the first stage amplify circuit. Move the C11 to cut the next circuit.

  1. When the input signal frequency is 50MHz and the amplitude varies from 100mV to 1V, the output magnification of N1's pin6 more than 10. The amplitude of the input signal is 100mV. The matching impedance of the oscilloscope is 1 Mega.
  2. when the amplitude of input signal is 100mV and the frequency varies from 1MHz to 100MHz, the magnification is more larger than the design value at 10MHz,20MHz,40MHz,60MHz. The output voltage is over 10V (The matching impedance of the oscilloscope is 1M Ohm).
  3. Set the matching impedance at 75 Ohm.The chip does not oscillate and the magnification is normal. The output signal waveform is shown as below and the input signal is 100mV.
  4. Short circuit the two ends of C7.Input signal 25MHz, 10mV to 100 mV.There will be zero drift of op amp. The blue waveform below is the output signal waveform of the input signal amplitude 50mV.
  5. Change R2 resistance from 51 Ohm to 510 Ohm. If 1 M Ohm is used as oscilloscope matching impedance, the chip will still vibrate. If 75 Ohmis used as an oscilloscope to match impedance, the output signal is normal.
  6. Increase the load resistance at the output of the first stage with a resistance of 102 Ohm. If 1 M Ohm is used as the matching impedance of the oscilloscope, the chip will vibrate.
  7. The front stage amplifier (negative input) is cascaded to the back stage amplifier (above schematic) input. Whether the positive input or the negative input, the oscilloscope matching resistance is set to 75 Ohm or 50 Ohm, the chip will vibrate.

Please help analyze how LMH6702 do not oscillate, and how can I control magnification within the normal range, thank you.

BRs,

Kun