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LMH6881: LMH6881

Arvind Patel
Prodigy 10 points
Part Number: LMH6881

We have cascaded 2 stages of LMH6881. Both Stages have supplies and all control lines with series ferrite beads to provide good isolation. When Gain is increased above 10dB we observe oscillation. 

50 Ohms Series resistor is used on Out+ & Out-  connecting to 2nd LMH6881 Input stage. The 2nd stage has 100 Ohms series resistor  connecting to 50 Ohms load (we need 1.2v DC at load)

All stage are DC coupled.

Any suggestions on what can be causing the oscillations. PCB is 8 layers and sufficient isolation is provided between stages. Layer 2 & 7 are GND planes.

  • 0
    Prodigy 10 points

    Does the VGA need a minimum/max load resistor? 

  • 0
    TI__Genius 17646 points

    Hi Arvind,

    Can you try taking out the capacitor loads? Sometimes that causes amplifiers to oscillate. Also, mybe you remove the series resistors and try loading the outputs of the LMH6881 with a differential load, which can also help stabilize an amplifier. Can you also show us some output waveforms? Is it a sustained oscillation or is it just gain peaking and instability?

    Best regards,

    Sean

  • 0 in reply to Sean Cashin
    Prodigy 10 points

    We have removed the 1pF caps at output of both VGA but it still gives oscillations. Initially we had 1 Ohm series resistor connecting VGA Out to next stage VGA Input. This had oscillations when gain of VGA1 was increased from 6dB to 10dB. After changing to 100 Ohms series resistor, the oscillations are happening at gain >14dB @ VGA1 Output pins. We observe oscillation at VGA 1 output pins but not at VGA 2 Input pin. It seems the series resistor seems to help tame the oscillations to some extent.

    Below is waveform of the type of oscillations that shows up when Gain is increased.Approx 100MHz.

    DC at both  + & - out is approx 2.5v.

    Do any of the LMH6881 pins need special treatment? Is this a case of VGA2 output feeding back into VGA1 input? currently both VGA1 & VGA2 are under same shield can. Should we split them into different shield cans?

  • 0 in reply to Arvind Patel
    TI__Genius 17646 points

    Hi Arvind,

    Can you keep increasing the series resistor >100Ω to stabilize the amplifier further? Can we stretch this solution to stabilize the highest gain required by your application? 

    As far as special treatment of pins, can you probe the power supply pins of the amplifier? Is there possibly a resonant frequency in your supply decoupling scheme that is activated by a higher output voltage amplitude? If so, we can damp a supply oscillation by adding a snubber to the supply rail.

    I don't think a parasitic capacitive coupling would create nearly enough feedback for oscillation of this magnitude.

    Best regards,

    Sean

  • 0 in reply to Arvind Patel
    TI__Expert 3620 points

    Hi Arvind,

    Can you share some images of your layout? I am particularly interested in the supply bypassing, input and output signal traces, and grounding.

    Best,

    Sam