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LMH6504

Michael Handy
Prodigy 10 points
Other Parts Discussed in Thread: LMH6504, TINA-TI, LMH6505

 

To Whom It Concerns,

The LMH6504 output oscillated when the input is near threshold. I built a bread board with the LMH6504device and the supply rails are +vc at +5 and -vc at -5v and decoupled with 25uF in parallel with 0.1uF on both rails. Rg is 100 Ohms and Rf is 10K Ohms with an input signal of  +/- 10 mV. I have utilized the TINA simulator with good results. Could someone please give me some ideas as to what could cause this oscillation?

Regards, Mike Handy

 

  • 0
    TI__Mastermind 22825 points
    Hello Michael,
    Is there anyway you can supply the entire schematic (TINA-TI *.tsc file works)?

    Regards,
    Hooman
  • 0 in reply to Hooman Hashemi
    TI__Mastermind 22825 points

    Hi Michael,

    While I'm waiting for your more complete schematic, here are some comments:

    1. LMH6505: The LMH6504 is no longer available (NRND) and the LMH6505 replaces it (fully compatible).

    2. EVAL board: The LMH6504 (and the LMH6505) have an unpopulated EVAL board that you can order and use, in order to make sure your board is not an issue:

    http://www.ti.com/tool/lmh730066

    3. RF Value / Type: Most of the LMH6505 evaluation is done with RF= 1kohm. The output amplifier is of the current feedback type which is sensitive to the feedback resistor value. Usually a larger feedback resistor (like 10k in your case) only slows down the part. But, a larger feedback resistor could have the undesirable consequence of adding additional phase shift that coulld lead to instability. So, I'd recommend initial testing using the datasheet conditions (RF=1k, RG= 100ohm, RL= 100ohm). I'd even use a surface mount type resistor for RF as the output amplifier inverting input (pin 7) is very sensitive to parasitics.

    4. VG pin Decoupling: Make sure the gain control input (VG) is decoupled very close to the pin (Pin 1) and ground.  A series R and shunt cap close by might be even better (remember that the VG pin has in excess of 100MHz of bandwidth and any modulation of it, will show up at the output).

    5. Output load / probing: For initial testing, add a series 50ohm to the output and plug that into a 50ohm scope input. so that you don't need to worry about the scope probe capacitance on the output pin.

    6. Input Considerations: Make sure the incoming signal at the non-inverting input (pin 2) is not driven from a long cable / coax without close by termination / decoupling. A 50ohm shunt to ground near pin 2 would do the trick with minimal trace hanging off the pin.

    7. Instability Sensitivity Probing: If none of these solve your issue, you could maybe use the scope probe tip on the device pins while monitoring the output instability and let me know which pin, if any, shows sensitivity to the probe tip / capacitance. I'm looking for a clue as to where to look for the source of the issue.

    Regards,

    Hooman