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Maximum Load Capacitor that LMV116 can drive

xiongjie dong
Prodigy 20 points
Other Parts Discussed in Thread: LMV116, LMH6645

Hi, 

I am currently working on a LC Colpitts oscillator circuit and I am using LMV116 op amp for the oscillator.

The following is the schematics for the oscillator:

The inductance is 2.9uH, In order to oscillate at the frequency of 800KHz or a little higher, the capacitor should be nF range.

I placed the 1nF cap (C27 and C28), then the circuit started oscillating without the inductor attached to it. My guess is the LMV116 cannot drive 

the cap with 1nF. So I am wondering what the maximum load capacitance LMV116 can drive.

If LMV116 cant drive nF range capacitors, are there any op amps will be more suitable for this application or can drive large cap?

Thanks,

Jay Dong

  • 0
    TI__Guru 53966 points

    Hello Jay,

    The LMV116 is an enhanced slew rate version of the LMH6645. The high speed LMH devices are very sensitive to output capacitance (normally they drive back-terminated loads) and normally start oscillating with direct capacitive loads as low as 50pF.

    You have a direct capacitive load path to ground through C29 and C28 - this is probably what is causing U9 to go unstable.

    Try placing a 50 to 100 ohm resistor in series with the output (pin 1). This is usually enough to squelch the oscillations.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 20 points

    Hi, Paul

    Thanks for the help.

    I just tested it out again placing a resistor in series with the load capacitors and it squelch the oscillations. But it also squelch the LC oscillations that I want.

    Just try to find a way that it can only squelch the oscillations causing by the big capacitive load and leave the LC oscillations.

    Thanks,

    Jay