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THS3062 Bandwidth at 1MHz begins amplifying signal

Nathan Pham
Prodigy 10 points

Other Parts Discussed in Thread: THS3062

Hi,

I am doing a class project and I'm having trouble along with a partner of mine testing the THS3062 Op-amp. We were testing the Op-amp with a direct line 2Vp-p AC signal into the positive input into the Op-amp with rails of +/- 10V rails, this is with no termination resistor on the input to the Op-amp but a direct signal. Secondly, I follow the datasheet in terms of RL=150 Ohms and Rf = 750 Ohms, and not including the RG resistor and ground. Essentially, the Op-amp is connected like a negative feedback circuit and this is for the purpose of achieving unity gain. At 1MHz, the signal on the output begins increasing in voltage which isn't supposed to happen according to the datasheet. I hope somebody has insight into this or clear up any misunderstanding I have on this.

Thanks,

Nate Pham

Portland State University Student

  • 0
    TI__Genius 13935 points

    Hi Nate,


    What kind of signal source do you have?  Without an input termination the amplifier input impedance is very high.  If the signal source is expecting 50 Ohms this can cause unusual behavior. 


    Please describe how you connect the signal source and the amplifier.  There is always something connecting the signal source and the amplifier.  If it is very short (<1") then you can neglect it.  If it is long (>12") then you'll need to at least know the parasitic capacitance and inductance.  It is possible that the connection between the amp and the signal source is causing instability.

    Also, unless you have a very clean board layout, your load may not be purely resistive.  Can you describe how you have connected the load to the amplifier?  How far apart are they?  If you are using something like a breadboard this can definitely cause issues. 

    Regards,

    Loren

  • 0
    TI__Genius 12631 points
    Hello Nate,
    Do the oscillations happen even without the ac signal being applied to the amplifier input? If you do check for oscillations without an input ac signal please ensure that the noninverting pin of the amplifier has a low impedance path to GND.

    Have you designed a PCB for this setup or are you breadboarding this? Can you please send me a photograph of your board? Also, please send me a scope of the oscillations.

    How are you checking for oscillations? Are you putting a scope probe directly on the output of the amplifier? If so then the capacitive load from the probe ( 10pF to 100pF) can cause the amplifier to become unstable. If you put a series resistor (even 100 ohm is enough) between the amplifier output and the scope probe, you will be isolating the amplifier from this load.

    Other things to check:
    1. Do you have sufficient power supply bypassing capacitors and have you followed the PCB layout techniques recommended on Page 14 of the datasheet.
    2. Finally you can also try to put a 50 ohm termination to GND on the non-inverting pin and see if that makes the oscillations go away. Just make sure that the signal source you use to drive the amplifier has the ability to drive the load. Most function generators have a 50Ohm output mode for this purpose.

    -Samir