Using AD8055 as a non-inverting op-amp

Hi there!

I am using an AD8055 in a circuit. The IC is currently being used as an non-inverting op-amp as in figure 38, page 12, of  the AD8055 's datasheet. The input is from a tuned circuit (inductor and  capacitor in parallel) and is at 200kHz with the peak to peak voltage being  about 40mV. However, whenever the tuned circuit is connected to the op-amp, the  output of the op-amp quickly "saturates" (and oscillates between +/- 3V) with an  input voltage of only 40mV and a gain less than 10. Strangely enough, this  oscillation does not occur at low gains (about +3). However, I require the  op-amp to have a gain of about 100 eventually.

Could someone help me sort out this problem please?

Thanks for the help!

AD8055_8056.pdf
  • 0
    •  Analog Employees 
    on Mar 4, 2011 7:45 PM

    Moving this question to the Operational Amplifiers community.  Please continue the discussion here.

    Thanks,

    AndyR

    EngineerZone Community Manager

  • Can you post you actual schematics and indicate what the supply voltages are? What construction method are you using, a two layer PCB, veroboard or something in between? Can you tell if there is any HF oscillation (by which I mean at > 10 MHz) on the output?

  • Thanks for the reply OliverSedlacek!

    I have built the exact circuit in Figure 38 of the document attached to the first post (I was previously having problems with the op-amp in another configuration, so I decided to build the circuit supplied by AD to eliminate any additional problems I was causing). The input signal, now I am testing the op-amp, is taken from a signal generator.

    The power is taken from +5V and -5V supply rails and I am using breadboard for building the circuit.

    Unfortunately the oscilloscope I am using doesn't allow me to measure the output frequency (the time base does not have a large enough range), but the frequency is large (>1Mhz as a rough estimate, but it could be higher/lower - I will obtain a better measure for you asap).

    I will try to use a low frequency input, or maybe just a dc signal, and see how this makes a difference to the output.

    (I also modelled my circuit on: http://designtools.analog.com/dtAPETWeb/dtAPETMain.aspx#RunAnchor. The web model of the circuit works fine, but the actual chip does not)

    Thanks for offering help, once again!

  • Your problem may well be the breadboard. A high speed opamp like the AD8055 needs a carefully designed PCB to work properly. 0.1uH of stray inductance on a ground or supply connection will completely nobble your circuit.

  • 0
    •  Analog Employees 
    on Mar 7, 2011 6:46 PM

    Hello,

    One way to eliminate problems is to use our evaluation board for the AD8055, it can be found in the ordering guide on the AD8055 product page. I'm not sure it is layout related, since the circuit apparently works for lower gains. There's a few things to check,

    1. Exactly what kind of breadboard are we talking about, if its "dead bug"  breadboarding, then yes there could be a layout problem.
    2. Is the part bypassed?, What are the bypass cap values?
    3. Are you using SMT or axial leaded components?
    4. What are the values of the gain and feedback resistors?
    5. What is the load?
    6. Have you tried using a function generator to feed the input signal to the circuit instead of the tuned circuit?
    7. What is the oscilloscope probe capacitance you are using?

    Regards

    John