using AD8067 as inverting amplifier

Hi,

I tried to work with AD8067 operational amplifier as inverting amplifier. My input resistance is 12k ohm and feedback resistance is 10k ohm. Now gain should be less than one. I tried to simulate it in LTspice with spice model of AD8067 from analog device website. I got very weird result. The frequency increased and the output is like a beam. I am attaching the circuit and output curve here. Can anyone please help me to solve this problem?

Thank you.

Niloy

attachments.zip
  • Hi Niloy,

    As i read through the datasheet (page 13), I found out the error in your circuit is the value for feedback resistor. 1kohm is recommended as feedback rather than 10kohm.

    In your case, you can change your RF value to 1kohm and your RG to 1.2105kohm (1.21kohm as nearest standard value). I already tried it and its working fine.

    I hope this helps.

    Best Regards,

    Jino

  • Hi Niloy,

    If I may add on my previous response, the AD8067 is stable at gains > 8. As for your application, i think AD8067 is not the best part for you.

    Can you share your schematic of your application so that I can assist you better? What are your input requirements if I may know? As what i understand, you want a gain of -0.826? At to what level do you expect your current output will be?

    Best Regards,

    Jino

  • Hi Jino,

    Thanks for your reply. I want to connect AD8067 with a sensor that has a resistance value around 12k ohm or even higher. Now input resistance of that high and feedback resistance 1k ohm, I still get very high oscillation. I am attaching the output waveform here. I need my current in input side to increase by 10,000 times at output voltage signal. It looks like AD8067 does not allow me to do that. Can you suggest me one fast amplifier with high gain bandwidth (my signal freq is around 500k Hz) which meets my specification?

    Thanks.

    Niloy.

  • Hi Jino,

    I am designing a biosensor which detects particle by sensing change in resistance thus change in input current. My normal sensor resistance is 242100/n ohm where n ranges from 1 to 20. Resistance change due to passing particle is in the range 1 to 300 ohm. This change in resistance will be superimposed on to my original sensor resistance. Small change in resistor causes small change in current. My feedback resistance needs to be very high so that I can get a reasonable amplification at the output and detect the change. Also amplifier noise needs to be very low. I hope it answers your question.

    What is your suggestion?

    Thanks.

    Niloy.

  • Hi Niloy,

    Sorry for the delayed response.

    With your noise gain less than 1.826 and a signal of 500 kHz, that makes a GBP of 913 kHz and ADA4627-1 (GBP = 19 MHz) can be appropriate for your application. It is a wide bandwidth precision amplifier that features low noise and very low offset. It's typical offset voltage is only 70 uV (130 uV lower than previous part) and a noise of 0.86 uVp-p. Unlike AD8067, it does not have recommended values for RF so you can play with values you want and get the same performance offered by the device. Just take note that ADA4627-1 is not Rail-to-Rail thus care is required to ensure inputs do not exceed the IVR.


    I hope this helps.

    Best Regards,

    Jino