Driving AD80066

I'm using AD80066 to digitize four channels at 4MHz each (so ADCCLK is running at 16MHz).  I'm finding that the buffer amplifier I'm using to drive the ADC is oscillating.  I've tried AD8061, ADA4897-1 and AD8065; they all seem to oscillate at around 100MHz.

The circuit has a 100R resistor between the sensor and the +ve input of the amp.  The amp is configured as a unity-gain buffer.  There's a 51R resistor in series with the output (though about 3 inches from it), then the ADC.
I've looked at various material describing the techniques that are generally employed in PCB layout to prevent stray capacitance (and consequent oscillation).  Unfortunately, one of our constraints is that we don't want significant areas of the board to have both planes missing because of stray light leakage.  And we already have a PCB layout... 

Can you suggest alternative amplifiers that are suitable for driving the ADC and that will be less susceptible to oscillation, or ways to stop the oscillation (preferably without significant changes to the PCB).

Thanks!

  • A bit quick on the 'Send' button there.

    When I mentioned 'both planes missing' I was referring to power and ground planes, which are currently continuous under the buffer amplifiers.  We don't want large gaps in both planes anywhere, because then light can shine through the PCB - we don't want stray light.

    And I should have mentioned that we have the possibility to slow the whole circuit even more, by a factor of about 4 (so the AD80066's ADCCLK would be 1MHz, and the individual channels would be running at 250kHz).

  • 0
    •  Analog Employees 
    on Jul 13, 2017 5:00 PM over 3 years ago

    Hi pixelPhile,

    Could you attach the scope shots of the oscillations? A schematic would help as well. What is your intended application?

    Regards.

  • I hope these are visible.

    The application involves a CMOS linear sensor (left hand side of the schematic).  The AD80066 is used in SHA mode.

    Thanks for your interest,

    Phil

  • 0
    •  Analog Employees 
    on Jul 18, 2017 6:36 PM over 3 years ago

    Hi Phil,

    Apologies for the late response. What we could do is help you troubleshoot the circuitry.

    First, disconnect the amplifier and the AD80066 by removing R1. Check for the amplifier output to see if the oscillation is caused by the AD80066 or the component on the left side. If the oscillation still persists, please install a Low Pass RC filter after the Video DAC. Make the amplifier for gain of 2 and add a voltage divider network at the output to bring it back to a gain of 1. A 100R combination would do it. Also, could you further expand the schematic? I want to see more of the left side of the circuit. Thank you!

    Regards,

    jino

  • Hi Jino, and thanks again.

    These are very worthwhile suggestions and I'll carry them out as soon as I can (will be a few days at least).

    I cropped the schematic because I didn't want to publish details of what we're doing.  Can I share this with you privately?

    Phil