Using AD8338 with AGC

Hello!

I'm using two AD8338 in series . I want to control second AD8338's gain using first AD8338's AGC block. For ex: If first AD8338's output is less or greater than the desired voltage , Then second AD8338 minimize the difference between these voltages, not first AD8338. I tried to activate only first AD8338's AGC feature and to tie first AD8338's deto pin to second AD8338's gain pin. But it doesnt work . How can i realize it? 

Also I tried to test one AD8338 without AGC. When Gain is greater than 50dB , oscillation occured. What should i do to obtain 50-80dB gain without oscillation?

Thanks

  • 0
    •  Analog Employees 
    on Feb 1, 2021 2:16 AM 25 days ago

    Hi Yaser,

    Good day! Thank you for considering AD8338 on your application.
    Can you show us the schematic on how you configure the cascaded AD8338 this will enable us to help you efficiently?
    Also, may we know your intended application for using this scheme for the AD8338?

    For your second inquiry in order to achieve a gain of 50dB and above, you need to set the Vagc to 0.72V (for 50dB) up to 1.1V(for 80dB) the mode pin is tied to VBAT. But I would think this is already what you're doing if you only see the oscillations at 50dB above. Can you show us a scope shot of this behavior and possibly the schematic as well.
    We have an evaluation board that you can use or can be a reference of your circuit https://www.analog.com/media/en/technical-documentation/user-guides/UG-559.pdf, the document contains the right configuration for normal and AGC configuration of the device.

    Hope this helps Yaser.

    Thank you and Best Regards,
    Dann