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AD8332 X-Amp VGA Input Range

Category: Hardware
Product Number: AD8332

The AD833x devices are essentially configured as an LNA followed by a resistor ladder variable attenuator (with added circuitry to make a 48dB VGA), and lastly a post-amp. There is direct access to the connection between the LNA and the VGA at the pins of the device. One could choose to directly connect these pins, such that the input range of the LNA is defined as 500mVpp, which after the 19dB LNA gain will supply roughly 4.5Vpp at the VGA input.

My question is, if I add more gain between the LNA and VGA such that the input range is still limited to 500mVpp but I go over 4.5Vpp at the output of my external amplifier, can the resistor ladder in the VGA handle overvolting? For example, assume I add 15dB of gain between the LNA and the VGA, and I supply a 500mVpp signal at the input of the LNA. There should be 4.5Vpp at the LOP/LON pins on the AD833x, and the output of my 15dB intermediate amp will be 25Vpp (let's just assume the worst-case). If the attenuator is set to 48dB, the output of the VGA should be 0.1Vpp followed by 1.13Vpp after the post-amp. If the VGA is indeed as ideal as the simplified diagram, then I should only have to worry about power dissipation in the resistor ladder? I am wondering if there are voltage limits to the switches that adjust the resistor adder in 6dB steps, and also the interpolation circuits. I do not believe there is much info in the datasheet about these parameters. Does anyone know more about the limitations of the VGA?

Thanks. 

Top Replies

    •  Analog Employees 
    •  Super User 
    Jan 16, 2024 in reply to CSousa +1 verified

    Hi  

    Thank you for your patience.

    Additional amplifier can be inserted between the LNA and VGA as long as it will not exceed 5Vpk-pk differential input swing of the VGA. The AD8332 VGA is designed…

  • Hello  , 

    I'm looking into your setup and have a few questions. Could you explain the purpose of the additional 15dB gain in your application? Also, what's the input voltage range you're working with; is it consistently 500mVpp? Lastly, where is the input sourced from—is it another device?

    Thanks,

    Mae

  • Mae,

    Sorry, my original question may have been a little too hypothetical, let me get specific:

    The output of the AD8332 LNA is about 4.5Vpp differential maximum, meaning each output measured single-ended is 2.25Vpp max. We would like to know if the part can handle putting a fully-differential between the LNA and VGA so that we can get roughly 9Vpp differential, or 4.5Vpp SE per output. Something like the AD8350-15. Basically using more of the 5V power supply in the VGA.

    The reason for this is we may have very small signals input to the AD8332, and we would like to push the maximum gain beyond the roughly 49.5dB end-to-end gain of the AD8332.

  • Hello  ,

    I am still confirming the input voltage range of the VGA so that we can make sure that the input voltage coming from the 15dB-output will not damage the unit especially that the expected input will be 4.5VVpp SE where the supply +/-5. You will be using a dual supply, right?

  • The design is all single supply, designed around a common-mode voltage of 2.5V

  • Hi  

    Thank you for your patience.

    Additional amplifier can be inserted between the LNA and VGA as long as it will not exceed 5Vpk-pk differential input swing of the VGA. The AD8332 VGA is designed for graceful overload recovery so it will still recover if the VGA input signal saturates the VGA output voltage range even if the VGA gain control voltage is set to high. We recommend having an AC couple in between all stages to eliminate the cumulative DC output offset voltage. However, it will still be quite difficult to achieve the performance in terms of noise and distortion. 

    I hope this information helps. 

    Thanks,

    Mae