AD8334 - Consequences of Exceeding the Input Voltage

     Good afternoon.  I have an A&D customer that is looking to replace the TI VCA2615.  One of the parts under consideration is the AD8334.  On page 25 of the data sheet, it states that "... the maximum input signal before saturation is ±275 mV or +550 mV p-p.  Overload protection ensures quick recovery time from large input voltages.  Because the inputs are capacitively coupled to a bias voltage near midsupply, very large inputs can be handled without interacting with the ESD protection."

     The customer needs something that has more control over the LNA instead of the fixed gain LNA in the AD8334.  The VCA2615 has this feature.  One option is the consideration of using a separate LNA and using that output to VGA of the AD8334.

QUESTION 1:  Does ADI make such a part as I did not find one using the Compass selection tool?

QUESTION 2:  How high of an input voltage can the part handle before interacting with the ESD protection?

 

QUESTION 3:  If they bypass the LNA and go straight into the VGA, what is the maximum input voltage on this stage before it is saturated?  Also, what is the maximum input on this stage after saturation but prior to any damage or ESD protection kicking in?

  • 0
    •  Analog Employees 
    on Mar 6, 2013 7:47 PM

    Hi Richard,

    Thank you for considering ADI products. These are my answers to your questions.

    1. If you are looking for VGA which the gain of the preamp can be vary, you may want  to consider the AD8337, AD8336 and AD604.

    2. Usually ESD protection are diodes that are connected between  the input pins and supply pins. Therefore these can be activated by applying input voltage that is equal or greater than to +Vs + 0.6V or -Vs - 0.6V. However you would not want to rely on the ESD cells to clamp your input voltage and therefore you could consider parallel diodes to ground before the AC coupling capacitor to limit signals into the LNA.

    3. If you are going to bypass the LNA and drive the VGA directly, each input can swing at +/-1.25V or +/-2.5V differentially given that your inputs are biased at mid supply and have a 200 ohms input impedance

    I hope it helps!

    Regards,

    Emman

  •   Emman,

      Good morning.  Thank you for the response.  I have advised the customer accordingly and will let you know if I need more information.

      Have a good day.  Thank you.

      Rich