over voltage protection circuit for CMOS IC

Discussion in 'General Electronics Chat' started by gotumal, Mar 24, 2008.

  1. gotumal

    Thread Starter Active Member

    Mar 24, 2008
    99
    0
    Hi reader,

    I am using IC AD7718 working on +5V to sense some analogue stuff. Before it, I have OP07 which works on 10V. During normal working conditions, output of OP07 wont go beyond 4.5V. But I have to give overvoltage protection for my ADC. I thought of using BAV99 in SOT23 package but the range, analogue pin supports is Vadd +100mV. What could be the best circuit?

    Could anybody help me in this regard?
     
  2. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    Checkout the Littelfuse transient voltage supressors (TVS).
    These devices clamp to Vcc + 0.6V and GND - 0.6V.
    For digital lines I add a series resistor to limit
    the current. Check the input specifications of your device
    to see if you need current limiting resistors.

    The part I use is an SP724AHT which protects four lines.
    There are a variety of application notes on the Littelfuse
    website.

    (* jcl *)
     
  3. rwmoekoe

    Active Member

    Mar 1, 2007
    172
    0
    i'll use a pair of 1n4148 and a resistor.

    ' vcc
    ' |
    ' 1n4148
    ' |
    in -- 1k -+- to adc
    ' |
    ' 1n4148
    ' |
    ' gnd
     
  4. gotumal

    Thread Starter Active Member

    Mar 24, 2008
    99
    0
    Thanks for your replies.
    Actually, BAV99 diodes are pair of schottkey diodes in a single package with low forward voltage drop than 1N4148. (of course it depends on current, but can be limited by a suitable resistor).
    Also as the forward voltage drop is more in TVS also compared to accepted high level of ADC input pin.
    So could not really find the solution so far.:(
     
  5. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    The absolute maximum for the AIN to AGND is -0.3V to AVdd + 0.3V.
    and the maximum current is 30mA. You should see if there is an application
    note that discusses the internal protection of the ADC inputs.

    You should be OK with a TVS or clamping diode as long as you limit
    the current to well below the 30mA. If you find an application note that discusses
    this (or talk to an Analog Devices application engineer) please post what you
    find out.

    (* jcl *)
     
  6. gotumal

    Thread Starter Active Member

    Mar 24, 2008
    99
    0
    In AD7708 datasheet, in the absolute maximum rating, they have mentioned that AVdd to Agnd voltage can be -0.3V to +7V and Analog input voltage to Agnd is -0.3 to AVdd+0.3V. I am using +5V for AVdd. So my maximum voltage will be 5+0.3V or 7+0.3V.

    Could anybody help me?

    I have attached datasheet for refernce. Please refer page no.10.

    Thanks for reading this thread!
     
  7. rwmoekoe

    Active Member

    Mar 1, 2007
    172
    0
    you can use any build-in clamp ic or the 1n4148 configuration (don't forget the 1k resistor though), plus (series it via) a resistor to the ain pin with a value of
    (Vdiode-drop - 0.3) / 30mA.
    with a clamp using diodes of 0.6v drop it will be 10ohm.
     
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