AD5422 - Transient voltage protection

Hello,

we want to use the AD5422 in single supply mode (AVDD=17VDC) with voltage and current output on same terminal.

There is a input protection example in the datasheet of the AD5422 (page 37, figure 76).

Right at the input terminal we want to use a bi-directional TVS SMBJ18CA. We also use a current limiting resistor 18 Ohm.

At the input of the AD5422 we have two protection diodes to AVDD and GND.

 

What kind of diodes should be used there (type and power rating) ?

Do I have to use schottky diodes or can I use normal silicon diodes, e.g. LL4148 ?

Should I take care of the higher reverse current when using schottky diodes ?

What is the clamping voltage of the AD5422 internal ESD protection diodes ?

Is another resistor necessary between external clamping diodes and the AD5422 input ?

 

Regards

  • 0
    •  Analog Employees 
    on Jan 9, 2014 11:26 AM

    Hi there,

    I have drawn a quick sketch of what you have described below:

    AD5422 Vo & Io shorted:

    • For the configuration with Vo & Io shorted together, use of the external Rset is recommended for better accuracy.

    AD5422 with Unipolar supply:

    • Is the external buffer also being driven with the unipolar supply rails? The reason for asking this is to do with your system performance requirements in terms of footroom. The AD5422 part itself will go to <20mV for zeroscale in 0-10V range with unipolar supplies (AVSS @ 0V). Depending on your choice of external buffer, this may limit the footroom further.

    AD5422 internal ESD protection diodes:

    • The AD5422 contains ESD protection diodes that prevent damage from normal handling.
    • The ESD structure at the Vout/Iout pad has:
      • An internal diode to the avss terminal and will conduct >0.7V approx.
      • A resistive internal diode 2x stack path to avdd and will conduct > 1.4V approx.

    Choice of protection components:

    • TVS – The TVS should be sized with the lowest breakdown voltage possible while not conducting in the
      functional range of the current output. The SMBJ18CA has a breakdown voltage of 20-22V.
    • Clamping diodes –  In terms of your choice of clamping diodes, both the current limiting resistor and the
      two clamping diodes must have appropriate power ratings for your output range. BAS70-04LT1 schottky diodes have already been proven on the CN0321 circuit note.
    • The use of Schottky diodes is recommended as they have  a lower conducting voltage thereby ensuring limited fault current flows on-chip.
    • If there is concern about using shottky diodes, then a normal silicon diode could be used. In this case, we would recommend a further resistor inside the diode to limit current on chip. 10 ohms should be sufficient to limit current in internal diodes to < 100mA.
    • With 18 Ohm current limiting resistor, for 4-20mA output, the compliance limit at the terminal is
      decreased by V = Imax X R = 0.36V.
    • In the sketch shown, there is also another 10kOhm resistor shown at the positive input to the external buffer. This protects the amplifier by limiting the current during a transient event.

    Best Regards,

    Tracey...

  • Hello,

    thank you for the quick response.

    We are using the OP2237 with single supply. The output of the OpAmp can swing to withing 10mV of ground.

    So with the clamping voltages of the AD5422 I think it is better to use a Schottky diode or a normal silicon diode plus resistor, as you mentioned.

    But is the higher reverse current of a Schottky diode a disadvantage ? Is this an additional error in the current output during normal operation ?

    I'm wondering the BAS70-04 used in the application CN0321  can withstand the power dissipation. There is a TVS diode used (SMBJ26CA) with breakdown voltage of 29-32V.  So with a voltage AVDD of 15V there are still 17V for the resistor and the protection diode.

    Regards

  • 0
    •  Analog Employees 
    on Jan 10, 2014 10:26 AM

    Any leakage from the TVS or diode will generate some error on the output, so it's important to keep these leakages small in relation to your output. Another option for diode is the BAV199, this device is a standard diode but has lower leakage.

    In either case, the extra resistor inside the diode to limit current on chip provides an additional level of protection to the part.

    The level of transient protection in any given design will depend on the environment in which it is operating and so it is difficult to give a specific recommendatioin which covers all cases.

  • 0
    •  Analog Employees 
    on Aug 2, 2018 2:32 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin