ADG409

I have an ADG409 selecting taps on a resistor divider string in the feedback path of an OpAmp, It is a +-12 volt circuit. And I am occasionally burning holes through the top of the chip package.?.  The top of the string is 3.5K before the top switch and the lowest switch has 539 ohms to ground so I am confident the switch itself is not the cause of the destruction.

My question is, how does the ADG409 handle loss of the Vss connection. 

Vss is normally connected to -12 volt in this circuit. I can conjecture a possible loss of the negative rail because of the pin placements on the boards backplane connector, but am unsure if this would harm the chip.?. Note also that the en (enable) pin is directly connected to +12 volts. If the loss of the -12 volt connection can cause this, would a limiting resistor on the enable line prevent damage? Would placing a reverse biased diode between Vss and ground (locking Vss to ~.3 to .6 volts above ground) alleviate the risk.  I appreciate your input. Thanks.    

  • Hi, Klantz571.

    There are a few things that you should consider to be having this problem:

    1. Unlikely, but it could be going through the multiplexer, in its normal operation, more than its maximum current, 20 mA for continuous current, and 40 mA for peak current.
    2. What I believe your problem is, when losing the -12 V connection, your Vss will be floating, and, as the analog input of the multiplexer is from rail-to-rail, this means that, for the multiplexer to be working properly, your analog input should be from Vdd, which is +12 V, to Vss, which is not -12 V anymore. Thus, if your analog input is negative and you lost connection to the -12 V to your multiplexer, you are going to damage it, since it will cause the internal diode of the multiplexer tying the analog input to Vss to forward-bias and current to flow.

    Remember that this part does not have overvoltage protection, which means that if your analog input is outside the range Vdd to Vss, it will damage your device, and even other components downstream.

    These are some observations and possible solutions for you to try:

    1. If your analog input is only positive, from ground to Vdd, you can tie the Vss pin of the multiplexer to the ground, which will change some of the device's parameters, but it will continue to work.
    2. Tying Vss to +0.3 V to +0.6 V could damage the part since, according the absolute maximum ratings of the ADG409, Vss maximum voltage is +0.3 V.
    3. Adding a resistor to limit the current when the fault occurs could help you prevent damaging the device. The current should be limited according to absolute maximum ratings of the ADG409, specified in the datasheet. However, this will increase Ron per channel and the overall system error.
    4. It could be added Schottky diodes connected from the analog inputs to the supplies, in order to prevent damage to the part. This would prevent the analog input signal from exceeding the supply voltage by more than the Schottky diode voltage drop, so the internal diodes do not forward-bias and current does not flow.
    5. Another option to help you with the overvoltage problem would be to place diodes in series with the supplies to block the current flow through the internal diodes. On the other hand, this method would reduce your analog signal range to the diodes on the supplies.

    Bear in mind that these possible solutions will add errors to your setup. Besides, always consult the datasheet, and consider the absolute maximum ratings of the device you are working on.

    Please, refer to https://www.analog.com/en/analog-dialogue/articles/switch-and-multiplexer-design-for-hostile-environ.html for more information about the overvoltage protection using external components.

    Yours sincerely,

    Igor Ono.