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TLC2274: Floating input

Paul Dye
Prodigy 180 points
Part Number: TLC2274

Hi,

We manufacture a product which uses the TLC2274ID.

Each op-amp stage is used as a voltage follower and usually has an external sensor connected.

It has been suggested that we have a variant which leaves off one of the external sensors which means that the input to one of the op-amp stages will be floating. There is no pull down resistor. There are 2 filter capacitors to ground. We can't really alter the PCB.

Common knowledge says that floating inputs can take current but my question is how much current? Could the device become damaged/ go into a fault condition?

Thanks for your help

Regards

Paul

  • 0
    TI__Guru 62700 points

    Paul,

    In order for the op amp to operate within its input/output linear range, the input terminal must be provided path for the input bias current, IB, to flow into the system ground.  For that reason, allowing the input terminal to float will result in the input (and output) to collapse on one of its rail - positive rail if the IB current comes out of the input terminal and negative rail in case IB flows into the input (TLC2274 may have IB flow in either direction). However, unlike in a bipolar op amp where saturating the output transistors results in the increase of the quiescent current, in case of CMOS amplifier like TLC2274 there is no such danger – the output will rail but the IQ should state unchanged. Having said that, even though such condition will not damage the part, operating op amps under any non-linear conditions for an extended period of time may decrease the reliability and/or shorten the life of the product.

    For more info on this topic, please read the blog: https://e2e.ti.com/blogs_/archives/b/thesignal/archive/2012/11/27/the-unused-op-amp-what-to-do

  • 0 in reply to Marek Lis
    Prodigy 180 points

    Hi Marek,

    Thanks for your response.

    Would any reliability or shortened life just affect the op-amp stage that is in the floating condition?

    I am trying to assess the risk to the rest of the product which uses some of the other stages in the quad package.

    Thanks for your help.

    Regards

    Paul

  • 0 in reply to Paul Dye
    TI__Guru 62700 points

    Yes, for the most part, the floating op amp may be affected the most but there may be secondary effects like possible increase in the total quiescent current.  Having said that, in case of CMOS op amp like TLC2274, I do not believe you need to worry about it.