OP4177 opamp is not working as voltage limiter.

Hi!

I am facing some issues with OP4177 (precision amplifier)  in a voltage limiting application. Earlier in the design LM-148 ( quad uA741) was used and there was absolutely no issue. In the same configuration OP4177 gives horrible results with absolutely no clue.

Am attaching the circuit and practical results for reference.

Please advice.

Regards

Ruchi

attachments.zip
  • Ruchi,

      AR4A, etc. are connected in such a way where under certain conditions, the diode is reverse biased and there is no feedback.  A lot of op amps don't like this.  The OP4177 is a precision op amp with protection diodes and resistors between the inputs to protect the diff pair.  See OP177 data sheet for a similar schematic.  In addition, Some quads have one, or two or four bias networks.  I don't know about the OP4177, but if it has one bias network, if one section is overloaded, it can affect the other three.

    The LM148 is a commodity, non precision op amp.  If it works in your system, why do you want to change it?

    Harry


  • Hello Harry!

    I selected OP4177 as a replacement for LM148 in my design based on the specs given in datasheet. I found it better in every aspect e.g. offset, bias current, supply current. Slew rate , gain BW were comparable too.

    Now when it is not working in my final board, I am in a fix. Even if I try to choose another opamp now, I really do not know which parameter to look for in the datasheet to ensure that the problem does not repeat.

    reason for changing LM148 was its non-availability in SOIC package with +125degC operating temp range which is required in my application.

    Ruchi

  • Ruchi,

    I think you might be stuck.  The precsion bipolar op amps usually have diodes between the inputs to protect the diff pair so you don't zener the EB junction.  You might look at some of the JFET op amps, such as the ADA4000-4 or the AD8513, but the Vos and TCVos will be higher than a bipolar.

    Harry


  • Thanks Harry!

    I would try for other opamps too as suggested. I am also working on alternative configurations for the voltage limiter

    circuit with the same OP4177 . Have you come across any alternate configuration for such application which you would like to suggest?

    Regards

    Ruchi

  • Ruchi,

      Most of the time when customers want a full wave rectifier, it is to find out the RMS value.

    Nowdays, a dedicated RMS to DC converter is used, such as the AD8436.

    Harry