Inverting op amp with 2/3 gain.

Discussion in 'General Electronics Chat' started by dustovich, Jun 1, 2015.

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  1. dustovich

    Thread Starter New Member

    May 31, 2015
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    Hello,
    This is my first post here on AAC, looks to be a pretty good resource for people learning, especially myself. Have a small amount of experience with basic electronics, but virtually nothing with op-amps and other more advanced things. After looking around here, and other places I am pretty sure I have found the configuration that I need. But I find it quite hard to select which op-amp would be best suited for my application, there are too many.

    First off a description of what I am trying to do:
    I am trying to read a 5v sensor with my 3.3v ADC on my micro. The sensor output is analog and will vary between ~0.2-4.8v and has an impedance of >50kohms according to the spec sheet. I have attached it here. Most micros that I have seen (arduino) prefer an input impedance of <10kohms to read properly without error, so regardless of the range shifting that I need with my new micro (teensy 3.1) I would have needed an op amp to begin with. I am going to be reading this sensor in parallel with the actual ecu in my car, so there can be nothing affecting the voltage level, which fits well with the op amp theory (infinite input impedance). I will have both a 5v and a 3.3v source, but I would assume that the op amp will require the 5v source only. There will be no AC/frequencies, and more or less a (relative to khz/mhz frequencies) very slow changing DC voltage relative to the pressure applied to the sensor. It will be subjected to outside temperatures (-30c-+40c) so this will have to be taken into account also. The micro will share a common ground (car chassis) with the ecu sensor, but will have a different power supply than the one internal to the ecu itself.

    After some research my plan was to have an op-amp configured to invert the signal with a 2/3 gain so I will have a perfect inverted linear voltage adjusted down to 3.3v. So this would require a single supply rail-rail, low noise?, low offset op amp with not much change due to temperature (my guesses that is). After searching around the forums, I have found the TLV34x(A) series of op amps which seem to fit the bill for what I am looking for.

    Questions that I have are:
    Is there a more suitable op amp out there for this application?, or is there a more suitable way of doing this period?
    Considering I don't want to affect the circuit that I am measuring at all, and taking into account the existing circuit impedance (and unknown circuits inside the ECU) what ranges of resistor values would be best to use for the input/feedback resistors?
    What else am I missing?

    Hopefully I have provided enough information to make this understandable.

    Thanks in advance for your help also.
     
  2. DickCappels

    Moderator

    Aug 21, 2008
    2,651
    632
    Welcome to the AAC forum.

    The owners of All About Circuits have elected not to host discussions of automotive electrical system modifications/enhancements due to safety concerns, the potential of legal ramifications and the possible circumvention of vehicle regulations at the state and federal level.

    This thread is against the AAC forum rules, Chapter 6, as seen here:

    6. Restricted topics. The following topics are regularly raised and however are considered “off-topic” at all times and will results in Your thread being closed without question:

    • Any kind of over-unity devices and systems
    • Automotive modifications <== This includes adding things to a car's electrical system.
    • Devices designed to electrocute or shock another person
    • LEDs to mains
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    • Transformer-less power supplies
    This can be found in our Terms of Service (ToS)

    Automotive modifications of any kind are strictly forbidden. Therefore, this thread will be closed.

    Please try to understand the reasons behind this action, and feel free to browse and use the forums.

    You might find answers to your questions in one of these forums:

    http://forum.allaboutcircuits.com/showthread.php?t=54400

    Another good forum that shares many of the same membership is http://www.electro-tech-online.com/ . Good luck.
     
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