Need help choosing a good op amp to make a differential op amp.

Discussion in 'The Projects Forum' started by c1rcu1ts, Nov 12, 2014.

  1. c1rcu1ts

    Thread Starter Member

    Oct 19, 2013
    62
    0
    Hi all, I need some assistance in choosing a good op amp for building a differential op amp. I am having problems using the LM741 with 4 x 27Kohm resistors to set the unity gain. The output is less than the difference of the voltages i am trying to subtract (for example i input 9V and 0V to the terminals and it outputs something like 6.3V). I even tried using the 1NA106KP differential amplifier integrated circuit but when i tried 10V and 9V , i keep getting roughly 1.3V as the output.

    Can anyone post some good op amps to create the differential amplifier that gives close to the actual value of the difference between the inputs?

    or/and

    Can you explain why i keep possibly getting that 1.3V?

    Or what to possibly look for in a datasheet when choosing the op amp.

    ~Thanks
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,088
    3,027
    You need to look at the "common mode voltage range", or the output voltage range. What you'll find is that an ancient op-amp like the 741 (there are none older!) cannot get close to either power rail at its output and cannot sense voltages on its inputs that are close to either rail either.

    This is true of most op-amps and you design around that, so you stay within the amp's range specifications.

    There are rail-to-rail op-amps (I have a TLV272IP in my parts box), as well as amps that can approach one rail or the other but not both (LM358N, for example). Lots of choices.
     
  3. c1rcu1ts

    Thread Starter Member

    Oct 19, 2013
    62
    0
    Thank you very much. I was doing a bit or research as well, i guess i should choose an op amp with a high common mode rejection ratio as well??
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,088
    3,027
    I think any op-amp will likely have "enough" CMRR, unless your application requires an unusually high ratio.
     
  5. c1rcu1ts

    Thread Starter Member

    Oct 19, 2013
    62
    0
    My application is to use it for a current sensing resistor. I am using a 1 ohm 25Watt resistor and i would like connect the two terminals of the resistor to the inputs of the differential amplifier and and pay attention to the output (to measure the pd of the resistor). This way i can easily know the current flowing in my circuit.

    Would an "unusually high CMRR" be required?

    is the LM308N a good choice?
    datasheet: https://www.futurlec.com/Linear/LM308N.shtml
     
    Last edited: Nov 13, 2014
  6. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    CMRR in excess of 70 dB would be unusually high.
     
  7. c1rcu1ts

    Thread Starter Member

    Oct 19, 2013
    62
    0
    Thanks, would it be suitable for my application though?
     
  8. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    CMMR is not what is limiting you. It is the limitation on the voltage on the inputs.

    Look for a current sense amp. They are designed to do exactly what you want, and they allow the voltage on the inputs to be outside the range of the power to the amplifier, which opamps do not.

    Here is some information about them from MAXIM:

    http://www.maximintegrated.com/en/p...lifiers.html?gclid=COXCn7Ch-MECFcTm7Aodr3UA9g


    Bob
     
  9. wayneh

    Expert

    Sep 9, 2010
    12,088
    3,027
    How will the two voltages compare to the voltage of the power rails of your circuit?

    I would pull out a LM358N for this application. (I have some from other projects.) These can sense voltages down to the negative power rail, which would be the low side of the shunt resistor if your sensing circuit shares a ground with the low side of the shunt resistor.
     
  10. c1rcu1ts

    Thread Starter Member

    Oct 19, 2013
    62
    0
    I am inputting this voltage into the inverting terminal of a comparator and putting a reference at the non inverting terminal.

    Thanks but i got the differential amp with unity gain to work. I used +15 and 0V to supply the amp which gave the weird readings but when i used +15V and -15V instead, i got the correct readings.

    ~Thanks everyone
     
  11. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    Hopefully, you understand why going to dual supplies worked, right?

    Bob
     
Loading...