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Choosing op-amp for the design

Shiranga Ariyasinghe
Expert 1145 points
Other Parts Discussed in Thread: LM318, OPA277, OPA191, OPA192

Hi all,

I have simulated below circuit to get the measurements from a sensor. Sensor changes it's voltage 0-5V range. I wanted to convert it to a fully differential signal( -5 to 5V) to transmit to another circuit board. Then converted it in to a single ended signal and scaled it down to 0-3.3V measurement to get reading to micro-controller. My simulation is as below and simulation results were as I was expected,

But after implementing circuit with 1% resistors, I couldn't even get close results as I was expected in simulations. I think I have made a mistake with choosing LM318 and seems it may not suit for this application. Now I want to change the LM318 from the circuit and test it back to get close results as simulations.

Voltage following (buffering) stage added some offset to the signal. My supply voltage is+15V/-15V.

1) How can I chose op-amp for voltage following stage and the scaling stage ? what are your recommendations?

2) What are the main differences between OPA,TLV,TLE,LM op-amp series?

3) Further I have voltage adding and subtracting circuits as well. The are also having same problem with LM318. How can I chose op-amps for those applications as well ?

Thank you.

  • 0
    Expert 1145 points
    Can anyone help please ?
  • 0 in reply to Shiranga Ariyasinghe
    TI__Guru*** 145336 points
    Lakshan,

    Everything makes sense except for "to another board". What are the characteristics of the cable and the load there? May I assume the sensor is slow (the signal is low frequency)
  • 0 in reply to Ron Michallick
    Expert 1145 points
    Hi Ron,
    Thank you for answering :)

    Maximum signal frequency will be 1.5kHz . To another board means, I have two PCBs. One of them has the sensor and the fully differential op-amp. The other one have voltage followers, single ended circuit with these three op-amps and MCU. Cable is a ribbon cable with low resistance.

    I was thinking about OPA277, OPA191, kind of op-amp which are having low offsets. Will they match with my requirement.? what are your recommendations? They other problem I am facing with LM318 (with +15V/-15V) is the increase of heat. Will it be fix with op-amp change?
  • 0 in reply to Shiranga Ariyasinghe
    Expert 1145 points
    Ron,
    Further, for the heating problem, I am expecting below results without considering load current, (assume ambient temperature = 30, SOIC package), V= 30V (+/- 15V)
    LM318 : Rja = 97C/W; Iq = 5.1mA then T = 5.1x30x97 + 30 = 44.81C
    OPA192: Rja = 115.8C/W; Iq = 1.5mA then T = 1.5x30x115.8 + 30 = 35.211C
    OPA277: Rja = 67C/W; Iq = 800uA then T = 0.8x30x67 + 30 = 31.608C
    OPA191: Rja = 115.8C/W; Iq = 250uA then T = 0.25x30x115.8 + 30 = 30.1185C

    Therefore can I expect low temperature rise by changing the device. ?
    Ron, I am expecting answers for above questions as well.
  • 0 in reply to Shiranga Ariyasinghe
    TI__Guru*** 145336 points

    Lakshan,

    Here is TI-TINA circuit with OPA277.

    It pretty basic. 0-5V input is gain =1 buffer to get 0V to 5V. A -1 gain amp give 0 to -5V. A simple resistor diver gets 0V to 3.3V

    OPA277.TSC