OP Amp selection help

Discussion in 'The Projects Forum' started by newhobby, Aug 15, 2014.

  1. newhobby

    Thread Starter New Member

    Mar 26, 2014
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    Hi Everyone,

    This is what I have:
    Single supply +48V - Can't be changed.
    LM2575HV-3.3 that brings down the 48V to 3.3V
    Mcu running at 3.3V
    Low pass filter to smooth down PWM to 0-3.3V analog signal.

    This is what I need:
    OP Amp with gain of 3 to give me a 0-10V signal.
    I used LM358 in the past when my supply was +12V without problems, but It can't be used due to the +48V of the current supply.
    I've seen quite a few that work on +/-30V, such as OPA551. My question is if I have single supply of +48V instead of dual +/-30V, is it safe to use?? Or I need to find something that would work +/-48V??

    Thank you,
    RI
     
  2. newhobby

    Thread Starter New Member

    Mar 26, 2014
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    0
    That's what I mentioned earlier... The input signal voltage is 0-3.3V and the output is 0-10V. So, I need a gain of 3 and a supply that is 10V or greater, right?
     
  3. newhobby

    Thread Starter New Member

    Mar 26, 2014
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    0
    Just to add to the discussion, the output signal current demand is very little. It is used as dimming signal for an LED fixture, so it doesn't need much current at all.
     
  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,378
    494
    OPA551 Datasheet.
    Absolute Maximum Ratings.
    Supply voltage, V+ to V-.............60V

    So. What does that mean. Lets say you have single supply setup. Your V- is 0V. Then your V+ is 60V. Why? Because the "distance" from V+ to V- is 60V.

    The same applies to the dual supply setup. The "distance" from -30V to +30V is... 60V.

    Now. Seeing that the absolute maximum is 60V, your 48V is well below the maximum, you should be fine using OPA551.

    I am seeing that OPA551 GBW is 3 MHz. With gain of 3, that gives you bandwidth of 1 MHz (3 MHz/3=1 MHz). Is that enough?
     
  5. newhobby

    Thread Starter New Member

    Mar 26, 2014
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    Thanks!!!
    My signal is a continuous analog signal.
    This signal would only change once per second or even once per minute, which is the most common.
    Would the bandwidth matter in this case?
    The bandwidth you are referring is how fast my input signal can be transfered to the output, right?
     
  6. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,378
    494
    Ok, that will be a problem for OPA551. The minimum input voltage is (V-)+2.5. So. If you have single supply setup, your V- is 0V. Which means that your minimum input should be 0+2.5=2.5V. Your input signal is 0-3.3V, the minimum being 0V.

    Here is what I think will happen:
    You will feed 0V to OPA551. It will automatically interpret it as 2.5V. Amplify it by 3. And output 7.5V. So you send in 0V, you expect 0V out but you get 7.5V out.

    You need to either use dual supply setup or you need a "finer" op amp that can get very close to your bottom rail (0V).
     
  7. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,378
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    No. The bandwidth here is the frequency of the input signal.

    Since you say signal will change in seconds, that is frequency in kHz, which is much less than 1 MHz, so it should work fine.

    See my other post about input voltage of OPA551. That one will be a problem.
     
  8. crutschow

    Expert

    Mar 14, 2008
    12,981
    3,221
    If you just need to power an op amp from the 48V then you can use a simple 15V zener shunt regulator (series resistor, zener to ground) to reduce the voltage. Select the series resistor value to allow about a few mA more current (for the zener) than the worst-case maximum current required by the op amp including any load with (48V - 15V) = 33V across the resistor. You need to use a single-supply op amp such as an LM324.
     
  9. newhobby

    Thread Starter New Member

    Mar 26, 2014
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    Let's see if I got it right...
    I get one of these zeners: SMAZ24-13-F
    Use it like the design attached and I could go with the same LM358 I have been using in the past. Is that correct?
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    Yeah, but think about the power dissipation in the 1K resistor...
     
  11. newhobby

    Thread Starter New Member

    Mar 26, 2014
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    Thanks for the heads up.
    I think 1w should be good enough, right?
    24v x 40mA =. 96w
    I'll never use 40mA, since this is just a signal booster for a control signal input.
     
  12. crutschow

    Expert

    Mar 14, 2008
    12,981
    3,221
    You need to also be concerned with the zener dissipation which would also be near 1W depending upon how much current the op amp circuit draws. For best reliability you should derate the dissipated power for both devices by at least 50% so I recommend a 1W resistor nearer to 2kΩ.
     
  13. newhobby

    Thread Starter New Member

    Mar 26, 2014
    24
    0
    Awesome!!
    Thanks everyone.
     
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