Voltage divider at op-amp output

Discussion in 'General Electronics Chat' started by Dritech, Dec 29, 2015.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi,
    I have an op-amp which its output vary from 0 to 5V. I need to perform data logging using a Data acquisition (DAQ) board.
    The DAQ analog input range is from 0 to 2.4V and so I need to divide the output of the op-amp by 2.
    The op-amp output waveform will be similar to the one below, with the baseline being at 2.5V and the frequency in the range of 5Hz to 1kHz.
    Will a simple voltage divider at the output of the op-amp (see diagram below) divide the signal with these parameters?
    What are the other options to achieve this without using complex circuits?
    Thanks.

    [​IMG]
     
  2. Bordodynov

    Active Member

    May 20, 2015
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    You can reduce the transmission the coefficient of the operational amplifier 2 times. The output impedance of the operational amplifier is less than 500 ohms. There will be fewer induced of spurious noise.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
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    I think what Bordo means is that if you can reduce the gain of the opamp so the output is within your desired range without an external divider, the lower output impedance of the opamp driving the A/D directly will reduce noise pickup and other circuit effects.

    ak
     
  4. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for the reply. The origital signal (input of the op-amp) baseline is at 0V and the op-amp is doing an offset of 2.5V. Therefore, when no signal is present at the input, the op-amp output is 2.5V. The circuit that forms the offset is fixed, which makes it difficult to vary the gain.
    Are there any other options please? will the mentioned option perform well?
    Thanks.
     
  5. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    It just might work, depending on your DAQ input requirements. Sometimes that sort of equipment requires a low input impedance to work within specs.
    Another option would be for you to install a second stage OpAmp with a gain configured at 0.5. That way you'd guarantee a low impedance output and do away with the resistors.
     
    PeterCoxSmith likes this.
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I do note that 5/2 is 2.5, which is greater than the limit of 2.4

    Generally you want to trade a little accuracy for some head room so you can detect times when the sensor has gone bad and giving either a constant min or max output which is outside of the expected range.

    Thus you can test and handle when you get bad data.
     
    Last edited: Dec 29, 2015
  7. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    If I reduce the resistors value to 220ohms, can that solve the problem of the high impedance at the input of the DAQ?
    Let's say I use two 220ohms resistors, am I right if I say that the DAQ will only see 220ohms at the input and that the op-amp output current will be around 12mA max. (5V / 440ohms) ?
     
  8. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Regarding gains of less than unity, few weeks ago I learnt that, even with an op amp that could cope with that, it is better to have a voltage divider at the output.

    The book "Op amps for everyone" - 3rd edition (Ron Mancini) available in the TI site has a concrete reference (and suggestions) to less than unity gain. (25.2 Op Amp Operated at Less than Unity).
     
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  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Actually the DAQ will see 110 ohms, or the thevenin equivalent resistance.

    The op amp sees 440 ohms.
     
  10. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    So here's another idea, which I've successfully used in the past.

    Place a resistor voltage divider at the output, just like the OP suggested initially. And then connect it to an OpAmp configured as a voltage follower, that is, with a gain of 1.
     
    PeterCoxSmith likes this.
  11. Bordodynov

    Active Member

    May 20, 2015
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    Add to your capacitor divider. This filters out high-frequency hindrance.
    See:

    Filtr.png
     
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