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  1. #1
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    Buffer Circuit Question

    Hi,

    I have a MCP6002 wired as a buffer circuit. A voltage divider with 1M and 20K to ground. When i feed the voltage divider output to input of the buffer. The output voltage of the Buffer
    when measured is off by few milli volts, and it changes at various voltages of the input.
    The input voltage was always well below the max Supply voltage of the opamp which is 5V.

    any help would be appreciated.

    Thanks,
    Jai.

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    Re: Buffer Circuit Question

    A few mV is well in the range of expectable MCP6002 offset voltage. When you say the error changes with input voltage, how do you measure it? DC or AC voltage?



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    Re: Buffer Circuit Question

    Hi,

    Thanks for reply.

    I am trying to scale a volatage of 0 - 15V to mcp3428. I am using a front voltage divider and buffering with mcp2002. I find that the error ouput from buffer is not constant it keeps changing with varying
    input voltage to buffer. I find when i directly connect the voltage from the powersupply to the buffer it is spot on. But when i use voltage divider a error is seen. and if that error is constant then it is ok.
    but it varies with input voltage change.



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    Re: Buffer Circuit Question

    What's the resistor tolerance?



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    Re: Buffer Circuit Question

    Tolerance 0.1%



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    Re: Buffer Circuit Question

    Hi,

    did you do an error calculation? or at least an error expectation?

    If you do this, you will recognize, that the buffer doesn´t give a benefit. It just introduces errors.

    Scaling down 0...15V with 1M/20k gives an output voltage of 0...294mV.
    Then the offset error of the MCP6002 is +/-4.5mV wich is 9mV in total. this is an error of 3% full scale.
    (or about 10% at referenced to 5V input, or about 50% at 1V input.)
    So the 0.1% resistors are useless.

    Then there is a 25mV OPAMP output voltage limit (maximum) to the rails.
    This makes a measurement of input voltages of 0...1.2V completely impossible.

    --> Use a voltage divider of about 75k and 10k and a capacitor (parallel to the 10k) without buffer and get far better performance. Especially with low input voltages.

    Klaus

    Btw: MCP6002 is a dual OPAMP. What did you do with the second OPAMP? Hopfully you did not leave the unused inputs floating...
    The same applies for the unused inputs of your ADC.
    Indeed it applies for any analog or digital circuit....if not mentioned otheriwse in the datasheets.
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



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    Re: Buffer Circuit Question

    Hi,

    Thanks for the reply.
    I want a higher input impedance. atleast 1M ohms. yes i did tie the other buffer to ground. any ideas. thanks.



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    Re: Buffer Circuit Question

    Hi,

    I want a higher input impedance. atleast 1M ohms.
    I currently see no need for this.
    The higher the impedance the more problematic.

    yes i did tie the other buffer to ground
    This means?
    The output? --> makes no sense
    Both inputs? --> maks no sense, because you still get saturated ouput stage. But better then than floating inputs...

    any ideas.
    * less attenuation (7 times higher voltage gives 7 times lower relative error)
    * better Opamp
    * negative supply (as an alternative you could add some fix offset at the analog side and subtract the offset at the digital side)
    (all with proper decoupling capacitors, proper PCB layout, stable power supply, low pass filtered signal)

    Klaus
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  9. #9
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    Re: Buffer Circuit Question

    Exactly what are the input and output voltages that show this offset?
    Zapper
    Curmudgeon Elektroniker


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