High resolution measurement with oscilloscope

Discussion in 'General Electronics Chat' started by savage67, Mar 19, 2014.

  1. savage67

    Thread Starter Member

    Oct 3, 2006
    Hi everyone,

    I have a problematic issue which is about verification of an op-amp circuit output. In this circuit, The opamp output has a 0.7V dc offset and a 1.4V peak to peak AC component. So max-min values: 1.4-0V

    the problem is that i am going to use that circuit to sample the grid voltage value so its accuracy is very important for me. To define the its accuracy i have to verify its output to input ratio (Gain).

    As you know a big portion of oscilloscopes has a 8 bit resolution. So measuring an ac voltage with an 8 bit oscilloscope will not give me a right knowledge about is Gain and i can't use a high resolution multimeter to measure an AC signal.

    Is there any simple way to measure the output with high accuracy by using an oscilloscope or another else?
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
    Let us review the facts.

    You think that 8-bit resolution o-scope is not accurate enough.

    You are not allowed to use high resolution multimeter.

    What makes you think that you are allowed to use high resolution o-scope?

    You could use ADC instead of the o-scope. There are 8, 10, 12, 14, 16-bit ADC. You will also need additional circuitry to record the data. And make sure ADC can take the input voltage and the input frequency.
  3. MrChips


    Oct 2, 2009
    Here is another solution. Take the AC voltage, rectify and smooth it. Then measure the DC voltage with an accurate DC voltmeter.
  4. savage67

    Thread Starter Member

    Oct 3, 2006
    Do you know any measurement device that has a high resolution and low bandwidth. Because High resolution and high bandwidth featured ones are very expensive.
  5. magnet18

    Senior Member

    Dec 22, 2010
    a high resolution dmm is the easiest, why can't you use one?
  6. shteii01

    AAC Fanatic!

    Feb 19, 2010
    And now you have found out why 8-bit o-scope is the one you have on hand.
  7. ifixit

    Distinguished Member

    Nov 20, 2008
    Hi savage67,

    You could use the cheaper scope in signal compare mode.

    1. Test signal goes to CH1.
    2. UUT (opamp) output goes to CH2.
    3. Invert CH2 unless the opamp already inverts the signal.
    4. ADD CH1 & CH2
    5. Only the difference between the two signals will be display
    6. Adjust the CH1 gain, the test signal, to match the CH2 signal amplitude, or you could also set the gain to what is expected out of the opamp and the difference will be displayed.

    What will be seen on the scope is the difference between the ideal test signal and the opamp output. Phase differences can be nulled out, but would require a way to delay the test signal slightly to match the opamp output phase.

    The 8-bit scope resolution is now use to measure differences, which give overall better resolution to your measurement. You will need to calibrate your setup to ensure accuracy is maintained as well.

    This may be more trouble than it's worth.