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Why isn't there a buffer on an oscilloscope?

  1. Nov 25, 2016 #1
    Today in my eletronics class, we were introduced to the amp op concept. After going through some fundamentals, the teacher challanged us to find v0 of a simple circuit with a battery and two resistances of 1M ohm each(v0 is in between these resistances). Afterwards he asked us if this same value would be correctly read by an oscilloscope. The answer would be no because of the internal resistance of the oscilloscope (1M ohm) that would then be parallel with one of the resistances from the circuit.
    One of my classmates then asked why wasn't there a buffer incorporated on the oscilloscope.

    I went home and looked out what a buffer would do and realized that my classmate's question made a lot of sense. I tried to look up if buffer was high energy consuming or if it could short circuit both the oscilloscope and our circuit but with no success. Or maybe sometimes we want to module our wave through some options from the oscilloscope itself and using a buffer might complicate that. So my question is: why is this?

    Thanks
     
  2. jcsd
  3. Nov 25, 2016 #2

    berkeman

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    Staff: Mentor

    A typical 'scope probe has a 10:1 input impedance ratio, so you get a 10MOhm input impedance into the 1MOhm 'scope input.

    There are reasons for not going to higher impedances, mainly noise pickup and reduced robustness. Some FET input probes have higher input impedance, but are very susceptible to damage by ESD. There are also some 100:1 'scope probes that I've used when I needed a higher input impedance, but they have reduced bandwidth, generally.
     
  4. Nov 25, 2016 #3

    Baluncore

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    Science Advisor

    The standard 1 megohm input resistance of an oscilloscope permits series resistors to be used as attenuators for higher voltages. A standard 10:1 probe adds 9 megohm in series, EHT probes add 99 megohms or more to give 100:1 ranges.

    Resistors over about 100k should not be used without protection from environmental contamination because unpredictable surface leakage currents may dominate the circuit. Most circuits have a much lower impedance than 1 megohm, hence it is rare that an oscilloscope needs a high impedance probe.

    External high impedance probes and buffers or “electrometer amplifiers” are available. They need a power supply and so are usually only used on signals having voltages less than about 15 volts. They may have input currents equivalent to a 10G or 100G equivalent input resistance, but obviously cannot be used with series attenuator resistors or poorly insulated standard oscilloscope probes.
     
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