Oscilloscope probe question

Discussion in 'General Electronics Chat' started by jody, Aug 17, 2013.

  1. jody

    Thread Starter Member

    Nov 12, 2012
    39
    0
    HI
    i have confused myself and cant find a clear simple description for an answer. on a digital scope i have the spec sheet quotes the input impedance as 1MEG. so does this mean that it already has this impedance (so i could just connect a basic croc clip probe) or does it means it needs a min 1 meg probe.
    Also if it already has a 1meg impedance and i connect a normal x1 1 meg probe does this not increae the impedance by adding the two together.:confused::confused::confused::confused:
     
  2. tindel

    Active Member

    Sep 16, 2012
    568
    193
    Those specs are usually pretty misleading - A 1x probe will usually have no passive components in it. The SCOPE usually has 1Mohm impedance from the input to ground.

    A 10x probe usually has a 9Mohm resistor in it to couple with the scope 1Mohm to give you 1:10 of the signal on the input of the scope. You're actually attenuating the signal instead of multiplying it.
     
  3. jody

    Thread Starter Member

    Nov 12, 2012
    39
    0
    hi
    thanks. i realize i should have asked if a x1 probe has a 1 meg impedance.
     
  4. Trevos54

    New Member

    Aug 19, 2013
    1
    0
    Industry convention is that passive attenuator probes are shown with the "X" after the attenuation factor. So 10X, 100X and so on.

    Probes that amplify the signal (active probes) have the "X" before the multiplication factor: X10, X100 etc.

    A 1X passive probe has no series resistor, so the device under test sees the 1 MOhm input resistance of the scope. But watch out - input capacitance of a 1X probe is usually high: 40pF or more. At medium and high frequencies that capacitance presents a significant load to your device, which can disrupt operation of the circuit. If your scope has sensitive enough volts/div settings it is often best to choose a 10X passive probe which has a lower input capacitance of 15 PF or so.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    Some probes have selectable X1 or X10 impedance. The X1 is used to get maximum sensitivity at lower frequencies where your circuit can tolerate the higher input capacitance. The X10 position is used at higher frequencies where the higher input capacitance of the X1 setting will affect the circuit operation adversely. There is no exact crossover frequency for when you use X1 or X10, it depends upon the circuit impedance.
     
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