Scope Probe Capacitance

Discussion in 'Test & Measurement Forum' started by brightnight1, Apr 26, 2018.

  1. brightnight1

    Thread Starter Member

    Jan 13, 2018
    40
    1
    I was recently given a Tektronix 2210 oscilloscope (from 1989) but I need to buy some probes for it. Next to the connector on the scope it says 1M Ohm and 25pF. I know that I want to match the resistance and capacitance of the probe to my scope.

    I can find a 1M Ohm probe pretty easily but cannot find a probe with a 25pF capacitance. How close of a matching capacitance do I need to get? I found a probe on Amazon that's 1M Ohm and a capacitance of 18.5-22.5PF (below). I read something about trimming capacitors on scopes but ideally I'd like to start with the correct probes.

    I also am a little confused about what the Hz rating is on a scope probe. The link below says "100MHz"..does that mean it's for measuring signals up to that speed?
    https://www.amazon.com/RioRand-PP15...3&sr=8-1-spell&keywords=oscilliscope+probe+1M

    Thanks in advance!
     
  2. ebp

    Well-Known Member

    Feb 8, 2018
    2,069
    726
    The spec you need to look at is
    "Compensation range 15-40 PF ..."

    This means that the probe can be adjusted for an input that has a capacitance of 15 to 40 pF. Since your scope's nominal input capacitance is 25 pF, the probes will work fine. Adjusting the trimmer capacitor in the probe sets the "frequency compensation" so that the attenuation of the probe is the same at high frequency as it is at DC. This is a critical adjustment and all attenuator probes require compensation adjustment. Sometimes you even have to make a very small tweak for best performance if you move the probe from one channel of the scope to another.

    have a look at these, which will explain probe fundamentals:
    https://www.tek.com/learning/probes-tutorial
    http://web.mit.edu/6.101/www/reference/ABCprobes_s.pdf
     
    Last edited: Apr 26, 2018
  3. BR-549

    AAC Fanatic!

    Sep 22, 2013
    4,074
    1,062
    Those will work. The cap adjustment on the probe lead connector.....is to cancel the capacitance of the probe lead. They call it compensation. Have you got a manual for your scope?
     
  4. RichardO

    Late Member

    May 4, 2013
    2,274
    890
    A x1 probe (1 "MegOhm" probe should have as small a capacitance as possible since this capacitance in in parallel with the scope's input capacitance. This capacitance loads your circuit at high frequencies.

    A x10 must be compensated (adjusted) to match the input capacitance of the scope input to get good square wave response. The probe you linked to says "Compensation range 15-40 PF". This is enough to compensate you scope which gas a 25pF input capacitance.

    The Tek 2210 has a bandwidth of 50 MHz. A 100 MHz probe will be a good match for that. (A 100 MHz probe will only degrade you system risetime by about 10%).


    edit: Others beat me with good answers. I type way too slow. :rolleyes:
     
  5. brightnight1

    Thread Starter Member

    Jan 13, 2018
    40
    1
    Thanks so much for all the answers, I really appreciate it!

    Don't have a manual for it. Was hoping to do more searching online to find a manual.
     
  6. danadak

    Well-Known Member

    Mar 10, 2018
    1,758
    362
  7. DickCappels

    Moderator

    Aug 21, 2008
    4,967
    1,512
    You want the compensation to be as close to perfect as you can get it. Think about how irritating it would be to always see poorly compensated waveforms.

    upload_2018-4-27_21-7-59.png
     
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