1X or 10X Passive Voltage Probe?
1X or 10X Passive Voltage Probe?
(OP)
Suppose you have applied 800kHz, 30ppV sine signal (from a pulse generator) to a 115pF, 10Ohm RC circuit and you want to know the value of the applied voltage as accurate as possible what passive voltage probe would you use, 1X or 10X? Is there a way to check the accuracy of the two measurements? What I do is I use a resistor of known resistance and I divide the current passing through it into the voltage across it measured alternatively with the 1X and the 10X probe. What I see is that throughout one period of the sine wave the 1X probe sustains an R value closer to the known R value than the 10X probe. This makes me conclude, although I'm measuring the voltage across the R and not the voltage across the RC, that the 1X probe is more accurate. Am I right and are there other ways I can objectively check the accuracy of the two probes with regard to measuring the voltage across the RC?





RE: 1X or 10X Passive Voltage Probe?
TTFN
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RE: 1X or 10X Passive Voltage Probe?
TTFN
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RE: 1X or 10X Passive Voltage Probe?
Thanks for the link. What do you think of the method I'm applying to check the accuracy (I wish it to be on the order of 1 mV) and do you have a suggestion for an experiment to study that in the concrete case?
RE: 1X or 10X Passive Voltage Probe?
Is this for school?
TTFN
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RE: 1X or 10X Passive Voltage Probe?
RE: 1X or 10X Passive Voltage Probe?
A oscilloscope?
Analog or digital?
Which model?
Which manufacturer?
Are you setting the V/Div to get maximum deflection on the screen?
If digital, are you using built in measurement functions?
I have found measurement variations from all of the above regardless of the probe used. Even well know popular scope models will give you different rms voltage measurement readings for just a 115VAC 60 Hz depending upon how many cycles are shown on the screen (time base or sweep-rate setting).
Even a good Fluke handheld will give you bad measurement if you don't realize that it's 3db AC measurement spec may only be 10 kHz.
If you don't know the specification and limitations of your instrument, then the probe used is the least of your measurement issues.
RE: 1X or 10X Passive Voltage Probe?
RE: 1X or 10X Passive Voltage Probe?
In the former case, you have around 1700 ohm impedance and in the latter case, the impedance is 10 ohms.
Loading 1700 ohms with a 1 Mohm probe or a 10 Mohm probe doesn't change much. The error will be less than 0.1 % in both cases (the capacitor's voltage is at 90 degrees and that makes the error very small).
Your biggest error source is probably how well the probes are compensated and the inherent accuracy of the scope. Worry about basic things first and you will find that the things you are asking about will be of very little importance.
Also, remember that your capacitance value probably has a 10 or 5 % tolerance. And that your test leads and wiring add tens of pF, which can't be ignored if you want to reduce errors.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: 1X or 10X Passive Voltage Probe?
2% tolerance on input resistance
Almost 20% tolerance on input capacitance
8-bit resolution on vertical, which means that its best accuracy with a 30V input is around 0.1 V
Anyway, back to your original question: neither, because an oscilloscope cannot support the accuracy you're asking for with any probe. Even a high performance meter would probably have trouble with an AC measurement near 1 MHz. Just in terms of digital measurement, you need at least 17 bits of single-ended resolution, which you're not going to find in most oscilloscopes.
TTFN
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