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why 10:1 probe?

Question asked by dsula on Jan 1, 2012
Latest reply on Jan 3, 2012 by Willco788

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Hi,
I was wondering if somebody could explain to me why the scope input is usually 1MOhm to ground. I understand that this makes a 10:1 divider when using a 9MOhm probe, however what I don't understand is, why the division, why not use a high impedance input (GOhms) and not use any divider at all ?
Thank you for any explanation.

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algoss Jan 3, 2012 9:39 AM
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The division value (1:1, 10:1, 100:) in a passive probe, is set by the relative impedance of the probe tip, and the input to the scope. If the probe tip is 5MOhm, and the scope input is 1MOhm, then the divider is 5:1. The voltage seen by the scope is 1/5 of the voltage seen at the probe tip, so the scope must use math to multiply the voltage it sees, by the inverse of the divider value (5 in this case) to show the user the actual voltage at the probe tip.

Also, the BW of the probe is inversely related to the tip impedance. The higher the tip impedance, the longer it takes to charge the inherent capacitance of the tip, so the probe has a lower BW. This is why higher BW probes have a lower impedance, and the really high-impedance probes have impedance values of 25-50K, feeding a scope input with 50Ohms. These, of course, are active, in that they have amplifiers in them.

This is somewhat simplified, but the concepts hold...

Al
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dsula Jan 3, 2012 10:16 AM
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Thank you, but it doesn't explain much to me. And the question is only related to passive probes.
If I read your response I don't see ANY advantage of using a 10:1 probe. I loose BW and I loose signal amplitude. Why should I use a 10:1 probe?

I see an advantage though in that the 10:1 division also removes the scope capacitive input load from the circuit under test. Is that the reason people use 10:1?
However if this is the reason, back to my original question. Why not build a scope with Giga Ohms input impedance and just a 10MOhm probe (no 1MOhm scope internal to ground). Wouldn't that be better? I don't loose signal amplitude and I still have the scope input cap removed from the circuit through the 10 MOhm in the probe head. Of course I also pay the BW penalty.

Or phrased a bit differently, I don't understand why the scope input is terminated to GND through a 1M resistor. What would happen if I use my 10 MOhm probe, but I removed the 1MOhm resistor to gnd in the scope?

Am I actually making sense here?
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algoss Jan 3, 2012 10:45 AM
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"
If I read your response I don't see ANY advantage of using a 10:1 probe. I loose BW and I loose signal amplitude. Why should I use a 10:1 probe?
"

There is a maximum rated voltage input to the scope. A 10:1 probe allows you to look at higher voltages.

"
I see an advantage though in that the 10:1 division also removes the scope capacitive input load from the circuit under test.
"

The load I was talking about was the load of the probe, not the load of the scope. If you look at your probes, they will list an input impedance AND an input capacitance. The combination of those 2 together sets the input BW. If we double the input impedance, without changing the capacitance, you cut the BW in half. If you double the capacitance, without changing the impedance, you cut the BW in half.

"
Why not build a scope with Giga Ohms input impedance...
"

For the same reason as above. Every device every built has complex impedance, with resistive (R), capacitive (C), and inductive (L) components to that impedance. Balancing the RCL is an important part of designing a scope, or other measurement instrument, with flat, well-defined response.

"
I don't understand why the scope input is terminated to GND through a 1M resistor
"

It's not. That's the impedance of the amplifier or attenuator that feeds the A/D that allows you to see the signal.

Al
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Willco788 Jan 3, 2012 9:16 PM
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I can't speak for other models but on my DSOX3024, there is a 1 megohm resistor at the channel's input that can be measured with an ohm meter, when DC coupling is selected. I agree with algoss that a 10:1 probe is needed to allow measurements at high voltage levels. To measure voltages as high as 1000v, I use a 100:1 probe. Using a 1:1 probe, my DSOX3024 would be totally destroyed at that voltage level.

There are times when I measure low level signals with a 1:1 probe but you have to be very careful that you don't connect the 1:1 probe to a 1000v power supply.
When I'm done making low level measurements I immediately disconnect the probe. Someone else can borrow your scope without you knowing it and make a HV measurement.
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