From: gherold@teachspin.com   
      
   On Tuesday, September 25, 2018 at 11:06:40 AM UTC-4, Tom Gardner wrote:   
   > On 25/09/18 15:34, George Herold wrote:   
   > > I was thinking of making a DC (slow) x100 'scope probe by adding a   
   > > series 100 meg ohm resistor.  Any thing I should look out for?   
   > >   
   > > George H.   
   > >   
   >   
   > Don't forget the point David Hess frequently makes...   
   >   
   > If you are relying on the scope's 10Mohm input resistance   
   > to form the lower leg of the potential divider, then consider   
   > what happens if you turn the scope to AC input coupling. The   
   > *entire* input voltage appears across the scope's AC coupling   
   > capacitor.   
   >   
   Huh, I'm not seeing that at all.  Don't I still have 1 meg to   
   ground inside the scope?  (putting my DMM across my 'scope input   
   I measure 1 meg ohm for both DC and AC coupling.)   
      
   George H.   
   > That can and should be avoided by having a probe with the   
   > potential divider made from two resistors, one for the upper   
   > leg and the other is (in parallel with the scope/s input)   
   > the lower leg. Then if the scope is in AC mode the voltage   
   > "seen" by the scope will be slightly wrong, but the voltage   
   > will be limited by the potential divider.   
      
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