From: gherold@teachspin.com   
      
   On Tuesday, September 25, 2018 at 3:25:36 PM UTC-4, Tom Gardner wrote:   
   > On 25/09/18 16:20, George Herold wrote:   
   > > 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.)   
   >   
   > Not on the scopes I'm familiar with.   
   >   
   > The BNC is connected to the capacitor, and the other side   
   > of that is connected to the 1Mohm (not 10Mohm, doh!)   
   > vertical sensitivity attenuators.   
   >   
   > In DC mode the capacitor is shorted out so the BNC is   
   > connected to the attenuators. In AC mode the BNC is   
   > "floating".   
      
   Thanks Tom,  I need to correct what I said about the keysight scope   
   (DSOX 1102G) On AC it's 1 meg ohm for gains of 500 mV/ div and less   
   and 1.2 Meg at 200 mV and higher.   
      
   George H.   
   >   
   >   
   > >   
   > > 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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