Rigol DS2072A termination issue

[SharpEars]

When I change the internal termination from 1 MOhm to 50 ohm and try to measure a DC voltage of 5V using the included 10X probe (with the scope itself properly configured for 10X and 5V per division), I am getting a reading of zero volts. The 1 MOhm setting is working correctly and showing the full 5V. What could be causing this issue?

[coppice]

When I change the internal termination from 1 MOhm to 50 ohm and try to measure a DC voltage of 5V using the included 10X probe (with the scope itself properly configured for 10X and 5V per division), I am getting a reading of zero volts. The 1 MOhm setting is working correctly and showing the full 5V. What could be causing this issue?

50/9000050 is a pretty small multiplier. Its hard to distinguish a signal that small from none at all.

[SharpEars]

When I change the internal termination from 1 MOhm to 50 ohm and try to measure a DC voltage of 5V using the included 10X probe (with the scope itself properly configured for 10X and 5V per division), I am getting a reading of zero volts. The 1 MOhm setting is working correctly and showing the full 5V. What could be causing this issue?

50/9000050 is a pretty small multiplier. Its hard to distinguish a signal that small from none at all.

I don't understand. I thought that the 50 ohm termination would have just placed a 50 ohm load on the voltage source I was measuring, but still measure correctly on the scope voltage wise assuming that the voltage source was not severely impacted by the 50 ohm load (i.e., could easily source 100 mA).

[wraper]

I don't understand. I thought that the 50 ohm termination would have just placed a 50 ohm load on the voltage source I was measuring, but still measure correctly on the scope voltage wise assuming that the voltage source was not severely impacted by the 50 ohm load (i.e., could easily source 100 mA).

If you would use just a coax cable instead of the probe, that would be true. Did you forget that it is 10:1 probe? It don't divide voltage by miracle. 10:1 probe have 9 MOhm resistance which combined with 1 MOhm input resistance results in 10 MOhm total input resistance with 10:1 dividing coefficient. When you activate 50 OHm termination, total input resistance will become 9M of probe resistance + 50 OHm of oscilloscope input resistance. Therefore you make a voltage divider with 10M 9M and 50R resistors. So you just made a 200 000:1 180 000:1 probe  .

[SharpEars]

I don't understand. I thought that the 50 ohm termination would have just placed a 50 ohm load on the voltage source I was measuring, but still measure correctly on the scope voltage wise assuming that the voltage source was not severely impacted by the 50 ohm load (i.e., could easily source 100 mA).

If you would use just a coax cable instead of the probe, that would be true. Did you forget that it is 10:1 probe? It don't divide voltage by miracle. 10:1 probe have 9 MOhm resistance which combined with 1 MOhm input resistance results in 10 MOhm total input resistance with 10:1 dividing coefficient. When you activate 50 OHm termination, total input resistance will become 9M of probe resistance + 50 OHm of oscilloscope input resistance. Therefore you make a voltage divider with 10M and 50R resistors. So you just made a 200 000:1 probe  .

DUH, I am a big idiot! 

[wraper]

Btw, don't play with termination resistor. It isn't of some infinite power type, just SMT. Easy way how to damage your oscilloscope by applying larger voltage to the input.

[Mark_O]

DUH, I am a big idiot! 

Don't feel too bad.  You could be the guy doing the "who's doin' dat?" ads for Teledyne on their LeCroy scopes. 

Now that would be a real reason for hanging your head in shame and humiliation.   

[miguelvp]

Like wraper mentions, don't play with the internal termination, purchase a t-connector and and external 50 Ohm terminator.

It just takes one time that you are in a hurry and forget you left the internal termination set to 50 Ohms and ruin your day the next day when you think it was set to 1Meg.