oscilloscope 50 ohm input termination

[alimjoco]

Hello,

I was thinking to use a 50 ohm input set in scope with a 50ohm BNC coaxial cable in measuring a voltage ripple from an output capacitor of a buck converter. The output voltage is 1.8v with a dummy resistor load of 3.7ohms. The BNC coaxial cable is properly terminated in scope input, will it still load the output of the converter?assuming I only have 1x attenuation dynamic range..

 

[kj.singh]

You are drawing a heavy current of ~500mA from the source. There won't be any loading by connecting to scope. The scope input impedance is at least in MOhms. I hope that I understood your query.

 

[alimjoco]

yes around 500mA. But take note that I will set the input impedance of the scope to 50ohms to in order for my BNC 50 ohm coax cable to be properly terminated and avoid reflections along line in higher switching frequencies..will that load the DUT..Any detailed technical explanation is fully appreciated

 

[kj.singh]

If you set the scope on AC coupling then it shouldn't load your DUT. Basically, if possible you should limit the higher frequency side too by limiting it to 20MHz if possible. Just keep the vertical divisions set to 5 or 10mV and time scale to 1/10 of a second, x1 on the probe is fine. You should be able to see the ripple waveform or further adjust the division knobs. Try to use the co-ax cable with a short length especially the ground to avoid heavenly noises otherwise 50Ohm won't do any harm. I hope this helps.