Author Topic: Input impedance oscilloscope  (Read 1814 times)

0 Members and 1 Guest are viewing this topic.

Offline alin_im

  • Regular Contributor
  • *
  • Posts: 164
  • Country: gb
Input impedance oscilloscope
« on: June 03, 2015, 10:49:07 am »
Hy guys,

I have a have an oscilloscope that has the imput impedance of 1M, do you think i can change the input impedance to 50 ohm if i put a 50 ohm resistor in parallel with the input, i also what to try with 75 ohm and 600 ohm. Do you think i could get some problems with this idea or it is perfectly fine.
One man's 'magic' is another man's engineering. 'Supernatural' is a null word. - Robert A. Heinlein
 

Offline Dago

  • Frequent Contributor
  • **
  • Posts: 659
  • Country: fi
    • Electronics blog about whatever I happen to build!
Re: Input impedance oscilloscope
« Reply #1 on: June 03, 2015, 11:24:27 am »
Hy guys,

I have a have an oscilloscope that has the imput impedance of 1M, do you think i can change the input impedance to 50 ohm if i put a 50 ohm resistor in parallel with the input, i also what to try with 75 ohm and 600 ohm. Do you think i could get some problems with this idea or it is perfectly fine.

You can buy a "pass-through terminator" which has an integrated 50 ohm termination. Or a simple T-adapter with a terminator in one leg works fine.

Do notice that your oscilloscope probe/cable is not matched to 50 ohm however. So you need to connect your circuit/device to the scope with a regular 50 ohm coax.
Come and check my projects at http://www.dgkelectronics.com ! I also tweet as https://twitter.com/DGKelectronics
 

Offline tggzzz

  • Super Contributor
  • ***
  • Posts: 12794
  • Country: gb
    • Having fun doing more, with less
Re: Input impedance oscilloscope
« Reply #2 on: June 03, 2015, 02:30:11 pm »
I have a have an oscilloscope that has the imput impedance of 1M,

No, you don't. You probably have 1Mohm in parallel with 20pF - calculate the impedance of 20pF at, say, 100MHz and you will see the 1Mohm is completely irrelevant.

Quote
do you think i can change the input impedance to 50 ohm if i put a 50 ohm resistor in parallel with the input, i also what to try with 75 ohm and 600 ohm. Do you think i could get some problems with this idea or it is perfectly fine.

It works well, and a few scopes already have a 50ohm termination option built into them.

A real advantage of a 50ohm input is that it allows "low" impedance Z0 probes to be used - which have a higher impedance than a traditional 1Mohm *10 probe, and have an upper frequency limit that is 10* higher than 1Mohm probes.

See https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/ for more information.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf