Active oscilloscope probe output and cable - Page 1
Forum section: electronics / projects

author: DanielDlouhy
author_rank: Contributor
timestamp: May 21, 2024, 07:22:18 pm
content: Hello everyone,I am currently making a differential probe for my oscilloscope, the main circuitry is finnished and tested working. But I am wondering about the output... I want my probe to drive a 1MOhm scope input, but I am quite lost here. Most designes I saw on the internet use a 50 Ohm resistor in series with the output, right after the main differential amp. But if I understand that corretly, by doing this, and using a 50 Ohm cable you have to use a 50 Ohm terminator to reduce reflections. Or am I wrong? So my question is, what to do on the output of my probe and what cable to use, to have a 1MOhm output?(The maximum frequency of my probe is about 100 Mhz.)Thank you in advance!

author: jonpaul
author_rank: Super Contributor
timestamp: May 21, 2024, 07:36:35 pm
content: Oscilloscope Probe circuitshttps://w140.com/tekwiki/images/6/62/062-1146-00.pdfOscilloscope Probe measurementshttps://w140.com/tekwiki/images/1/19/062-1120-00.pdfhttps://w140.com/tekwiki/wiki/Concepts_SeriesRead and learnENJOY!Jon

author: bdunham7
author_rank: Super Contributor
timestamp: May 21, 2024, 07:50:54 pm
content: Quote from: DanielDlouhy on May 21, 2024, 07:22:18 pmYou definitely don't want your DP to have a 1M output impedance. If you intend to drive a 1M input impedance at the scope, then you'd want the output impedance of the amplifier to be low but not necessarily any precise number. If you are using a cable of any signficant length to the scope input, then you want it to be exactly 50R so it is at least back-terminated. 50R covers both scenarios and also allows you to use a scope with a 50R input.

author: TimFox
author_rank: Super Contributor
timestamp: May 21, 2024, 08:00:52 pm
content: Double termination (50to 50) is ideal, but has a gain of 1/2.Series or back termination (50output impedance from source, sum of amplifier output impedance and discrete resistor) is usually good enough and has a gain (ignoring load capacitance) of 1,000,000/1,000,050.

author: DanielDlouhy
author_rank: Contributor
timestamp: May 21, 2024, 08:49:56 pm
content: Quote from: bdunham7 on May 21, 2024, 07:50:54 pmI am sorry, I wrote that incorrectly... Yes I want to drive a 1M input impedance at the scope. So a 50 ohm series resistor on the output of my probe and a 50 ohm cable (about a 1m long) should do the trick?

author: TimFox
author_rank: Super Contributor
timestamp: May 21, 2024, 09:02:15 pm
content: You might want a bit less than 50, subtracting the low inherent output impedance of your amp.

author: David Hess
author_rank: Super Contributor
timestamp: May 22, 2024, 12:26:07 am
content: Quote from: DanielDlouhy on May 21, 2024, 07:22:18 pmTermination is only required on one side of the cable to stop reflections at the ends. When the 50 ohm series resistance drives the cable, the signal gets divided in half by the 50 ohm series resistance and the 50 ohm cable impedance. When the wave reaches the open 1 megohm end, the halved signal doubles restoring it to its original amplitude.In practice the series termination resistance is a little lower to make up for the driver's output resistance, and the input capacitance of the 1 megohm input causes some aberrations.