Author Topic: Active oscilloscope probe output and cable  (Read 465 times)

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Offline DanielDlouhyTopic starter

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Active oscilloscope probe output and cable
« on: May 21, 2024, 07:22:18 pm »
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!
« Last Edit: May 21, 2024, 08:50:53 pm by DanielDlouhy »
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Offline jonpaul

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Re: Active oscilloscope probe output and cable
« Reply #1 on: May 21, 2024, 07:36:35 pm »
Jean-Paul  the Internet Dinosaur
 
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Offline bdunham7

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Re: Active oscilloscope probe output and cable
« Reply #2 on: May 21, 2024, 07:50:54 pm »
I want my probe to have a 1MOhm output, but I am quite lost here :(

You 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.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 
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Online TimFox

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Re: Active oscilloscope probe output and cable
« Reply #3 on: May 21, 2024, 08:00:52 pm »
Double termination (50 \$\Omega\$ to 50 \$\Omega\$) is ideal, but has a gain of 1/2.
Series or back termination (50 \$\Omega\$ output 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.
 
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Offline DanielDlouhyTopic starter

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Re: Active oscilloscope probe output and cable
« Reply #4 on: May 21, 2024, 08:49:56 pm »

You definitely don't want your DP to have a 1M output impedance.
I 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?
« Last Edit: May 21, 2024, 08:59:21 pm by DanielDlouhy »
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Online TimFox

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Re: Active oscilloscope probe output and cable
« Reply #5 on: May 21, 2024, 09:02:15 pm »
You might want a bit less than 50 \$\Omega\$, subtracting the low inherent output impedance of your amp.
 
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Offline David Hess

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Re: Active oscilloscope probe output and cable
« Reply #6 on: May 22, 2024, 12:26:07 am »
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?

Termination 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.
 
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