10x osciloscope probe - 2.2 Megaohm input resistance - Question - Page 1
Forum section: electronics / beginners

author: BinaryBits
author_rank: Contributor
timestamp: May 01, 2023, 12:11:42 pm
content: I am new with scopes, and i have tried to search the internet, but haven´t found an answer to my question.So i have this Agilent 10073C 10x probe which has an input impedance of 2.2 Megaohm.The specsheet for this probe also says "2.2 Megaohm input impedance when terminated into 1 Megaohm"So everywhere i read it says that 10x probes usually has 9 Megaohm input resistance. So if i use this particular probe with my Tektronix TDS220 scope, will the 2.2 Megaohm input impedance affect the readings on the scope, since 10x probes usually has 9 Megaohms, for a total of 10 Megaohms together with the scope, so maybe the scope is "expecting" 9 Megaohms when the scope is set to 10x?Or doesn´t the input impedance have anything to do with the times attenuated?Also, does it mean that i can´t measure as high of a voltage with this probe, since the impedance is lower than on the usual 10X probes?I haven´t noticed any obvious differences on the readings compared to my cheapo 1x/10x probe, but maybe i have missed something?So my question is, will my Tektronix scope see the readings through this 2.2 MOhm probe exactly the same way as through any 9 MOhm probe?Thanks in advance.

author: alm
author_rank: Super Contributor
timestamp: May 01, 2023, 12:27:01 pm
content: Theprobe datasheetsays attenuation ratio 10:1, so it will look like any other 10:1 probe to your scope. A 10:1 probe with 10 MOhm input resistance is (very simplified) a 9 MOhm resistor in series with the 1 MOhm input of the scope, forming voltage divider that divides by 10.The 10073C has been designed for a lower input impedance. This will have been achieved by putting something like 280 kOhm in parallel with the scope input for a total resistance of about 220 kOhm, and then a 1.98 MOhm series resistance to give a 10:1 attenuation ratio. So the only effect you'll notice is a slightly higher load on your circuit at DC. For maximum voltage check the probe datasheet / manual. There is not necessarily a direct relationship between impedance and maximum input voltage.You'll have to check if the probe compensates to your scope. The probe is specified as compensating to a scope input capacitance of 6 - 15 pF, and from memory the TDS has a higher input capacitance. But then the trimmer caps they use aren't known for their low tolerances, so you might get lucky and if you already have the probe you can easily test this by connecting the probe to the probe compensation output and see if you can trim it for a clean rising / falling edge without overshoot or undershoot.

author: HighVoltage
author_rank: Super Contributor
timestamp: May 01, 2023, 03:09:21 pm
content: If I remember right, the Agilent 10073C was the fastest passive scope probe that Agilent offered at the time.To get the fast rise times, the resistance was lowered.https://teledynelecroy.com/doc/probes-probing

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: May 01, 2023, 04:17:05 pm
content: alm has a good pointYour scope will not work correctly with the wrong capacitance probe. Your scope has a 20 pF input, your probe has a max 15 pf spec.Maybe the comp will work to 20, as alm suggests.If the probe is not matched your square wave will not be square and your higher freq responses will not be accurate. Things like rise time will not be accurate at all.But the probe will still "work"The resistance of that particular probe is not that important, except for very accurate readings.The cheap Chinese probes I have bought are just fine and in some ways better than the much more expensive HP probes that I have ( also made in China ) I do not recommend the switched probes 1X-10X. Besides the probes internal problems, operator problems like leaving the probe in 1X mode while sampling high volts may damage the scope. Don't ask me how I know this HA!! People do not use 1X much at all anyway.The important thing is that the scope capacitance matches the probe capacitance.After you are sure the scope works well, I would buy a pair for your nice new scope.

author: Eraldo
author_rank: Regular Contributor
timestamp: May 02, 2023, 04:28:10 pm
content: Leaving aside the capacitance problem, if you were to plug that probe into a tektronix (1meg imput impedance) you would basically have an x3.2 probe since x10 probes are just voltage dividers (the ratio between the measured voltage and displayed voltage in this case is 3.2).So if you were to set the oscilloscopes internal probe selection to x1, you would just need to multiply to 3.2 every voltage value of interest to find the real voltage you're measuring. (correct me if i'm saying something wrong here since i'm not sure if the true rms voltage can be found this way or for anything else)so in the end it would work but it would be very inconvenient because of the reason above( it not being a standard value like x10 or x100) and the oscilloscope doesn't display the correct values neither in x1 nor in x10. Not sure if there is a feature where you can set your own ratio of resistors in your device though of if it even exist as a feature in any other oscilloscope.Yeah just read the first comment of alm or a quick explanation below

author: alm
author_rank: Super Contributor
timestamp: May 02, 2023, 05:02:04 pm
content: Quote from: Eraldo on May 02, 2023, 04:28:10 pmNo, the ratio between the measured voltage and displayed voltage would be 10, since it's specified as a 10:1 probe when terminated in 1 MOhm. Read the specs more carefully.

author: Eraldo
author_rank: Regular Contributor
timestamp: May 02, 2023, 08:08:00 pm
content: Yeah i get it now that i reread your first comment again. So they are basically not only putting a resistor in series but one in parallel with the scope too. That makes more sense. First time i hear it though, and it being specified as only "2.2meg imput resistance" in the datasheet was a bit misleading (they didn't even add one of those notes at the end of the page)Thanks for correcting me

author: BinaryBits
author_rank: Contributor
timestamp: May 13, 2023, 09:41:14 am
content: Thanks, that clarifies it. I already have this probe, i got it with the TDS220. The trace looks normal when connected to the 1kHz calibration signal, i had to compensate the probe a bit though.But i haven´t used the probe for any other measurements, so i don´t know if it would start showing erroneous readings when making real measurements?Also, the voltage reading shows 5V, when connected to the calibration signal, as it should do.Exactly, the compensation range is 6-15pF on this probe according to the datasheet.Thanks for your answers, that was helpful.

author: pdenisowski
author_rank: Super Contributor
timestamp: May 13, 2023, 10:51:43 am
content: I made a video that discusses how 10X probes work (including the voltage divider part) and another on probe compensation)