getting lower voltage readings on an oscilloscope - Page 1
Forum section: electronics / beginners

author: sahko123
author_rank: Frequent Contributor
timestamp: June 13, 2020, 01:01:49 pm
content: I want to know if setting the 10x or 100x scope probe compensation on a digital oscilloscope is a good idea to get from 1mv/div down to 100uv/div or even 10uv/div. Is this compensation normally done digitally or using analog circuitryor does it depend on the scope? (I need this low a voltage to read the voltage coming out of my transistor matching circuit and my current multimeter is only 2000 count at 200mv min (Ive got a better rigol 240,000 count one coming))

author: bdunham7
author_rank: Super Contributor
timestamp: June 13, 2020, 02:19:38 pm
content: No, it works backwards from what you are thinking. The 10X or 100X settings tell the scope that you are using a 10X or 100X voltage-dividing probe. A 10X probe is not an amplifier, it is an attenuator, so if you set the scope to 1V/div and 10X probe, it actually measures 100mV/div and displays it as 1V/div.I don't know your scope model, but unless it is extraordinary, you are unlikely to get anything more sensitive than about 2mV/div, and if it is an entry-level DSO, noise will consume most of that. You would need a low-noise, reduced-bandwidth preamp to do what you want with a scope.

author: rstofer
author_rank: Super Contributor
timestamp: June 13, 2020, 02:28:37 pm
content: Compensation usually refers to an adjustment of the frequency response of the probe. It has to do with overshoot on fast edges.Here is a video that tackles the probes Dave has done similar videos, Google for them.

author: sahko123
author_rank: Frequent Contributor
timestamp: June 13, 2020, 02:36:22 pm
content: I know that if you set the 10x on the probe it will attenuate but dont scopes have a compensation to read the voltage correctly? Like you set the 10x probe to 10x from 1x which divides the input by 10 and then in the scope multiply it by 10 again to show the correct reading on screen except in this case you set the probe to 1x instead of 10x but leave the internal multiplier in the scope to show lower voltage readings. I also only need to do dc readings so i can turn on averaging and reduce the noise

author: sahko123
author_rank: Frequent Contributor
timestamp: June 13, 2020, 02:37:21 pm
content: i also didnt mean frequency compensation i meant attenuation compensation

author: bdunham7
author_rank: Super Contributor
timestamp: June 13, 2020, 02:40:57 pm
content: Quote from: sahko123 on June 13, 2020, 02:36:22 pmNo, it doesn't amplify anything, it just changes the numbers on the screen. So 100mV/div @ 1X and 1V/div @ 10X use exactly the same vertical gain but display a different scale number. You can see that if your minimum 1X scale is 1mV/div, then your minimum 10X scale will be 10mV/div.

author: sahko123
author_rank: Frequent Contributor
timestamp: June 13, 2020, 02:47:57 pm
content: would it not be 1x = 1mv/div to 10x = 0.1mv/div (probes disregarded just the scopes internal rescaling) or 1mv/div visibly rescaled to be 1mv/10div or 0.1mv/div

author: bdunham7
author_rank: Super Contributor
timestamp: June 13, 2020, 03:00:07 pm
content: No, like I said you have it backwards. The purpose of telling the scope what type of probe you have connected is so that it displays the correct scale.Let's say you have two channels, both set to 1V/div and 1X and the zero point right in the center. On the first, you connect it directly to 1 volt. On the second, you connect it to 10 volts, but with a 10X probe--but you don't tell the scope about the 10X probe. Both channels will display 1 volt and their lines will overlap. The 10X probe reduces the 10 volts by a factor of 10.Now you change the probe setting to 10X--you 'tell' the scope about the 10X probe. You may have to readjust, depending on the scope, but you should now still have two lines at the same spot, but with channel #1 reading 1V, but channel #2 reading 10V. Both vertical amplifiers are seeing the same 1 volt and displaying it at the same spot--just the number on channel #2 has changed.

author: sahko123
author_rank: Frequent Contributor
timestamp: June 13, 2020, 03:15:49 pm
content: But if you tell the scope it has a 10x probe without actually plugging in the 10x probe just a 1x probe would it just change the number on the screen without scaling up the visual representation? like a signal going from being about a tenth of a division up from the center line to being a whole division up? (assuming a division is divided into 10)(At this point i think I'm being dumb but just not realising how)

author: bdunham7
author_rank: Super Contributor
timestamp: June 13, 2020, 03:19:49 pm
content: Yes, if you change to 10X without adding a 10X probe, it will indeed display a reading as 10 times the actual voltage.However, the flaw in your plan will become obvious when you try it because the minimum scale setting on your scope will be 10 times as high. So you can have say 1mV/div at 1X or 10mv/div at 10X. So your maximum actual sensitivity will be 1mV/div (or whatever your scope has) no matter what. Try it. You don't even need any probes or signals to verify this.