Keysight Labs | Filters, Impedances, and Comparators - Oscilloscope Front End Design (part 4) @KeysightLabs | Uploaded July 2017 | Updated October 2024, 11 hours ago.
Learn about signal conditioning and triggering techniques used in modern oscilloscopes!
Click to subscribe! ► bit.ly/Scopes_Sub ◄
Part 1: What is an Oscilloscope?
youtu.be/o-JvRlKCOaM
Part 2: Hardware Layout - What's Behind the BNC?
youtu.be/EaBSN6dpqP8
Part 3: Attenuators and Preamps
youtu.be/gPQRSWcRaFg
Part 4: Signal Offsets and Filters
youtu.be/1sVEWzMjkBo
Part 5: ADCs and Acquisition Boards
youtu.be/pdbzIwelCL4
Part 6: Probing and Q&A
Link coming soon!
------------------------------
Offset Subtraction
------------------------------
Offset subtraction is done in hardware. If you have a sine wave with a DC offset and want to only see the AC component, you can move the signal vertically on the oscilloscope screen. This happens even before the signal goes into the ADC and uses voltage subtraction. The voltage subtraction allows you to scale the signals differently without over saturating your oscilloscope's front end.
The caution is that you have to be careful with you power supply rails. That's why offset is limited in some oscilloscopes.
You can also use current subtraction instead of voltage subtraction. This uses a summing junction op amp, and the scope can subtract off the DC portion of the signal. But, too much loading causes the system to not be ideal, so again range is limited.
--------------------------------------
Impedance conversion
--------------------------------------
This allows you to drive the capacitance of the ADC. There's usually a button that allows you to change between the 50 ohm path and the high impedance path.
You pick the 50 ohm or the Mohm path based on your signal's impedance and the probes you have available.
There's also some buffering involved to protect your ADC.
-------------------------------
Hardware filtering
-------------------------------
Oscilloscope hardware filtering allows engineers to filter out noise, power supply coupling, or other noise sources in your system. The oscilloscope's hardware filter intentionally decreases the bandwidth of your acquisition system, which decreases the acquired noise.
Digital oscilloscopes can also do software filtering in the digital domain, and gives a lot more flexibility to engineers.
For Keysight, this also means the same front end can be used in different bandwidth oscilloscopes. The downside of digital filters is that it doesn't filter the signals before going into the ADC. So, a best practice is to also apply the hardware filter along side the software filter.
--------------------------------------------------------
Trigger comparators and filtering
---------------------------------------------------------
There also has to be some signal conditioning before the signal gets to the triggering system. For example, a signal with a lot of noise may be passed through a low pass filter to get a cleaner trigger.
Also, sometimes signals can be passed through a comparator to provide a cleaner edge to the oscilloscope trigger circuitry.
In analog oscilloscopes, there was a vertical deflection amplifier and a horizontal deflection amplifier. The horizontal deflection amplifier used a comparator to initiate the signal sweep across the screen.
If you add a second comparator, then you can start to get creative and digitally synthesize some triggers. For example, runt pulse triggers, pulse with triggers, rise time triggers, etc. This is one of the advantage of digital oscilloscopes over analog oscilloscopes.
---------------------
Links
---------------------
Twitter: @DanielBogdanoff twitter.com/DanielBogdanoff
Learn more about using oscilloscopes:
oscilloscopelearningcenter.com
Check out the EEs Talk Tech electrical engineering podcast:
youtube.com/playlist?list=PLzHyxysSubUnAMeCIi2-S0Vm7YtSAGqx_
The 2-Minute Guru Season 2 playlist:
youtube.com/playlist?list=PLzHyxysSubUlqBguuVZCeNn47GSK8rcso
The 2-Minute Guru Season 1 playlist:
youtube.com/playlist?list=PLzHyxysSubUkc5nurngzgkd2ZxJsHdJAb
More about Keysight oscilloscopes:
bit.ly/SCOPES
Check out our blog:
bit.ly/ScopesBlog
Like our Facebook page:
facebook.com/keysightbench
#tutorial #electronics #oscilloscopes #oscilloscope #engineering
#design #IC #ASIC #electrical #test #trigger #impedance
Learn about signal conditioning and triggering techniques used in modern oscilloscopes!
Click to subscribe! ► bit.ly/Scopes_Sub ◄
Part 1: What is an Oscilloscope?
youtu.be/o-JvRlKCOaM
Part 2: Hardware Layout - What's Behind the BNC?
youtu.be/EaBSN6dpqP8
Part 3: Attenuators and Preamps
youtu.be/gPQRSWcRaFg
Part 4: Signal Offsets and Filters
youtu.be/1sVEWzMjkBo
Part 5: ADCs and Acquisition Boards
youtu.be/pdbzIwelCL4
Part 6: Probing and Q&A
Link coming soon!
------------------------------
Offset Subtraction
------------------------------
Offset subtraction is done in hardware. If you have a sine wave with a DC offset and want to only see the AC component, you can move the signal vertically on the oscilloscope screen. This happens even before the signal goes into the ADC and uses voltage subtraction. The voltage subtraction allows you to scale the signals differently without over saturating your oscilloscope's front end.
The caution is that you have to be careful with you power supply rails. That's why offset is limited in some oscilloscopes.
You can also use current subtraction instead of voltage subtraction. This uses a summing junction op amp, and the scope can subtract off the DC portion of the signal. But, too much loading causes the system to not be ideal, so again range is limited.
--------------------------------------
Impedance conversion
--------------------------------------
This allows you to drive the capacitance of the ADC. There's usually a button that allows you to change between the 50 ohm path and the high impedance path.
You pick the 50 ohm or the Mohm path based on your signal's impedance and the probes you have available.
There's also some buffering involved to protect your ADC.
-------------------------------
Hardware filtering
-------------------------------
Oscilloscope hardware filtering allows engineers to filter out noise, power supply coupling, or other noise sources in your system. The oscilloscope's hardware filter intentionally decreases the bandwidth of your acquisition system, which decreases the acquired noise.
Digital oscilloscopes can also do software filtering in the digital domain, and gives a lot more flexibility to engineers.
For Keysight, this also means the same front end can be used in different bandwidth oscilloscopes. The downside of digital filters is that it doesn't filter the signals before going into the ADC. So, a best practice is to also apply the hardware filter along side the software filter.
--------------------------------------------------------
Trigger comparators and filtering
---------------------------------------------------------
There also has to be some signal conditioning before the signal gets to the triggering system. For example, a signal with a lot of noise may be passed through a low pass filter to get a cleaner trigger.
Also, sometimes signals can be passed through a comparator to provide a cleaner edge to the oscilloscope trigger circuitry.
In analog oscilloscopes, there was a vertical deflection amplifier and a horizontal deflection amplifier. The horizontal deflection amplifier used a comparator to initiate the signal sweep across the screen.
If you add a second comparator, then you can start to get creative and digitally synthesize some triggers. For example, runt pulse triggers, pulse with triggers, rise time triggers, etc. This is one of the advantage of digital oscilloscopes over analog oscilloscopes.
---------------------
Links
---------------------
Twitter: @DanielBogdanoff twitter.com/DanielBogdanoff
Learn more about using oscilloscopes:
oscilloscopelearningcenter.com
Check out the EEs Talk Tech electrical engineering podcast:
youtube.com/playlist?list=PLzHyxysSubUnAMeCIi2-S0Vm7YtSAGqx_
The 2-Minute Guru Season 2 playlist:
youtube.com/playlist?list=PLzHyxysSubUlqBguuVZCeNn47GSK8rcso
The 2-Minute Guru Season 1 playlist:
youtube.com/playlist?list=PLzHyxysSubUkc5nurngzgkd2ZxJsHdJAb
More about Keysight oscilloscopes:
bit.ly/SCOPES
Check out our blog:
bit.ly/ScopesBlog
Like our Facebook page:
facebook.com/keysightbench
#tutorial #electronics #oscilloscopes #oscilloscope #engineering
#design #IC #ASIC #electrical #test #trigger #impedance
