PicoScope 7 Automotive
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Shared Ground Question
Shared Ground Question
Hi All,
I'm still fairly new to oscilloscope use and I no longer work on cars so this is just a hobby for me and it's a lot to learn part time.
My question relates to floating and shared grounds on the scope, I have a pico 4423 which uses shared grounds as opposed to the newer scopes that have floating grounds.
I'm struggling to get my head around the issues I need to be aware of when undertaking multiple measurements due to the shared ground.
I fully understand that all 4 inputs share a ground so I can't try to use it to measure something referenced to ground at the same time as something that isn't. I.e it's not 4 separate multimeters.
How does this effect my measurements if I wanted to measure and inductive crank sensor, and a pair of hall effect cam sensors?
Assuming I connected both leads of input 1 to the crank sensor and then the earth leads of channel 2 and 3 to the battery ground and the hall effect output would that give me an incorrect reading on the crank or would I be connecting it wrong?
Many thanks
Andy
I'm still fairly new to oscilloscope use and I no longer work on cars so this is just a hobby for me and it's a lot to learn part time.
My question relates to floating and shared grounds on the scope, I have a pico 4423 which uses shared grounds as opposed to the newer scopes that have floating grounds.
I'm struggling to get my head around the issues I need to be aware of when undertaking multiple measurements due to the shared ground.
I fully understand that all 4 inputs share a ground so I can't try to use it to measure something referenced to ground at the same time as something that isn't. I.e it's not 4 separate multimeters.
How does this effect my measurements if I wanted to measure and inductive crank sensor, and a pair of hall effect cam sensors?
Assuming I connected both leads of input 1 to the crank sensor and then the earth leads of channel 2 and 3 to the battery ground and the hall effect output would that give me an incorrect reading on the crank or would I be connecting it wrong?
Many thanks
Andy
-
Steve Smith
- Pico Staff Member
- Posts: 1594
- Joined: Sun Aug 25, 2013 7:22 am
Re: Shared Ground Question
Hello Andy and thank you for the post
A great place to start regarding common ground (shared ground) scopes and floating input scopes is here https://www.picoauto.com/library/traini ... ing-inputs
Coming back to your scenario of simultaneously measuring Hall Effect Cam and Inductive Crank signals with the awesome common ground 4423, you have a number of options depending on the style of inductive crank shaft sensor/circuit architecture
With an Inductive crank signal using PCM/vehicle ground
You can measure all these signals reference to chassis ground as we describe here https://www.picoauto.com/library/automo ... e-running/
With an Inductive crank signal using a floating architecture (no ground reference on the crankshaft sensor wiring)
Again, your can measure both sides of the crank sensor wiring using channels A and B (if required) as we describe here https://www.picoauto.com/library/automo ... g-running/
This is only necessary if you wish to know the true amplitude of the crankshaft sensor signal. (i.e., if this is important to the measurement)
If amplitude is relevant, you can use the built-in math channel A-B to reveal (See below)
In the majority of cases, you may only be carrying out a correlation measurement (between Cam and Crank) and therefore just measuring one side of the floating crank sensor (referenced to chassis ground) is fine. Remember to measure the "Main" crank signal and not the "Sub" return floating signal as below (PicoScope 6 image used but refer to waveform structure only)
Regarding inductive crank signals, you may wish to measure DC coupled rather than AC coupled if you are focused upon crossing points and bias voltages as these will be changed when AC coupled. (They will be centered about 0 V)
A word to the wise about measuring inductive crankshaft sensors (either PCM ground referenced or floating)
There may be occasions when measuring such sensors, the engine may stall, run uneven, or set a DTC. This is down to the architecture of the PCM / Crank sensor circuit and in particular their sensitively to intrusion from a scope probe (even with 1 MΩ impedance)
In such a scenario (I remember a Mercedes Benz B class W245) a high impedance probe may be required https://www.picoauto.com/products/test- ... 00-mhz-bnc to measure the signal, or, if you need to determine amplitude, a differential probe will help here https://www.picoauto.com/products//elec ... e-x20-x200 where you can measure across the crank sensor and capture the differential voltage on a single channel of your scope
I hope the above information helps and please feedback for any clarification
Take care……Steve
A great place to start regarding common ground (shared ground) scopes and floating input scopes is here https://www.picoauto.com/library/traini ... ing-inputs
Coming back to your scenario of simultaneously measuring Hall Effect Cam and Inductive Crank signals with the awesome common ground 4423, you have a number of options depending on the style of inductive crank shaft sensor/circuit architecture
With an Inductive crank signal using PCM/vehicle ground
You can measure all these signals reference to chassis ground as we describe here https://www.picoauto.com/library/automo ... e-running/
With an Inductive crank signal using a floating architecture (no ground reference on the crankshaft sensor wiring)
Again, your can measure both sides of the crank sensor wiring using channels A and B (if required) as we describe here https://www.picoauto.com/library/automo ... g-running/
This is only necessary if you wish to know the true amplitude of the crankshaft sensor signal. (i.e., if this is important to the measurement)
If amplitude is relevant, you can use the built-in math channel A-B to reveal (See below)
- Image 1
- Image 2
A word to the wise about measuring inductive crankshaft sensors (either PCM ground referenced or floating)
There may be occasions when measuring such sensors, the engine may stall, run uneven, or set a DTC. This is down to the architecture of the PCM / Crank sensor circuit and in particular their sensitively to intrusion from a scope probe (even with 1 MΩ impedance)
In such a scenario (I remember a Mercedes Benz B class W245) a high impedance probe may be required https://www.picoauto.com/products/test- ... 00-mhz-bnc to measure the signal, or, if you need to determine amplitude, a differential probe will help here https://www.picoauto.com/products//elec ... e-x20-x200 where you can measure across the crank sensor and capture the differential voltage on a single channel of your scope
I hope the above information helps and please feedback for any clarification
Take care……Steve
Re: Shared Ground Question
If the newsletter hadn’t highlighted this post, I would have missed it. I’m really grateful this excellent question was asked: Steve’s answer has cleared up a misconception I had and has given me a much better understanding of this topic.
Steve’s mention of the possibility of the ‘scope’s loading the CKP signal made me revisit a problem I had a while ago. When I connect this CKP sensor breakout lead, the car runs rough or won’t start.
Steve’s reply had me wondering if this breakout lead, together with the rudimentary x1 probe was somehow causing a similar problem.
So I connected the breakout lead and captured a CKP waveform of the engine not starting with the breakout lead fitted:
On noticing that the signal looked normal except fot the induced secondary-ignition spikes (3 shown in screenshot), I routed all breakout and probe cables well away from the HT ignition. This time, the car started, ran perfectly, and showed no HT pickup:
So the morals of the story are: if a CKP breakout lead is used, or the CKP sensor is back-probed, make sure all cables are routed away from sources of induced-voltage pickup. And, if a measurement technique produces strange or unexpected results, investigate why: it could turn out to be an interesting learning opportunity.
Thanks Andy and thanks Steve.
Martin
Steve’s mention of the possibility of the ‘scope’s loading the CKP signal made me revisit a problem I had a while ago. When I connect this CKP sensor breakout lead, the car runs rough or won’t start.
- CKP breakout lead
Steve’s reply had me wondering if this breakout lead, together with the rudimentary x1 probe was somehow causing a similar problem.
So I connected the breakout lead and captured a CKP waveform of the engine not starting with the breakout lead fitted:
- Breakout lead picking up HT
On noticing that the signal looked normal except fot the induced secondary-ignition spikes (3 shown in screenshot), I routed all breakout and probe cables well away from the HT ignition. This time, the car started, ran perfectly, and showed no HT pickup:
- Breakout lead away from HT leads
So the morals of the story are: if a CKP breakout lead is used, or the CKP sensor is back-probed, make sure all cables are routed away from sources of induced-voltage pickup. And, if a measurement technique produces strange or unexpected results, investigate why: it could turn out to be an interesting learning opportunity.
Thanks Andy and thanks Steve.
Martin
Re: Shared Ground Question
Steve Smith wrote: ↑Mon Feb 07, 2022 10:48 amHello Andy and thank you for the post
A great place to start regarding common ground (shared ground) scopes and floating input scopes is here https://www.picoauto.com/library/traini ... ing-inputs
Coming back to your scenario of simultaneously measuring Hall Effect Cam and Inductive Crank signals with the awesome common ground 4423, you have a number of options depending on the style of inductive crank shaft sensor/circuit architecture
With an Inductive crank signal using PCM/vehicle ground
You can measure all these signals reference to chassis ground as we describe here https://www.picoauto.com/library/automo ... e-running/
With an Inductive crank signal using a floating architecture (no ground reference on the crankshaft sensor wiring)
Again, your can measure both sides of the crank sensor wiring using channels A and B (if required) as we describe here https://www.picoauto.com/library/automo ... g-running/
This is only necessary if you wish to know the true amplitude of the crankshaft sensor signal. (i.e., if this is important to the measurement)
If amplitude is relevant, you can use the built-in math channel A-B to reveal (See below)
Image 1.png
In the majority of cases, you may only be carrying out a correlation measurement (between Cam and Crank) and therefore just measuring one side of the floating crank sensor (referenced to chassis ground) is fine. Remember to measure the "Main" crank signal and not the "Sub" return floating signal as below (PicoScope 6 image used but refer to waveform structure only)
Image 2.png
Regarding inductive crank signals, you may wish to measure DC coupled rather than AC coupled if you are focused upon crossing points and bias voltages as these will be changed when AC coupled. (They will be centered about 0 V)
A word to the wise about measuring inductive crankshaft sensors (either PCM ground referenced or floating)
There may be occasions when measuring such sensors, the engine may stall, run uneven, or set a DTC. This is down to the architecture of the PCM / Crank sensor circuit and in particular their sensitively to intrusion from a scope probe (even with 1 MΩ impedance)
In such a scenario (I remember a Mercedes Benz B class W245) a high impedance probe may be required https://www.picoauto.com/products/test- ... 00-mhz-bnc to measure the signal, or, if you need to determine amplitude, a differential probe will help here https://www.picoauto.com/products//elec ... e-x20-x200 where you can measure across the crank sensor and capture the differential voltage on a single channel of your scope
I hope the above information helps and please feedback for any clarification
Take care……Steve
Steve,
Probing the topic a little further, with the 4423 as an example, am I right in thinking you could safely and reliably use up to 4 current clamps (self-powered from 9V batteries) at any one time? I believe this would be fine; is that correct?
What about a passive, standard x1 or x10 probe along with one or more current clamps; my instincts tell me that would also be fine?
And, of course, you could safely use up to 4 differential probes at any one time, but, other than ensuring you stay within the differential, common-mode and maximum voltage limits for the probes, is there anything else you’d need to think about before hooking up a differential probe?
Best wishes
Martin
-
Steve Smith
- Pico Staff Member
- Posts: 1594
- Joined: Sun Aug 25, 2013 7:22 am
Re: Shared Ground Question
Hi Martin, I hope you are well and sorry for my late reply
You can use 4 x current clamps with the 4423 or any Automotive scope if required and this is something I have done in the past when looking at 4 x injector current events
The beauty of such measurements is that they are non-intrusive and therefore their impact upon the circuit under tests is not applicable
Where you may see an issue is with ambient noise intruding upon the clamps/cables which you may have to filter accordingly. Remember, there will be no ground reference on a 4423 with 4 x current clamps connected and therefore the entire measurement system will be “floating” (No reference to chassis ground)
With that said, depending on the style of PC/Laptop the USB may be grounded via the ground pin of the mains power supply or charger which has the potential to improve ambient noise.
Equally, depending on quality of all components involved (mains, chargers and PC/Laptop) there could also be the complete opposite effect of the introduction of noise into the USB line and so BNC ground!
Your best bet here Martin is to determine which set-up delivers the optimal results with the 4423 and 4 x current clamps.
A floating laptop top with all clamp cables routed away from sources of noise sounds like the most probable solution
Sticking with the 4423 and a standard x1 and x10 probe along with one or more current clamps, this combination is fine as would be any mix and match of such probes.
As soon as you introduce a chassis ground reference to any of the scope channels, all channels will be referenced to chassis ground as the 4423 is a common ground scope. Our measurement system then becomes “grounded” to the vehicle ground (i.e., at the same potential as chassis ground)
Remember the introduction of a x10 probe includes an increase in the input impedance of the measurement system for the signal under test (i.e., the signal measured by the probe will be less “influenced” thanks to an increase in probe and scope impedance)
E.g.,
Input impedance with a x1 probe = 1 MΩ (Scope impedance)
Input impedance with a x10 attenuator = 8 MΩ (Scope & attenuator impedance)
Input impedance with a TA375 passive probe (x10) = 10 MΩ (Probe impedance)
There has been an occasion where a high impedance probe (TA375) measuring the Mercedes crank sensor (mentioned above) prevented the vehicle running uneven but still set a DTC for the crank sensor signal! How sensitive must that circuit have been & could this be the result of a non-OE crank sensor?
Using multiple differential probes (e.g., 4) in automotive applications was typically unheard of, however, “times they are a changing”
If we now think about EV’s and the likely measurements we are about to take with 3 phase motors, I can see the potential for requiring at least 3 differential probes (one for each phase reference to HV negative)
Remember, this is “live working” and the relevant certification and qualifications are required.
This style of measurement may be required if anomalies are found within the current of the motor phases (measuring current is the simpler and safer solution if access permits) i.e., you may wish to monitor the output of the IGBT’s against phase current
As you mention Martin, stay within the speciation’s of the diff probes regarding differential, common-mode, maximum voltages and be conscious of the impedance of multiple probes which are in parallel with one another when connected to the scope
I hope this helps, take care……Steve
You can use 4 x current clamps with the 4423 or any Automotive scope if required and this is something I have done in the past when looking at 4 x injector current events
The beauty of such measurements is that they are non-intrusive and therefore their impact upon the circuit under tests is not applicable
Where you may see an issue is with ambient noise intruding upon the clamps/cables which you may have to filter accordingly. Remember, there will be no ground reference on a 4423 with 4 x current clamps connected and therefore the entire measurement system will be “floating” (No reference to chassis ground)
With that said, depending on the style of PC/Laptop the USB may be grounded via the ground pin of the mains power supply or charger which has the potential to improve ambient noise.
Equally, depending on quality of all components involved (mains, chargers and PC/Laptop) there could also be the complete opposite effect of the introduction of noise into the USB line and so BNC ground!
Your best bet here Martin is to determine which set-up delivers the optimal results with the 4423 and 4 x current clamps.
A floating laptop top with all clamp cables routed away from sources of noise sounds like the most probable solution
Sticking with the 4423 and a standard x1 and x10 probe along with one or more current clamps, this combination is fine as would be any mix and match of such probes.
As soon as you introduce a chassis ground reference to any of the scope channels, all channels will be referenced to chassis ground as the 4423 is a common ground scope. Our measurement system then becomes “grounded” to the vehicle ground (i.e., at the same potential as chassis ground)
Remember the introduction of a x10 probe includes an increase in the input impedance of the measurement system for the signal under test (i.e., the signal measured by the probe will be less “influenced” thanks to an increase in probe and scope impedance)
E.g.,
Input impedance with a x1 probe = 1 MΩ (Scope impedance)
Input impedance with a x10 attenuator = 8 MΩ (Scope & attenuator impedance)
Input impedance with a TA375 passive probe (x10) = 10 MΩ (Probe impedance)
There has been an occasion where a high impedance probe (TA375) measuring the Mercedes crank sensor (mentioned above) prevented the vehicle running uneven but still set a DTC for the crank sensor signal! How sensitive must that circuit have been & could this be the result of a non-OE crank sensor?
Using multiple differential probes (e.g., 4) in automotive applications was typically unheard of, however, “times they are a changing”
If we now think about EV’s and the likely measurements we are about to take with 3 phase motors, I can see the potential for requiring at least 3 differential probes (one for each phase reference to HV negative)
Remember, this is “live working” and the relevant certification and qualifications are required.
This style of measurement may be required if anomalies are found within the current of the motor phases (measuring current is the simpler and safer solution if access permits) i.e., you may wish to monitor the output of the IGBT’s against phase current
As you mention Martin, stay within the speciation’s of the diff probes regarding differential, common-mode, maximum voltages and be conscious of the impedance of multiple probes which are in parallel with one another when connected to the scope
I hope this helps, take care……Steve
Re: Shared Ground Question
That’s excellent, Steve, thank you very much. This gives me added confidence in my use of the 4823. And this forum topic should become the reference answer for questions on this subject: I think you must have covered everything, thank you.
“ Remember the introduction of a x10 probe includes an increase in the input impedance of the measurement system for the signal under test (i.e., the signal measured by the probe will be less “influenced” thanks to an increase in probe and scope impedance) ”
I retained the 4 x10 probes when I sold my Fluke ‘scope and always use them in preference to a x1 for the very reasons you state. (And all the more so after I watched Dave (the Aussie) on his EEV blog video explaining the differences between x1 and x10.)
“ Remember, this is “live working” and the relevant certification and qualifications are required.”
“ Remember the introduction of a x10 probe includes an increase in the input impedance of the measurement system for the signal under test (i.e., the signal measured by the probe will be less “influenced” thanks to an increase in probe and scope impedance) ”
I retained the 4 x10 probes when I sold my Fluke ‘scope and always use them in preference to a x1 for the very reasons you state. (And all the more so after I watched Dave (the Aussie) on his EEV blog video explaining the differences between x1 and x10.)
“ Remember, this is “live working” and the relevant certification and qualifications are required.”
-
Steve Smith
- Pico Staff Member
- Posts: 1594
- Joined: Sun Aug 25, 2013 7:22 am
Re: Shared Ground Question
Thank you for the feedback Martin
Just a point to clarify re x10 attenuators and attenuated leads, these are fine for high voltage spikes of low energy such as primary ignition and induced voltages from injectors /solenoids etc., but when it comes to EV live working (i.e. measuring of High Voltage AC/DC) only a differential probe is to be used
I hope that helps, take care.....Steve
Just a point to clarify re x10 attenuators and attenuated leads, these are fine for high voltage spikes of low energy such as primary ignition and induced voltages from injectors /solenoids etc., but when it comes to EV live working (i.e. measuring of High Voltage AC/DC) only a differential probe is to be used
I hope that helps, take care.....Steve