EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: gkmaia on September 13, 2019, 05:24:05 am
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I got an Agilent 54622D which the LA is capturing pulses that do not exist.
None of the pulses you see on D0 or D2 of the attached image exist when probing with a passive probe on CH1.
My digital probe setup is all HP gold plated stuff oiled with D5Gold 100%. Would not expect it to have bad contact.
I am wondering what it could be...
ALSO if there is a threshold option to stop the digital to capture pulses shorter than 5ns for example.
Any help is appreciated.
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First rule of fault analysis is check the PSU is to spec.
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Do you mean the PSU of the DUT or the test gear?
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What happens if you remove the probe from the scope? Does it still capture the ghost signals?
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I see that you are measuring multiple channels simultaneously , maybe some crosstalking betwwen channels . If you use just D0 or D2 is the same ?
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What is the duration of high time of those pulses?
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I'd try a different threshold. Maybe the threshold is close to the floating/open input voltage. I've seen similar behaviour on logic analysers in general.
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Hey guys sorry for my delay... have been quite busy lately.
Answer to your questions:
PSU of DUT and test gear are spot on and calibrated
I did try some different thresholds and I can remove the ghost signals at the cost of removing the real signal as well. So that do not work.
High time duration of these pulses is around 5-10ns
Indeed you are correct about crosstalking. It is mostly caused by the 4 bit data bus. As soon as the bus is disconnected the ghost signals go away.
Removing the probe from the scope removes the ghost signals.
I have 3 of these 54622D. All of them same ghosting signal issue.
Yesterday I was probing with a new 40pin ribbon cable I made over the weekend and it has shown some improvement. Also when I connected the ground of CH1 probe to the DUT the ghost signals improved significantly.
If you look at the scans bellow on "better grounding" you can see 3 ghost signals on the VSS line. While on "initial issue" scan D2 is the Vss line and has way more ghost signals.
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I'd suspect a coupling/decoupling issue on vss and vdd rails that bounce the rails to the LA thresholds.
Probing those rails with a scope could confirm or not if this is the problem.
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If you look at the top photo you will see there is a Vss channel at the bottom which is LA with ghost signals. And at the top the same Vss rail but being probed with an analog passive probe in CH1 and it does not pick absolutely nothing.
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If you see spikes on VSS and VDD then there is something seriously wrong with your test setup. I've never seen this behaviour with these scopes and I have a couple of hundred hours of digital probe use under my belt. Are you using the official HP/Agilent cables or home-brew?
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Non official 40pin ribbon cable and HP original pod.
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I have the previous model of the 54622D with a similar custom made ribbon cable setup... I never saw this effect. There must be something wrong with the probe you made. Check the pinout... all one side of the 40-pin cable is GND... there is also 5V.
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Non official 40pin ribbon cable and HP original pod.
Are they a IDE PC drive cable ?
If so swap it out for an 80 conductor one to minimize the chance of crosstalk.
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I managed to eliminate the ghost signals completely.
My ground hook was faulty. New ground hook, no ghost signals.
Thanks for the help guys!
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Great - glad you found the problem.
One comment on using home-made cables for the logic inputs. The HP/Agilent/Keysight supplied cables are not ordinary ribbon cables. They are lossy, similar to scope probe cables, to improve signal fidelity by reducing reflections.