Agilent 54622D capturing false pulses - Page 1
Forum section: electronics / testgear

author: gkmaia
author_rank: Frequent Contributor
timestamp: September 13, 2019, 05:24:05 am
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: September 13, 2019, 05:37:16 am
content: First rule of fault analysis is check the PSU is to spec.

author: gkmaia
author_rank: Frequent Contributor
timestamp: September 13, 2019, 06:19:06 am
content: Do you mean the PSU of the DUT or the test gear?

author: TK
author_rank: Super Contributor
timestamp: September 14, 2019, 12:53:43 am
content: What happens if you remove the probe from the scope? Does it still capture the ghost signals?

author: CDaniel
author_rank: Frequent Contributor
timestamp: September 14, 2019, 06:29:15 am
content: I see that you are measuring multiple channels simultaneously , maybe some crosstalking betwwen channels . If you use just D0 or D2 is the same ?

author: oPossum
author_rank: Super Contributor
timestamp: September 14, 2019, 06:34:52 am
content: What is the duration of high time of those pulses?

author: nctnico
author_rank: Super Contributor
timestamp: September 14, 2019, 07:01:47 am
content: 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.

author: gkmaia
author_rank: Frequent Contributor
timestamp: September 16, 2019, 08:30:58 pm
content: 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 calibratedI 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 5-10nsIndeed 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.

author: tautech
author_rank: Super Contributor
timestamp: September 16, 2019, 08:40:19 pm
content: 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.

author: gkmaia
author_rank: Frequent Contributor
timestamp: September 16, 2019, 09:51:10 pm
content: 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.

author: nctnico
author_rank: Super Contributor
timestamp: September 16, 2019, 11:28:58 pm
content: 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?

author: gkmaia
author_rank: Frequent Contributor
timestamp: September 16, 2019, 11:53:30 pm
content: Non official 40pin ribbon cable and HP original pod.

author: TK
author_rank: Super Contributor
timestamp: September 17, 2019, 12:14:06 am
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: September 17, 2019, 12:30:13 am
content: Quote from: gkmaia on September 16, 2019, 11:53:30 pmAre they a IDE PC drive cable ?If so swap it out for an 80 conductor one to minimize the chance of crosstalk.

author: gkmaia
author_rank: Frequent Contributor
timestamp: September 17, 2019, 07:33:15 am
content: I managed to eliminate the ghost signals completely.My ground hook was faulty. New ground hook, no ghost signals.Thanks for the help guys!

author: MarkL
author_rank: Unknown
timestamp: September 17, 2019, 01:19:13 pm
content: 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.