Logic Inverter "noise" - Page 1
Forum section: electronics / general

author: staze
author_rank: Frequent Contributor
timestamp: July 15, 2013, 07:20:01 pm
content: All,I'm working on diagnosing a troubled Multimeter (here:https://www.eevblog.com/forum/chat/bench-multimeter-(another)/) and one thing that's come up is the hex inverter seems to be generating runt/crap pulses now and again (see image). The input to the inverter is "perfect".So, I'm curious since I can't find anything anywhere... does it make sense that an inverter would only randomly produce crap (rather than always)? What would cause a failure like this?Thanks!

author: c4757p
author_rank: Super Contributor
timestamp: July 15, 2013, 07:25:58 pm
content: Check the rails powering the inverter. Could be power supply noise due to a faulty cap. Failing that, probe both input and output at the same time; see what the input does precisely during generation of these crap pulses. If all the inputs are fine (including power) and the output is not, the only one to blame is the inverter itself; replace it.

author: SeanB
author_rank: Super Contributor
timestamp: July 15, 2013, 07:32:34 pm
content: If it is in a socket it could have an intermittent ground pin, or the bond wire internally is broken and barely making contact.

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 15, 2013, 07:40:52 pm
content: Is this the output of U49E? Did you replace the chip?

author: staze
author_rank: Frequent Contributor
timestamp: July 15, 2013, 08:21:37 pm
content: Wow, stupid non-standard notify.I haven't done both input and output at the same time (lack hands), but the input never wavers a bit. The output is the troublesome part. I'm just confused why only occasionally it would be bad.No, it's not in a socket, but I've ordered a replacement chip (NTE74HCT04) and plan to install it in a socket (since there's room). So no, haven't replaced the chip yet. And yeah, it's the output from U49E.The rails look perfect. No noise at all.I guess my only concern was some loading on the output that was causing the noise, but it doesn't sound like that's likely. So I'll just wait for the replacement to arrive and get it swapped. Either way, my understanding is TTL level signals really need to be within about 5% in order to work, so those noisy/runty ones probably aren't doing their job?

author: AndyC_772
author_rank: Super Contributor
timestamp: July 15, 2013, 09:01:32 pm
content: That output looks like it's struggling to pull down to GND. Check the ground connection to the IC, and if that's OK, change the device. I'd speculate that the lower (N channel) output FET in the inverter has died.

author: staze
author_rank: Frequent Contributor
timestamp: July 15, 2013, 09:04:23 pm
content: Quote from: AndyC_772 on July 15, 2013, 09:01:32 pmOkay, that makes sense... except, why would it produce SOME good TTL pulses, and some bad ones?I'll check the ground (resolder), and see what I get. But it sounds like it's an issue with the IC, and not something further down the line. So I'll replace if it's still showing as bad.Any idea what would cause this? Just, random, or static discharge, or...?

author: AndyC_772
author_rank: Super Contributor
timestamp: July 15, 2013, 09:13:24 pm
content: Speculation again, of course... maybe an internal connection has failed, but a transistor gate is still getting a weak signal by capacitive coupling. It looks as though each pulse is slightly stronger than the one before it, which suggests that something is happening which depends on time. It's the sort of behaviour you see when a high impedance node is floating, and picks up signals by capacitive coupling from nearby.

author: staze
author_rank: Frequent Contributor
timestamp: July 15, 2013, 10:52:53 pm
content: Quote from: AndyC_772 on July 15, 2013, 09:13:24 pmOh, that makes perfect sense. okay. Anyway, I'll post back once I've replaced it (once the part gets here). I just don't have a pile of logic chips around (sadly). I'm guessing, and maybe I'm wrong, this modern version will be a bit more robust as well... I know it's got protection from ESD, where I bet this original one didn't... but I don't recognize the brand mark on there, so I have no idea.

author: staze
author_rank: Frequent Contributor
timestamp: July 17, 2013, 09:22:06 pm
content: As another note, since I'm still waiting for the replacement part, the output of the inverter on another "channel" is extremely noisy (the "strobe" channel on the Keithley 199. The input is clean, but the output is crap. Rather than being a 5V "0" and a 0V "1" or whatever, that 5V "0" is just high speed oscillation. So yeah, clearly something is wrong.I'm going to unsolder and put in a 14pin socket while I wait for the part.

author: PedroDaGr8
author_rank: Super Contributor
timestamp: July 17, 2013, 10:59:35 pm
content: This same IC failed on my Keithley 199. Except unlike his it failed with a constant 5V output. Since I wasn't sure if the 5V came from this IC or was injected later, I removed the IC, probed the same line and it's back fluctuating at around 1mV (which I am assuming is just noise I have picked up). The input is nice and clean with no real obvious issues.

author: staze
author_rank: Frequent Contributor
timestamp: July 17, 2013, 11:02:00 pm
content: Quote from: PedroDaGr8 on July 17, 2013, 10:59:35 pmSo, when you tested out of circuit, you provide a 5V rail, ground the other (well, negative rail on the power supply), and then feed it some TTL level pulses from an FG?Thanks!

author: PedroDaGr8
author_rank: Super Contributor
timestamp: July 17, 2013, 11:15:08 pm
content: Quote from: staze on July 17, 2013, 11:02:00 pmNo I didn't go that far into it. I just checked to see the pin to see if 5V was still present (which it would be if the 5V was injected from farther up). I actually planned on testing that tonight; I found an exact match of this IC on a scrap board at work, out of an old fraction collector. I planned on testing the two side by side. Though in hind sight I should have tested it earlier because it is easy to do.

author: staze
author_rank: Frequent Contributor
timestamp: July 17, 2013, 11:18:01 pm
content: ah, okay then. =)I'm actually ordering some basic logic chips to have around (74HC stuff) for cases like this... waiting for stuff from China/Thailand, or in this case, New Jersey, is a bit annoying. The Aussies on this forum are spoiled with having a Jaycar/Farnell. Best we have is Frys, if you're close enough to one (closest for me is about 100 miles). =/ I miss the good ol' days of Radioshack.

author: PedroDaGr8
author_rank: Super Contributor
timestamp: July 17, 2013, 11:21:03 pm
content: Quote from: staze on July 17, 2013, 11:18:01 pmI'm about 20min away from a Fry's. Oddly enough I didn't even think of them for getting the part. Despite the fact when Im there I often pick up an IC or two just on principle.

author: staze
author_rank: Frequent Contributor
timestamp: July 17, 2013, 11:23:58 pm
content: Do they have much in the way of Logic IC's? If I was heading to Portland anytime soon, I would have just done that.

author: PedroDaGr8
author_rank: Super Contributor
timestamp: July 17, 2013, 11:28:16 pm
content: Quote from: staze on July 17, 2013, 11:23:58 pmI just checked and the have the NTE74HCT04 and it is in stock near me for $1.http://www.frys.com/product/1003903

author: staze
author_rank: Frequent Contributor
timestamp: July 17, 2013, 11:31:22 pm
content: Quote from: PedroDaGr8 on July 17, 2013, 11:28:16 pmboo....

author: MacAttak
author_rank: Unknown
timestamp: July 18, 2013, 12:19:39 am
content: Frys out here hasn't been so great. Almost all of their stock is wildly overpriced (if they have what you want at all) and ancient. And frequently with crushed pins and/or suspicious packaging.Easier and cheaper to just buy from digikey.

author: staze
author_rank: Frequent Contributor
timestamp: July 18, 2013, 12:22:11 am
content: Quote from: MacAttak on July 18, 2013, 12:19:39 amYeah, I actually just ordered from RS over eBay. Free shipping, and reasonable prices. they do free FedEx Ground, which kind of sucks since it's slow. But, it is what it is.There is a small shop here in town, but I'm guessing they'd want $5 for a 74HTC04. =/ I'm willing to pay double for something small like that locally... but sometimes it's just crazy.

author: staze
author_rank: Frequent Contributor
timestamp: July 18, 2013, 05:42:19 am
content: So, being a glutton for learning (or at least, confirming), I pulled U49 (the hex inverter), and breadboarded it with a +5V rail, and ground (from my bench PSU), then fed it a 5Khz 5Vpp square wave to the 4 inputs I could see were used on it (guess I could check the two unused ones, but why?). For interest, I set my FG to HiZ "destination" (50R source), put a Tee on the output, and put one output directly into channel 2 on the scope (blue trace), and the other half of the T goes to some minigrabbers that I hooked to the input/gnd.Something is DEFINITELY wrong. Two of the inverters (1A and 5A) give basically no output (although it still looks capacitive), one (2A) gives something that definitely looks capacitive, and the other output (3A)... god, hell if I know what it's doing. I do know on a couple of the inverters, when I'd feed it a signal, the IC would draw about 30mA of power. On the other 2, it would draw about 10mA. I do know the 3A output was basically what I was seeing in circuit (drives the strobe channel).Also of interest, all of them seem to either offset the input voltage, drag it down, or completely destroy it. This could be my fault for not understanding something required... maybe the FG can't drive enough... but I would think the inverter inputs would take VERY little current to drive.Definitely failed.

author: staze
author_rank: Frequent Contributor
timestamp: July 18, 2013, 07:03:57 am
content: I just realized I did something dumb. I was feeding in a square wave... with a 0V offset... these are CMOS/TTL chips... so the signal shouldn't go negative. doh.either way, chip definitely seems borked. will retest tomorrow with a proper signal. gah.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: July 18, 2013, 07:07:18 am
content: Quote from: staze on July 18, 2013, 05:42:19 amYep. 74HC CMOS inputs are very high impedance ... as long as the input voltage stays between the power rails.

author: staze
author_rank: Frequent Contributor
timestamp: July 18, 2013, 07:08:06 am
content: Quote from: ElectroIrradiator on July 18, 2013, 07:07:18 am.......*sigh*........

author: staze
author_rank: Frequent Contributor
timestamp: July 18, 2013, 07:23:30 am
content: not being able to sleep, I fired the scope, fg, and psu back up, and tested again with a +pulse this time... and largely similar results. Could be I toasted it with my square wave testing previously... ah well. Also note from looking online, CMOS logic chips should have all their inputs and outputs connected, even if not in use... so maybe that's part of the issue too.Either way, looks like the chip is still bad. Wish the replacement would hurry up and get here. I found RS has 74LS series, but I understand you generally don't want to downgrade your logic. Maybe I'll get impatient and go shopping locally tomorrow. =P

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 18, 2013, 07:25:53 am
content: I still think the chip was bad in the first place, so all you could have done is make it a little worse... good luck being patient

author: staze
author_rank: Frequent Contributor
timestamp: July 18, 2013, 07:32:17 am
content: Quote from: PA0PBZ on July 18, 2013, 07:25:53 amTotally agree. And, yeah... I just want to get it off the bench and onto the shelf. I got other projects to tackle! =)

author: smashedProton
author_rank: Frequent Contributor
timestamp: July 19, 2013, 04:35:59 am
content: Please forgive me if I say something dumb, I am a beginner.Wouldn't it be best to just use a 74hc14 schmitt inverter and call it good?

author: David_AVD
author_rank: Super Contributor
timestamp: July 19, 2013, 04:40:55 am
content: Quote from: staze on July 18, 2013, 07:23:30 amUnused CMOS inputs should always be tied high or low. Outputs don't give a rats.

author: staze
author_rank: Frequent Contributor
timestamp: July 19, 2013, 05:10:25 am
content: Quote from: David_AVD on July 19, 2013, 04:40:55 amAh, okay, that makes sense. just for S's and G's I'll try that tomorrow and see if my tests change at all.

author: David_AVD
author_rank: Super Contributor
timestamp: July 19, 2013, 05:30:12 am
content: Floating inputs (even on "unrelated" gates) can cause all sorts of hell to break loose.

author: staze
author_rank: Frequent Contributor
timestamp: July 19, 2013, 06:00:40 am
content: Okay, that didn't seem to make much of a diff. I tied all the inputs to my signal (10Khz, 2.5V +pulse), and all the outputs look pretty much the same... about 800mV, and not even inverted. =/Is there something wrong with doing it that way? Interestingly, the input from my FG was DC offset by 1V (FG to Tee, one half to my scope, other to the circuit, FG set to HiZ). So the 74HCT04 was pushing 1V BACK into my FG output and offsetting the pulse. =/ Guessing that shouldn't be the case...

author: David_AVD
author_rank: Super Contributor
timestamp: July 19, 2013, 06:17:57 am
content: Sounds like a fundamental issue at play. Got a picture of the setup?

author: staze
author_rank: Frequent Contributor
timestamp: July 19, 2013, 06:39:34 am
content: Sure.Inputs are 1,3,5,9,11,13. All are tied to the bus on the side, which I'm feeding with the FG. Red clip is from the FG, Black clip is the "gnd" from the FG.Red wire heads off to my PSU positive with +5V rail. Black wire (where the black clip is) also heads off to the PSU gnd (negative). My scope probe is clipped to that wire out of frame. Obviously pin 7 is gnd, pin 14 is +5V (Vcc).Just testing the output pins with my scope. Once I got it all hooked up, the chip was pulling 20mA of current at 5V from my PSU.I don't THINK there's any issue with this, but I'm just a beginner, so I could be completely off base. Mind you, this is the chip that appeared to be bad from all other testing, and in circuit.

author: David_AVD
author_rank: Super Contributor
timestamp: July 19, 2013, 06:47:12 am
content: Two problems:1) No decoupling capacitor from Vss to Vdd.2) It looks like you've gotten the pinout of the chip wrong. Check the data sheet again. (Vcc is pin 14 and Vss in pin 7)

author: staze
author_rank: Frequent Contributor
timestamp: July 19, 2013, 07:01:17 am
content: http://www.nxp.com/documents/data_sheet/74HC_HCT04.pdfInputs are 1,3,5,9,11,13. Outputs 2,4,6,8,10,12. Vss is pin 14, Vdd is pin 7. Oh, son of a... how the hell did I get the impression the pins went clockwise... thinking about it... gar....Didn't know you needed a decoupling cap between those. I just looked at this:http://www.vagrearg.org/content/decouplingStill not sure what size cap would be appropriate... that said, the issue doesn't seem to be stalled output (failing to rise fully) (as seems to be indicated on the page above), it's that it just doesn't really come up at all... but I'll add one and see what I get.Alright, I'll retest once I know what size cap to use. It better still be bad... I KNOW I was testing it right in circuit (since I referenced everything off Vcc and Vdd, and the inputs were all clean, and outputs bad).Gar.... thanks, can't believe I did that.

author: David_AVD
author_rank: Super Contributor
timestamp: July 19, 2013, 07:04:49 am
content: The screwed up pinout meant that +5 and ground weren't even going to the correct pins.Use a 100n decoupling capacitor between pins 7 and 14.EDIT: To be clear, pin 14 is Vdd (+5) and pin 7 is Vss (ground).

author: staze
author_rank: Frequent Contributor
timestamp: July 19, 2013, 07:16:39 am
content: okay, just fixed it. Used a 100nF (tested at 92.6nF) cap across pins 7 and 14.Inverter A1, A2, and A5 just give solid 5V output. A3 and A5 give 0V output. A6 comes close with an inverted pulse, but rather than being 5V to 0V, it's 5V to about 1V.So, still dead dead dead. Though I'm sure driving it the way I did in previous tests probably did something bad. Replacement doesn't arrive until Tuesday. Gar.

author: David_AVD
author_rank: Super Contributor
timestamp: July 19, 2013, 07:19:33 am
content: Can you post a picture of the corrected layout please?

author: staze
author_rank: Frequent Contributor
timestamp: July 19, 2013, 07:23:05 am
content: Here ya go.

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 19, 2013, 07:25:29 am
content: That looks ok now.

author: David_AVD
author_rank: Super Contributor
timestamp: July 19, 2013, 07:26:16 am
content: Looks correct now, but you may have toasted the chip with that arse about wiring earlier!

author: staze
author_rank: Frequent Contributor
timestamp: July 19, 2013, 07:29:08 am
content: eh, like I said, it was seemingly bad in circuit (producing all kinds of weird output), so I pulled it reading for a replacement, and tested just because, well, why not. =)But yeah, no doubt I could/may have toasted it. =) It's how we learn... potentially bust crap while testing it. =) I guess the biggest downside is not knowing how broken it was, since there's no way to know whether it's current state is my doing. Ah well... bad troubleshooting, good learning experience.Thanks for the help! Now I just sit back and wait... =)

author: staze
author_rank: Frequent Contributor
timestamp: July 21, 2013, 06:08:05 am
content: Interestingly, I just fired my test back up this evening, and the inverter seems to work.... so... not quite sure about that. I do note that the rise time is pretty quick, but the fall time is pretty slow...The schematics for my meter don't seem to show any decoupling cap across the Vcc and Vdd. But maybe the 7805 regulator and it's filter caps fill the need?So, I'm at a bit of a loss. Replacement part arrives Tuesday... guess we'll have to see. =/

author: Rerouter
author_rank: Super Contributor
timestamp: July 21, 2013, 06:13:34 am
content: your inverters edges move fast which means its a decent spike of current, so the closer the decoupling the better...if your main supply capacitance is more than 5cm away, a local decoupling cap is really in order,

author: staze
author_rank: Frequent Contributor
timestamp: July 21, 2013, 06:46:17 am
content: Makes sense.And David_AVD, I'd really like to thank you, seriously. Once I tested that chip again, with the proper pin-out, it worked. I dropped it back in the meter, and the meter now works.So... either the chip is intermittent, or there was a bad solder on the Vcc or Vdd pins which was fixed by me putting in a socket. Either way, thank you for the reality check. I'm still going to replace the IC since it could be intermittent, but thanks again!

author: SeanB
author_rank: Super Contributor
timestamp: July 21, 2013, 06:55:49 am
content: Likely the IC has a cracked lead frame or bond wire, and the heat of desoldering and the mechanical handling has remade the connection for now. Best to replace it.

author: staze
author_rank: Frequent Contributor
timestamp: July 21, 2013, 06:59:25 am
content: Quote from: SeanB on July 21, 2013, 06:55:49 amThat makes sense. But the fact that I verified the inverter was the cause of the malfunction is extremely pleasing. =)Thanks everyone for all your help!

author: David_AVD
author_rank: Super Contributor
timestamp: July 21, 2013, 07:04:43 am
content: A sanity check is always a good thing when things don't seem to be adding up.

author: staze
author_rank: Frequent Contributor
timestamp: July 21, 2013, 07:05:56 am
content: Quote from: David_AVD on July 21, 2013, 07:04:43 amIndeed. Love the avatar btw... very appropriate. =)