Could ALL these opamps be dead?? 0.5mA into inputs? - Page 1
Forum section: electronics / projects

author: Jon86
author_rank: Frequent Contributor
timestamp: October 06, 2019, 12:36:29 pm
content: I've been having some very strange issues over the last day or two, which started with a simple comparator circuit as part of a project.I noticed one of the inputs dragging the other down, and after messing around with different configurations and measuring currents, I could see around 0.5mA flowing IN to one of the inputs!Now I checked that the opamp was definitely getting power, and there was no short between the pins on the breadboard.I've tried 8 different MAX492 dual opamps (from the same tube I might add), and a 5532 I had lying around, and they all seem to exhibit the same behaviour.Potentiometer going into one input, voltage going into the other. That's it. When changing the pot position, the other input seems to track with it. Putting a reasonable resistance value on the other input to tie it down does help, but the problem is still there.The only thing I've found that works is my MCP608s, but I've only got two and I'm already using them elsewhere.So my question here is, are there circumstances where the inputs can accept a significant current? Is it a quirk of these particular parts? Or have I just been real unlucky and these old parts are all dead?Hopefully someone can enlighten me here, happy to take measurements and try different things if anyone thinks it'll help!Cheers

author: langwadt
author_rank: Super Contributor
timestamp: October 06, 2019, 12:53:15 pm
content: look at the datasheets, the MAX492 and 5532 have diodes between the + and - inputs

author: Jon86
author_rank: Frequent Contributor
timestamp: October 06, 2019, 12:58:14 pm
content: Damn, you're right! Can't believe I've never come across this before... Guess I almost always use a for-purpose comparator when I need one.So I take it these are only designed to be used in a configuration where the inputs are within a few hundred mV of each other? As in, amplifier type circuits?

author: magic
author_rank: Super Contributor
timestamp: October 06, 2019, 04:00:09 pm
content: Yes, that's what they are made forThis is to protect BE junctions of the inputs from reverse breakdown. A few old opamps like 741/4558/358 use special input transistors capable of withstanding 30V or so but that's rarely seen in high performance chips, presumably due to compromises involved.editAlways check absolute maximum differential input voltage.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 06, 2019, 04:06:00 pm
content: Quote from: magic on October 06, 2019, 04:00:09 pm

author: Jon86
author_rank: Frequent Contributor
timestamp: October 07, 2019, 09:14:05 pm
content: Could've had me scratching my head for weeks, I'm far too used to the 741 and similar parts, built like a brick dunny!Glad I understand this aspect of op-amp circuit design now though.

author: langwadt
author_rank: Super Contributor
timestamp: October 07, 2019, 10:08:32 pm
content: Quote from: SiliconWizard on October 06, 2019, 04:06:00 pmcheck this weird one,https://datasheets.maximintegrated.com/en/ds/MAX9691-MAX9693.pdf

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 08, 2019, 12:19:51 am
content: Quote from: langwadt on October 07, 2019, 10:08:32 pmQuote