Bizzare op amp buffer behavior - too much gain - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 23, 2019, 10:06:23 pm
content: Hi! I'm messing around with an instrumentation amp board i'm designing, and I'm using an op amp to buffer the Vbias set by dividers.The problem is, the op amp outputs WAY too high a voltage. It's set up as a unity gain buffer, but outputs 3,something volts with a 0.5v input.I've tried different single op amps i had lying around (to rule out bad parts, and since the op amp choice wasn't that critical), I've tried an LM741, CA3100, and TL071, the latter throwing my voltage all the way up to V+! (24V).The only logically thing being able to cause this over multiple op amps is that the feedback loop is open, or high impedance, but it ohms out ok. I've checked extensively for wiring faults and solder bridges and such, don't think that's the problem.It shouldn't be a complex circuit at all, and I'm out of ideas to try. Help.See attached schematic.Thanks in advance!

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 23, 2019, 10:54:18 pm
content: None of those opamps are rail-to-rail. There is no way they can output 0.5V with a 24V single supply AFAIK.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 23, 2019, 11:23:40 pm
content: Damn, didn't consider that. But now I've bumped the input up to 2.5V and the output is still above 3V+. Wouldn't that be within range?Thanks for your insight. That's definitely something I completely forgot to look into.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 23, 2019, 11:47:39 pm
content: I've replaced the op amp with an AA battery as 1.5V DC reference and everything works beautifully. I'll order some pin-compatible rail-to-rail op amps.What a blunder. But thanks for the quick help.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 24, 2019, 12:12:12 am
content: Quote from: ChristofferB on January 23, 2019, 11:23:40 pmThe TL071 as well as the 741 are both given with as much as 3V dropout from either rail (worst case, +/-12V max swing with a +/-15V supply), and a similar if not worse operating common mode input voltage range.On top of that, the TL071 is known to suffer from phase reversal in non-inverting configurations when the input is too close to the negative rail (which is ground for you), which is exactly why the output went up to the positive rail.

author: StillTrying
author_rank: Super Contributor
timestamp: January 24, 2019, 12:43:19 am
content: Quote from: ChristofferB on January 23, 2019, 11:47:39 pmYou need onlysingle supplyop amps, because your input and output is well less than the 24V.I nearly always start with a CA3140.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 24, 2019, 01:43:36 am
content: Roger, makes sense, i'll look into it. Thanks!

author: Zero999
author_rank: Super Contributor
timestamp: January 24, 2019, 08:53:54 am
content: The LM358 will probably do but is a buffer needed? The impedance of the AD8226 reference is 100k with a bias current of 7μA and your potentiometer has a maximum impedance of 10k.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 25, 2019, 02:32:21 am
content: The data sheet is very adamant about buffering. I just tried to belt-and-braces it. I think I have some 358s somewhere.. right now my taped-on AA battery works a treat as reference!

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 25, 2019, 02:35:37 am
content: Do you really need to be able to trim the reference? If not, a fixed reference could be just made out of a small LDO instead.Note that you could still trim it using an adjustable LDO. It's another way of doing it.