will an Op Amp be damaged if input voltage is higher than supply? - Page 1
Forum section: electronics / beginners

author: John_doe
author_rank: Regular Contributor
timestamp: January 28, 2019, 02:13:23 pm
content: Sorry for the newbie question, I have googled this and found a plethora of answers, however, either none of those that I read answered my specific situation, or they did but I didn't understand the explanation.My question seems pretty straightforward: If I supply 20Vdc to the non inverting input of an LM358 configured as a unity gain voltage follower, and supply it with only a single 5.5Vdc source, will the LM358 be damaged?I amNOTasking if the Op Amp will work, only if it will be damaged.If the answer is "Yes", additional questions:1. Is there another Op Amp model thatcanbe used this way?2. What if V+ (positive supply) is not connected at all, will this help?If the answer is "No", additional question:1. Can the Op Amp output exceed 5.5V ?Note: If it helps R1 and R2 can be increased to 1M.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 28, 2019, 02:41:56 pm
content: You should learn to read datasheets carefully - all this information is there. For the LM358 the maximum input voltage (without damage to the device) could be up to +32V from the negative supply rail, independent from the actual supply voltage, so for this particular opamp and circuit it is perfectly safe to put +20V on the input with only 5.5V supply.CheersAlex

author: Peabody
author_rank: Super Contributor
timestamp: January 28, 2019, 02:50:25 pm
content: This is what the Absolute Maximum Ratings section of the datasheet is for. Surprisingly, it appears you can apply up to 32V to an input without damaging it, and it also appears the maximum is not related to the power supply voltage (it often is), which is also 32V maximum.However, the output voltage can never exceed the actual power supply voltage, 5.5V in your case. It will actually be a bit less than that, and this will also be described in the datasheet.If it could output 20V, then you would run up against the maximum ratings of the 328P's A0 input, which is less forgiving.

author: rstofer
author_rank: Super Contributor
timestamp: January 28, 2019, 02:59:08 pm
content: However, the output voltage can not exceed the upper rail voltage. See the schematic on page 20 here:https://www.sparkfun.com/datasheets/Components/General/LM358.pdfAnd why would you want a high voltage output? The maximum analog input to the ATmega328p is Vcc + 0.3V Page 325 here:https://www.sparkfun.com/datasheets/Components/SMD/ATMega328.pdfIt would seem to me that the graph of Input Range in the upper left corner of page 6 of the LM358 datasheet limits the input voltage to be <= absolute value of the respective rail voltage. For negative inputs, the voltage can closely track the V-rail but for positive inputs, the voltage must be somewhat less than the V+rail. So, while using high voltages may not destroy the device, I certainly wonder if the op amp works under the stated conditions.

author: iMo
author_rank: Super Contributor
timestamp: January 28, 2019, 03:17:17 pm
content: The inputs of an opamp (and other chips as well) are protected by clamping diodes (2 or 4) and a resistor, something like 200-400ohm. That is the basic ESD protection.Opamps usually have got another diodes in between the INV and NINV inputs.In case the voltage at an input is higher than Vcc or Vss, the respective diodes start to conduct, and they divert the current into the Vcc or Vss.The diodes can usually survive 5mA.It depends on how you LIMIT that current.When you have external resistors wired at the inputs, for example 10k, the max voltage which will not smoke the ESD diodes would beVmax_input = Vcc (or Vss) +/- 0.6V +/- 0.005A*10kThat would work when the LM358 has got an ESD protection.PS: better go with 1mA when doing the calculationhttps://www.analog.com/en/analog-dialogue/articles/esd-diodes-as-voltage-clamps.html

author: Zero999
author_rank: Super Contributor
timestamp: January 28, 2019, 03:43:55 pm
content: Quote from: Alex Nikitin on January 28, 2019, 02:41:56 pmYes, that's the one really good thing about the LM358: it won't be damaged by voltage above the supply voltage, up to a maximum of 32V, with respect to 0V.However, the common mode and output voltage range is only 2V less than the supply voltage, so the maximum input and output voltage the LM358 will work at with a 5.5V supply is 3.5V. If operation to 5.5V is required, then run the LM358 off <7.5V, if possible.

author: rstofer
author_rank: Super Contributor
timestamp: January 28, 2019, 04:40:00 pm
content: All this is very interesting but what's the real question? You have a 40V signal that you want to stuff into an Arduino?If so, Chapter 4 of "Op Amps For Everyone" does a terrific job of describing how to offset and scale a signal in the context of a single rail op amp. The idea, of course, is to sell rail-to-rail input/output op amps. "Rail-to-rail" is a marketing term, not an engineering fact. But some op amps come pretty darn close. The LM358 isn't one of them!http://web.mit.edu/6.101/www/reference/op_amps_everyone.pdf

author: iMo
author_rank: Super Contributor
timestamp: January 28, 2019, 04:42:39 pm
content: While you already work with LTspice, you may simulate the situation with the internal ESD clamping diode as follows:

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 28, 2019, 04:54:07 pm
content: If you *really* require to have the common mode input voltage to exceed the supply voltage by a wide margin, you can always use some of Linear Tech's (now Analog Devices) "Over-the-top" opamps.For instance, the LT1490A is a dual and avail in both DIP and SOIC packages.Needless to say, LT offerings are pricey.

author: John_doe
author_rank: Regular Contributor
timestamp: January 28, 2019, 09:15:49 pm
content: Thanks guys, your help is much appreciated.Reading the datasheet was the 1st thing I did, but then I searched the web and got confused by posts likethis one.Quoteand:QuoteI've ordered some generic 358s from China, it's New Year's there so they'll probably only ship it out in a month or so. I have an ancient LM224J (National Semiconductor, ceramic package, probably from the eighties) which I've used many times and I felt bad about destroying it (hence this thread), but after reading your responses I did try it and it works great, 6.5v single supply gives an output of 5.22V with input at 17V (and the same with input at 5.22V).Thanks again.

author: rstofer
author_rank: Super Contributor
timestamp: January 29, 2019, 12:29:02 am
content: Yes, but...You won't have 'a stiff supply' if you have any resistors on the input pins. You probably don't want to wire them to a string of car batteries.

author: hsn93
author_rank: Regular Contributor
timestamp: January 29, 2019, 07:14:42 am
content: Quote from: Zero999 on January 28, 2019, 03:43:55 pmhello, I'm looking at ti lm358, isn't it 1.5V less than supply voltage?

author: spec
author_rank: Frequent Contributor
timestamp: January 29, 2019, 08:05:26 am
content: Quote from: rstofer on January 28, 2019, 04:40:00 pmThere are many opamps with true rail to rail input voltage common mode ranges. In fact, there are quite a few opamps with inputs that go beyond each rail, typically by 100mV and 200mV (TSX711 for example).It is quite right that rail to rail output is not exactly to the rails- how could it be- but it is not far off, certainly close enough to be effective in designs: turn off a MOSFET or BJT for example. And RRO opamps are generally far superior to opamps that run out of steam at some indeterminate voltage roughly a couple of volts from the supply rails, not only because of dynamic range, but also because RRIO opamps ease power supply requirements and simplify design. RRIO opamp outputs also limit in a more controlled and predictable way and tend not to suffer oscillations and phase reversal. RRIO opamps have also transformed low voltage nanopower circuits which would be virtually impossible without RRIO opamps, and RRIO parts have also played a big part in the development of handheld equipment: mobile phones, etc. RRO video amps have also simplified video designs.RRIO opamps have transformed analogue design, and would also avoid the many errors that newbees make, because they are not experienced enough to read the data sheet and often get into a pickle with input and output ranges, especially with the old 709 and 741 opamps.I find it difficult to understand why anybody bothers with non RRIO amps for general work, or opamps that have relatively large input voltage offsets and high input bias currents. It is a different mater for specialist applications like audio, or where extreme low cost is needed, and them the LM358 (input includes the negative supply rail and output approaches the negative supply rail) saves the day. When the LM358 type opamps came out, they opened up many avenues in design, and RRO opamps have continued that move, but to a greater extent.RRIO opamps tend to be more expensive than jellybean opamps, but when you weigh up the costs of other components in a circuit, the opamp cost often is not that significant.The TSX711, OPA191, OPA192 are good examples of RRIO opamps. Not only are they RRIO, but they have very low input offset voltages and negligible input bias currents, which gives you great freedom in design. Also, for newbees, they are 'perfect' opamps.https://www.st.com/en/amplifiers-and-comparators/tsx711.htmlhttp://www.ti.com/lit/ds/sbos701a/sbos701a.pdfhttp://www.ti.com/lit/ds/symlink/opa192.pdfhttp://www.ti.com/lit/ds/symlink/lm158.pdfhttp://www.ti.com/lit/ds/symlink/ua741.pdf

author: John_doe
author_rank: Regular Contributor
timestamp: January 29, 2019, 10:20:17 am
content: rstofer, spec:I know that that the LM358 has limitations. I did look at the input offset voltage, input bias current and input common mode voltage and found them acceptable, albeit far from perfect.Honestly, I pick components based on the price I find in eBay/Aliexpress. The LM358 and LM324 are $0.29 and $0.46, respectively, for 10 pieces deliverd to my mailbox:http://www.aliexpress.com/item//32851918659.htmlhttp://www.aliexpress.com/item//32790960436.htmlElectronics for me is a hobby, not my profession, and I refuse to pay exorbitant amount of money for my hobbies.Having said that, a rail-to-rail output opamp will save me a step up converter so I wouldn't mind paying a little extra for one, but of the three opamps you suggested (@spec), the OPA192's price of $2 apiece is the lowest but I can't find one in a DIP package (a must). Also, if I understand the datasheet correctly, it can't have inputs higher than supply.If anyone can suggest a reasonably priced RRO opamp that fits my requirements I'll be very happy to hear about it!

author: Zero999
author_rank: Super Contributor
timestamp: January 29, 2019, 12:29:27 pm
content: Quote from: hsn93 on January 29, 2019, 07:14:42 amYou can probably get away with an input and output voltage range of 1.5V less than the supply voltage, for the LM358, under light loads and at room temperature, but allowing for 2V will be more robust and guarantee it'll work over the full temperature range.The original poster quite likely has a higher supply voltage than 5.5V available anyway, so he might as well use it for the LM358 as well as the regulator powering the MCU.

author: amyk
author_rank: Super Contributor
timestamp: January 29, 2019, 12:58:35 pm
content: Quote from: imo on January 28, 2019, 03:17:17 pmLooking at the datasheet linked above (in particular, Note 7 on page 5), the LM358 appears to have no upper ESD diode.

author: spec
author_rank: Frequent Contributor
timestamp: January 29, 2019, 08:55:48 pm
content: Quote from: John_doe on January 29, 2019, 10:20:17 amHI John_doeI did not mean to imply anything about your design in my post about RRIO etc opamps. You will notice that I did say that for low cost applications the LM358 is the way to go, so your choice is fine.It is correct that none of the opamps I listed are available in DIL packs, but they are available in SOT3 which is not too bad to handle. By the way the TSX711 is around $0.70 from Digikey.You ask about a low-cost DIL RRIO CMOS opamp. The cheapest suitable opamp I could find is the Microchip MCP6281 family of 1, 2 & 4 opamp packs. The twin version in a DIL8 package is around $0.60 from Digikey- but you may find it cheaper elsewhere. Be aware though that the MCP6281 only operates from supply rails of 2V2 to 6V, but its input will operate with an input voltage that is 300mV higher than the positive supply rail, and 300mV more negative than the negative supply rail. It also has CMOS inputs, so input bias currents are negligible and will cause minimal input offset errors in an application.http://ww1.microchip.com/downloads/en/DeviceDoc/21811e.pdf

author: spec
author_rank: Frequent Contributor
timestamp: January 29, 2019, 09:12:01 pm
content: + John_doeI see that the other members have advised you that your circuit is quite safe, so that answers your initial question.But just a few observations about the LM358 performance in your application. When the LM358 is operated with 5V supply rails, its input and output voltage ranges are quite limited (note that there are also 'A' and 'B' versions of the LM358 with improved input characteristics, but not improved input voltage range):Input Voltage Range: 0V to 3V5Input Offset Voltage: 7mV (300uV, or less, desirable)Input Bias Current: 250nA (100pA, or less, desirable)Output Voltage Range: 20mV to 3V5, sink 250uA and source 500uA (for driving light loads you can connect a pull-up resistor from the LM358 output to the 5V supply rail. A 10k pull-up resistor would give an output voltage range of about 60mV to 5V).Gain Bandwidth Product: 700kHz (low)Slew Rate: 300mV/usec (slow)So you must be careful to take the above parameters into account when designing your circuits using an LM358.http://www.ti.com/lit/ds/symlink/lm158.pdfCompare that with the MCP6281 operating from a 5V supply rail:Input Voltage Range: -300mV to 5V3input Offset Voltage: 3mV (300uV, or less, desirable)Input Bias Current: 50pAOutput Voltage Range: 15mV to 5V985 with 250uA source and sink currents.Gain Bandwidth Product: 5MHzSlew Rate: 2V5/usechttp://ww1.microchip.com/downloads/en/DeviceDoc/21811e.pdfWhich clearly illustrates the advantages of MOS RRIO opamps, especially when used on low-voltage supply rails.By the way, always fit a 100nF X7R ceramic capacitor across the supply line pins of the LM358 or MCP6281, using as short leads as possible.

author: Zero999
author_rank: Super Contributor
timestamp: January 31, 2019, 09:10:22 am
content: Quote from: spec on January 29, 2019, 09:12:01 pmYou're mistaken here. A 10k pull-up resistor on an LM358 powered from 5V will not bring the output swing all the way up to 5V. It'll get a little closer to 5V, at the expense if the minimum output voltage increasing to around 700mV.This approach is flawed because the higher the output voltage, the less current flows through the resistor, so it will lose its ability to pull-up. The LM358 contains an internal current sink which also makes the situation worse. Connecting a current source, with a higher current than the LM358's internal sink, to the output and +V would be much better, as long as it can saturate near the positive rail. A current mirror with two PNP BJTs will do a much better job of going to +V, but the lower saturation voltage will still increase to around 700mV.