| Electronics > Beginners |
| will an Op Amp be damaged if input voltage is higher than supply? |
| << < (3/4) > >> |
| rstofer:
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. |
| hsn93:
--- Quote from: Zero999 on January 28, 2019, 03:43:55 pm --- --- Quote from: Alex Nikitin on January 28, 2019, 02:41:56 pm ---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. Cheers Alex --- End quote --- Yes, 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. --- End quote --- hello, I'm looking at ti lm358, isn't it 1.5V less than supply voltage? |
| spec:
--- Quote from: rstofer on January 28, 2019, 04:40:00 pm ---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 --- End quote --- There 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.html http://www.ti.com/lit/ds/sbos701a/sbos701a.pdf http://www.ti.com/lit/ds/symlink/opa192.pdf http://www.ti.com/lit/ds/symlink/lm158.pdf http://www.ti.com/lit/ds/symlink/ua741.pdf |
| John_doe:
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.html http://www.aliexpress.com/item//32790960436.html Electronics 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! |
| Zero999:
--- Quote from: hsn93 on January 29, 2019, 07:14:42 am --- --- Quote from: Zero999 on January 28, 2019, 03:43:55 pm --- --- Quote from: Alex Nikitin on January 28, 2019, 02:41:56 pm ---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. Cheers Alex --- End quote --- Yes, 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. --- End quote --- hello, I'm looking at ti lm358, isn't it 1.5V less than supply voltage? --- End quote --- You 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. |
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