Electronics > Beginners
will an Op Amp be damaged if input voltage is higher than supply?
amyk:
--- Quote from: imo on January 28, 2019, 03:17:17 pm ---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 be
Vmax_input = Vcc (or Vss) +/- 0.6V +/- 0.005A*10k
That would work when the LM358 has got an ESD protection.
PS: better go with 1mA when doing the calculation :)
https://www.analog.com/en/analog-dialogue/articles/esd-diodes-as-voltage-clamps.html
--- End quote ---
Looking at the datasheet linked above (in particular, Note 7 on page 5), the LM358 appears to have no upper ESD diode.
spec:
--- Quote from: John_doe on January 29, 2019, 10:20:17 am ---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!
--- End quote ---
HI John_doe
I 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
spec:
+ John_doe
I 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 3V5
* Input 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.pdf
Compare that with the MCP6281 operating from a 5V supply rail:
* Input Voltage Range: -300mV to 5V3
* input Offset Voltage: 3mV (300uV, or less, desirable)
* Input Bias Current: 50pA
* Output Voltage Range: 15mV to 5V985 with 250uA source and sink currents.
* Gain Bandwidth Product: 5MHz
* Slew Rate: 2V5/usechttp://ww1.microchip.com/downloads/en/DeviceDoc/21811e.pdf
Which 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.
Zero999:
--- Quote from: spec on January 29, 2019, 09:12:01 pm ---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).
--- End quote ---
You'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.
Navigation
[0] Message Index
[*] Previous page
Go to full version