Author Topic: Question about Op Amp's negative non-inverting input on single supply.  (Read 868 times)

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Offline alexg

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Hi All, this seems to be very simple question and I already did some research about it but this one question that I still can't find very definitive answer.
I have  MCP601, it is powered by single supply and wired into voltage follower/buffer configuration. On non-inverting input I have a piezo disk and when I tap piezo disk it sends waveform into Op Amp's input and apparently even though waveform has both, negative and positive components, on Op Amp's output I can only see the positive half of waveform, which is good and what I need. What are the op-amps input voltage limits?  For example if Op-Amp is powered by 3.3v, can input voltage be 5v, 10v?  Can it be -5 or -10v, is it a simple matter of Op-amp just not being able to make output voltage high/low enough, it I can actually damage it and need to protect inputs from overvoltage?
 

Offline bktemp

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Re: Question about Op Amp's negative non-inverting input on single supply.
« Reply #1 on: October 10, 2016, 04:34:57 pm »
Most datasheets have a section called "Absolute Maximum Ratings":
For the MCP601 it gives a maximum input voltage range of Vss-0.3V to Vdd+0.3V. So for single supply 3.3V this gives -0.3V to +3.6V.
The input pins tolerate a current of up to 2mA, so it has build in protection diodes, limiting the voltage if the current is not higher than 2mA.
If you exceed one of both values, the chip may get damaged.
 

Offline alexg

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Re: Question about Op Amp's negative non-inverting input on single supply.
« Reply #2 on: October 10, 2016, 04:49:31 pm »
Thanks for reply, how would you recommend to ensure that opamp does not get fried then? I'm thinking about this course of action:

1. Smack piezo bender as hard as possible without damaging it and see how much voltage it outputs, then use divider to make sure that this voltage stays below OP AMP's max input voltage? But since there are built in protection diodes inside of an Op Amp, then probably no need in lowering input voltage in case if piezo does not output more than 2mA of current.
2. Measure how much current piezo bender outputs and if that current less than 2mA then it's safe. How can I measure its output current with oscilloscope? I am googling it right now but if someone does not mind posting suggestion directly here, I'd sincerely appreciate that.

Thanks again.
 

Offline bktemp

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Re: Question about Op Amp's negative non-inverting input on single supply.
« Reply #3 on: October 10, 2016, 04:57:06 pm »
Dividing the voltage probably doesn't work, because piezo discs can produce hundreds of volts if you hit them really hard.
Since the MCP601 has a high input impedance, the easiest solution is adding a series resistor: Let's say you use 10k, the input voltage can go up to +23.6V or down to -20.3V before reaching 2mA. You can probably go even higher, but if you go to high it may attenuate the signal.
Since 20V is not enough, you can add additional diodes capable of much more than 2mA directly to the piezo disc, limiting the voltage to Vcc+0.7V and Vcc-0.7. After that only a small resistor like 1k is necessary to protect the opamp.
 


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