Electronics > Beginners
Over-voltage protection for inverting amplifier
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nsrmagazin:
This is the most used circuit for MCU ADCs.
Zero999:

--- Quote from: David Hess on December 02, 2018, 01:28:07 am ---Yes, it will work fine and transistors operating as transistors are often used in place of diode clamps for lower output impedance.  Even more commonly, a transistor is used to provide a low impedance point to *drive* a low leakage diode clamp because the Vce transistor connection has additional leakage.  Internally, precision bipolar operational amplifiers usually have sets of anti-parallel diodes across their inputs which do the same thing and especially prevent damaging base-emitter breakdown of the input transistors. 

Watch out for the added capacitance from the transistors or diode clamps at the inverting input which lowers the phase margin of the operational amplifier.  A small amount of capacitance across the feedback resistor will cancel the shunt capacitance and increase phase margin if this is a problem.

--- End quote ---
Thanks for providing an explanation as to why the parasitic capacitance of the diodes and transistors can be a problem and what can be done about it.
JPortici:

--- Quote from: Hero999 on November 28, 2018, 03:55:35 pm ---It should not be necessary as the MCP602 already has protection diodes.

--- End quote ---

Actually, there are diodes only between VSS and the input. the protection over the positive rail needs to be external.
http://ww1.microchip.com/downloads/en/devicedoc/21314g.pdf (section 4.1  this is common to many RR opamps from microchip)
Zero999:

--- Quote from: JPortici on December 03, 2018, 04:11:12 pm ---
--- Quote from: Hero999 on November 28, 2018, 03:55:35 pm ---It should not be necessary as the MCP602 already has protection diodes.

--- End quote ---

Actually, there are diodes only between VSS and the input. the protection over the positive rail needs to be external.
http://ww1.microchip.com/downloads/en/devicedoc/21314g.pdf (section 4.1  this is common to many RR opamps from microchip)

--- End quote ---
I didn't know that. I wonder why?

A good thing about this inverting configuration is it can already deal with a significant overvoltage already. R2 and R6 form a potential divider with an attenuation factor of 11, so if the output saturates at 0V, then the input voltage will have to go up to 55V, before the inverting input voltage will rise above +5V.
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