Hi,
Will the micro be damaged by the (non series resistor protected) opamp input?
LTspice and jpeg attached.
The micro has a -3v3 rail, and a 20V rail.
Micro has rail +3v3 to 0V.
Its a TL084C opamp.
What would be the worst circumstance for the micro?
The opamp output current is clamped to some 26mA, as you know....maybe micro can withstand a short blast at 26mA?
A SMT resistor cost is fractions of a penny. And if you employ a 0201 or even a 0402 package, the required board real state is close to negligible.
Ultra cheap insurance. Use it.
D2, D3 input protection diodes have a max current rating. Read the data sheet. Or some how (resistor) limit the current. I would pretend the op-amp output goes from rail to rail and pick a resistor based on that.
There is a school of thought that the opamp is a buffer...and that since its non-inv pin is never <0V, then neither should be the output.....though is there something with offset voltages of certain batches?...that when inv and non-inv are both 0V......it could be that non-inv "looks like" it is less than 0V?...so opamp output goes to negative rail?
The attached explains it better...yes we need the buffer....its too high Z divider to send several cm's across the board back to the micro.
the attached sim is nearer what we are doing with the circuit........in the attached LTspice sim, after a few ms, the 20V rail goes down as the product is switched OFF......but the neg_3V3 rail stays about neg_3v3...its at this point where the big injection of current goes through the ADC pin 's ESD diodes....because the opamp output goes negative.
I think this is the danger point of the circuit...would you agree?
A SMT resistor cost is fractions of a penny. And if you employ a 0201 or even a 0402 package, the required board real state is close to negligible.
Ultra cheap insurance. Use it.
thanks
[i must confess, i never encountered this before, as i always put a series resistor there...but this board has already been done and there's no room for the eight series resistors that would need adding......the whole board would have to be totally ripped up and started over. (a consultancy actually sent us this as they are overloaded at the mo)]
There are two primary problems with injection current into a CMOS device,
the input diode protection network and latchup. Latchup generally blows the internal
supply bond wires to the die, eg. catastrophic failure.
isn't accurate. The datasheet lists 26mA as the TYPICAL output short-circuit current... it's not the max nor is it "clamped"
Even if it doesn't blow anything up usually injecting a negative voltage causes all sorts of problems with any analog circuitry inside the uC. I wouldn't rely on just limiting the current with a resistor.
isn't accurate. The datasheet lists 26mA as the TYPICAL output short-circuit current... it's not the max nor is it "clamped"
Even if it doesn't blow anything up usually injecting a negative voltage causes all sorts of problems with any analog circuitry inside the uC. I wouldn't rely on just limiting the current with a resistor.