Re: Driving an OP AMP input with no VCC


I have a similar question to one posted a few days ago. I am
thinking of
driving an OP AMP running at 5V configured as a follower with a
4.0V
input
via a 10K and there could be times where Vcc is removed while the
input
remains. Is it possible that this could damage the IC and should
I
use a
74LCX541 buffer as previously discussed?

Best regards,


AJ


Depends on the OpAmp. Read the maximum input voltage spec. If it
defines maximum input as a relationship with VCC, then you may have
problems.

Be more precise in you question... OpAmp type, etc.

...Jim Thompson


I am using and MCP6044 running at 5V and I have the input clamped
via
BAT54S
diodes to a 3.3V rail and ground. The problem is that my board may
loose
power while an input remains. I have a TVS on the 3.3V rail to
prevent it
going above 3.3V but this would mean that an input would be clamped
at
3.6V
while the OP AMP's Vcc would be 0V.

Best regards


AJ

I am unable to view a data *** from Microchip's site. Firefox just
hangs and sucks 100% CPU. Sorry.




Thanks anyway mate, I am not committed to the MCP6044, can you
recommend
one that would be suitable for my application or tell me what I should
be looking for?

Best regards


AJ



All inputs and outputs................... VSS -0.3 V to VDD +0.3 V

I think you will find most, if not all OpAmps have this restriction,
so you need some way to protect it.

Post your schematic on a.b.s.e and everyone here will take a look.

...Jim Thompson


So it's time for some update.
Have a look at "over the top" opamps or comparators like LT1716 or
LT1782...
Bias current is going high over the supply rail, but it is OK for some
applications.


--
Thanks,
Fred.


Here is a link to the schematic to make things a bit clearer....
http://61.9.815.52/OP_IN.JPG
^^^
?
?


. The output connects to a PIC analogue
input and the idea is to be able to detect a high, low or float condition.
The MCP6044 has VSS -0.3 V to VDD +0.3 V so maybe I will be ok with this
configuration. I have done a bit of testing and I haven't blown an OP
yet.

I have seen these "over the top" OP's, didn't think of them, thanks for
the
suggestion.

Best regards,


AJ


Sorry, wrong address, I tend to do these things at 3am. Try this one
http://61.9.185.52/OP_IN.JPG

Regards


AJ


.