Re: Driving an OP AMP input with no VCC

AJ a écrit :
"Fred Bartoli" <fred._canxxxel_this_bartoli@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message news:4551f17a$0$3869$426a74cc@xxxxxxxxxxxxxxx
Jim Thompson a écrit :
On Wed, 08 Nov 2006 02:18:41 GMT, "AJ" <itisme33@xxxxxxxxxxxxxx>
wrote:

"Jim Thompson" <To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx> wrote in message news:kea2l21ebbdkp37cb058475pnslsuunvdi@xxxxxxxxxx
On Wed, 08 Nov 2006 00:37:57 GMT, "AJ" <itisme33@xxxxxxxxxxxxxx>
wrote:

"Jim Thompson" <To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx> wrote in
message news:7h81l2hib1ioqpss59k0lba5t7lrr8urk3@xxxxxxxxxx
On Tue, 07 Nov 2006 11:38:23 GMT, "AJ" <itisme33@xxxxxxxxxxxxxx>
wrote:

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.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
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.


Buffering with an opamp is unnecessary for a simple detection. And with your 30V input, you still have the same pb, whatever the 3.3V supply status. And thanks to the high leakage of schottkies, a floating input could well read a false high or low at high temperatures, depending luck.

That one will clamp your input voltage and provides an output within the supply rails.
If you're really cheap you can get rid of the 2 leftmost 100K resistors.
Use any NPN-PNP signal transistors.

1M
VCC ___
.----+----|___|-.
| | |
.-. | |
100K | | \| | ___
| | |-+------+-|___|-< 0-30V
'-' <| | |
| | | | 100K
to PIC <---+----+ | |
| | | |
.-. >| | |
| | |-' |
100K | | /| |
'-' | 1M |
| | ___ |
'----+----|___|-'
|
===
GND



--
Thanks,
Fred.
.

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