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Driving an OP AMP input with no VCC

A

AJ

Jan 1, 1970
0
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
 
D

default

Jan 1, 1970
0
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
When you have the potential for input to exceed VCC or ground it is
good practice to just clamp the signal at the input pin with diodes to
carry the current up/down to VCC or ground. The diodes are reversed
biased during normal ops.

Good practice for long wire runs to inputs to keep induced voltages
out and in industrial and automotive environments to clamp EMI.
 
J

Jim Thompson

Jan 1, 1970
0
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
 
A

AJ

Jan 1, 1970
0
Jim Thompson said:
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
 
J

Jim Thompson

Jan 1, 1970
0
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 sheet from Microchip's site. Firefox just
hangs and sucks 100% CPU. Sorry.

...Jim Thompson
 
A

AJ

Jan 1, 1970
0
Jim Thompson said:
I am unable to view a data sheet from Microchip's site. Firefox just
hangs and sucks 100% CPU. Sorry.

...Jim Thompson

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
 
K

Kingcosmos

Jan 1, 1970
0
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

What is your application? If you are looking to use an op-amp as a
follower, you could look at the OPA333 from Texas Instruments. Not
that fast but really good spec's in terms of low power, low voltage,
and offset (rail to rail too!).

With any op-amp (just about anyway) you need to pay attention to the
data sheets absolute maximum ratings. Usually there will be a maximum
rating on the input voltage;i.e. Vcc + 0.3V using the OPA333 as an
example. If you go beyond this then you need to heed the maximum input
current rating. Again using the OPA333 as an example, it is listed as
+/-10mA and it is explicitly stated in the datasheet as well. A simple
series resistor can be used to do this. You can also use clamps on the
inputs too.
As far as I am aware, this limited input current is continuous and
would not damage the part and I would go as far as to say that it would
not cause worry in terms of degradation or reliability either. The
real fun is what happens when you are using a dual or a quad and one
channel experiences an electrical overstress event while the others are
driven normally. In your case if you are not going to have power
connected, the point is moot; however, through my discussions with
several colleagues and looking at test data it appears that you can
treat each channel as a separate block within the IC. It is true that
they will all ultimately share the same power rail and that there are
many paths for current to flow during an EOS event. I will have to
look at the test again to see how it was set up if you are interested.
 
J

Jim Thompson

Jan 1, 1970
0
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
 
F

Fred Bartoli

Jan 1, 1970
0
Jim Thompson a écrit :
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.
 
J

Jim Thompson

Jan 1, 1970
0
Jim Thompson a écrit : [snip]
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.

It just occurred to me that the LM324 can take inputs > VCC.

...Jim Thompson
 
A

AJ

Jan 1, 1970
0
"Fred Bartoli"
Jim Thompson a écrit :

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.

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
 
A

AJ

Jan 1, 1970
0
Kingcosmos said:
What is your application? If you are looking to use an op-amp as a
follower, you could look at the OPA333 from Texas Instruments. Not
that fast but really good spec's in terms of low power, low voltage,
and offset (rail to rail too!).

With any op-amp (just about anyway) you need to pay attention to the
data sheets absolute maximum ratings. Usually there will be a maximum
rating on the input voltage;i.e. Vcc + 0.3V using the OPA333 as an
example. If you go beyond this then you need to heed the maximum input
current rating. Again using the OPA333 as an example, it is listed as
+/-10mA and it is explicitly stated in the datasheet as well. A simple
series resistor can be used to do this. You can also use clamps on the
inputs too.
As far as I am aware, this limited input current is continuous and
would not damage the part and I would go as far as to say that it would
not cause worry in terms of degradation or reliability either. The
real fun is what happens when you are using a dual or a quad and one
channel experiences an electrical overstress event while the others are
driven normally. In your case if you are not going to have power
connected, the point is moot; however, through my discussions with
several colleagues and looking at test data it appears that you can
treat each channel as a separate block within the IC. It is true that
they will all ultimately share the same power rail and that there are
many paths for current to flow during an EOS event. I will have to
look at the test again to see how it was set up if you are interested.

Thanks for your reply mate, the MCP6044 does have vcc + 0.3V and I am
connecting to the input via 10K so with any luck I will be ok, I haven't
managed to blow one yet.
 
J

Jim Thompson

Jan 1, 1970
0
"Fred Bartoli"


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


...Jim Thompson
 
F

Fred Bartoli

Jan 1, 1970
0
AJ a écrit :
"Fred Bartoli"


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
 
A

AJ

Jan 1, 1970
0
"Fred Bartoli"
AJ a écrit :

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 a lot for your input Fred, very helpful.

Best regards


AJ
 
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