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Looking for opamps with ability to tolerate input voltage above V+ ?

C

Christopher Ott

Jan 1, 1970
0
I have a board which is currently using several LT1491's, specifically for
their ability to tolerate up to 44 volts above V+ at the inputs. The down
side, is that they are expensive, and frequently out of stock. I'm curious
is anyone out there is familiar with a similar device which might be a bit
cheaper. I'm using this on heavy equipment with a 24v system. Commonly the
LT1491 has a V+ of 5v, however the inputs could momentarily be as high as
28v (due to miswires in electrical assembly).

Any ideas?

Thanks,

Chris
 
T

Tilmann Reh

Jan 1, 1970
0
(setting a Fup2 s.e.c)

Christopher said:
I have a board which is currently using several LT1491's, specifically for
their ability to tolerate up to 44 volts above V+ at the inputs. The down
side, is that they are expensive, and frequently out of stock. I'm curious
is anyone out there is familiar with a similar device which might be a bit
cheaper. I'm using this on heavy equipment with a 24v system. Commonly the
LT1491 has a V+ of 5v, however the inputs could momentarily be as high as
28v (due to miswires in electrical assembly).

If the high input voltages are only due to miswires, you might as well
add protection circuitry to a more common, standard OP.

When looking for OPs that accept voltages significantly above V+, they
all are rather esoteric, expensive and/or hard to get.
The good ol' TL072, however, at least accepts input voltages /up to/ V+,
which might be sufficient in some cases.
 
C

Christopher Ott

Jan 1, 1970
0
Tilmann Reh said:
(setting a Fup2 s.e.c)



If the high input voltages are only due to miswires, you might as well
add protection circuitry to a more common, standard OP.

When looking for OPs that accept voltages significantly above V+, they
all are rather esoteric, expensive and/or hard to get.
The good ol' TL072, however, at least accepts input voltages /up to/ V+,
which might be sufficient in some cases.

I do have a 30v TVS (400Watt SMA package) protecting the LT1491 inputs from
ESD and short duration transients such as alternator noise. So I'm really
just trying to protect the inputs from 5 volts to the point where the TVS
turns on (around 33 volts.) Miswires in electrical assembly are
unfortunately frequent enough that I have to accommodate for them. They will
always be +24v to +28v depending on if the engine is running. The LT1491
works very well for this, since it's inputs are so robust, but at $4.25
each, I feel like I'm getting raped.

Any elegant ideas to protect a normal opamp while keeping the parts count
low?
 
R

Richard

Jan 1, 1970
0
Christopher Ott said:
I have a board which is currently using several LT1491's, specifically for
their ability to tolerate up to 44 volts above V+ at the inputs. The down
side, is that they are expensive, and frequently out of stock. I'm curious
is anyone out there is familiar with a similar device which might be a bit
cheaper. I'm using this on heavy equipment with a 24v system. Commonly the
LT1491 has a V+ of 5v, however the inputs could momentarily be as high as
28v (due to miswires in electrical assembly).

Any ideas?

Thanks,

Chris
Zener diodes??
 
D

default

Jan 1, 1970
0
I have a board which is currently using several LT1491's, specifically for
their ability to tolerate up to 44 volts above V+ at the inputs. The down
side, is that they are expensive, and frequently out of stock. I'm curious
is anyone out there is familiar with a similar device which might be a bit
cheaper. I'm using this on heavy equipment with a 24v system. Commonly the
LT1491 has a V+ of 5v, however the inputs could momentarily be as high as
28v (due to miswires in electrical assembly).

Any ideas?

Thanks,

Chris

Add a clamping diode from the input to the positive supply rail of the
op amp, and add a small resistor on the input signal to keep it from
frying the diode (assuming there isn't already one there for the
circuitry)

That sort of problem could be designed out.
 
J

Jasen Betts

Jan 1, 1970
0
["Followup-To:" header set to sci.electronics.]
I have a board which is currently using several LT1491's, specifically for
their ability to tolerate up to 44 volts above V+ at the inputs. The down
side, is that they are expensive, and frequently out of stock. I'm curious
is anyone out there is familiar with a similar device which might be a bit
cheaper. I'm using this on heavy equipment with a 24v system. Commonly the
LT1491 has a V+ of 5v, however the inputs could momentarily be as high as
28v (due to miswires in electrical assembly).

the way I'm reading that, they just need to survive, they don't need to
function correctly in those conditions - right???
Any ideas?

two:

build your own op-amps from discrete transistors.
pass labs have an article on their web site.

use series resistors (eg 1K) and shunt diodes to VCC or to a 3.9V zener,
 
C

CWatters

Jan 1, 1970
0
in message
Add a clamping diode from the input to the positive supply rail of the
op amp, and add a small resistor on the input signal to keep it from
frying the diode (assuming there isn't already one there for the
circuitry)

That sort of problem could be designed out.

and might eliminate need for a fancy ESD protection device as well.
 
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