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Subject: Exceeding OpAmp input voltage range

Date: 10/07/02 at 11:14 AM
Posted by: Alfonso Baz
E-mail: fonso@hotkey.net.au
Message Posted:

Hi
I'm using an LM324 quad op amp which is designed for single supply operation.

In the data sheets, under "absolute maximum ratings" it states "Input voltage range (either input) -0.3 to 32V"

I have it wired up as a DC coupled inverting op amp with a gain of -2.5(Av) the signal it is amplifying is a dc signal from 0 to -10V with it's output ranging from 0 to 25V.
Vcc is at 28V and GND is 0V (GND)

Believe it or not it appears to work fine but when the input signal steps from 0v to -10V the output overshoots causing the output to temporarily hit saturation and then drop down to just below GND (-0.5V) with the input still at -10V.

If I lower the input to -10V slowly the output follows the input to +25V.

What I found was that the feedback delay (on a high dv/dt input signal) caused the output to latch up and I soon fixed this problem by placing a 0.1uF cap in the feedback path and now all is well.

My question is, if the absolute maximum’s state that the input range is from -0.3 to 32V how am I getting away with a -10 Volt input signal? Tests revealed that it takes approx. -11V input to latch the thing up.
I get the same results in spice modeling as well as in the real design.
What long term damage can such negative inputs cause the op amp?
Am I misinterpreting the “Absolute Maximum” values?
Is this a bad design and should I look for an alternative method?

Alfonso Baz


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