LM324 inverting amplifier, works but shouldn't?

[MikeK]

Okay, so a while ago I built a display for my Elenco xp-720 power supply.  It uses an LM324, a PIC, and an LCD.  It works well as it quite accurate.

But after going through the LM324 datasheet again recently, I'm wondering why part of it works at all.  Specifically, I used one of the op-amps in the LM324 to invert and scale the power supply's negative output.  This ranges from 0 down to -17V.

Since I power the LM324 with +5V and 0V, and the datasheet says that the minimum common mode input voltage is 0V, aren't I exceeding the spec by a large amount by allowing the input to be negative?

Or am I misreading the datasheet?

[c4757p]

With an inverting amplifier, the input to the inverting amplifier and input to the op amp are not the same thing.

See the attached schematic. -15V is applied to the input, +15V at the output, and the op amp only sees 0V and +15V. Remember that the op amp tries to make both inputs equal - by applying the opposite voltage to the other end of a voltage divider, the center will be zero.

Of course, using different resistors can scale the -15V to something under 5V. If you use R1=75k and R2=20k, than -15V becomes 4V.

One bad side to this approach is that the op amp will be unhappy if accidentally powered down. The LM324 is unique in being able to handle inputs up to +32V even when not powered, but that does not apply for negative inputs, only positive.

[MikeK]

Ohhhhh.

Great.  Thanks for explaining that.  I did forget about the inputs being made equal.  I thought I was on to some cold fusion hyperspace thing there for a second.

This is why I love op-amps...They're bloody awesome!

[c4757p]

This is why I love op-amps...They're bloody awesome!

Yes, they are. Probably the single most useful active component in analog electronics. There's not much you can't do with some op amps, resistors, and caps.