# How do I find op amp specifications regarding voltage limits?

This question is perhaps a bit unusual and is about how to read/interpret specifications. I have been doing some testing with a common difference amp (LM324) and have also used LTSpice to double check that my actual breadboarded results are what the simulation expects. The testing I have been doing is to measure the effect of changing Vcc on the value of Vout. I am comparing 2 input voltages, say 29V and 26V and expect Vout to be 3V (I am using a unity gain configuration with the op amp). The power for the op amp is varied from 27V down to zero and what I see is that as the voltage gets to around 14-15V, the Vout drops from a steady 3V to below 1V. The same holds true if my input voltages are 26V and 23V so I'm clearly hitting some limiting value of the op amp.
My question is this. Where in the LM324 documentation can I find the details that explain this behaviour and how can I use this to predict the behaviour before I start building the circuit? I'm guessing it's something to do with common rail details, but this is only a guess. My motivation is to find out what is the lowest Vcc I can drive my LM324 with if I am measuring a whole range of different input voltages from 28V down to 0V.

As per suggestion, I have now added a screenshot showing the circuit and the test results in LTSpice.

• This post could be much better with a schematic and fewer words since it's a bit rambly. First, there is a conflict because you say you are subtracting two voltages but then say you are using unity-gain buffer configuration so right there we already don't know what you really mean or have as a circuit. Then it sounds like you are giving the op amp 15V of supply but applying a whopping 27.5V common mode input signal to it and expecting it to work. You need to make your post much clearer about what it is you have and what you are doing. Commented May 22, 2020 at 4:11
• Use the snipping tool or the schematic editor on this website. Commented May 22, 2020 at 4:15