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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.enter image description here

Thanks for your guidance. Mike

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    \$\begingroup\$ 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. \$\endgroup\$ – DKNguyen May 22 at 4:11
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    \$\begingroup\$ Use the snipping tool or the schematic editor on this website. \$\endgroup\$ – DKNguyen May 22 at 4:15
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for sensible results the signals on the input pins of the op-amp must be between Vee -0.3 and and about Vcc -2 (this from memory it will be in the data-sheet sowewhere)

Assuming you're using the normal unity gain circuit with four equal resistors you'll hit that limit when the high input is about twice the supply voltage.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thanks for your help Jasen. I've found the section in the data sheet - it has 0 to Vcc-1.5 or 2, depending on temp. However, in my testing I can use inputs which are far ABOVE vcc, for example Vcc = 20V, and inputs of 25V and 27V will still give me an output of 2V. Clearly I still don't understand how you calculated the Vcc limit of around half the high input voltage. I'm still not clear how you worked that out from the specs. \$\endgroup\$ – NomadAU May 22 at 6:18
  • \$\begingroup\$ @NomadAU The input voltages to the CIRCUIT are irrelevant : the opamp doesn't know what R4 is. The inputs that matter are those at the opamp input pins : attenuated by R4/R3 and R2/R1 respectively, so about HALF your input voltages. Which is consistent with your "as the voltage gets to around 14-15V, the Vout drops from a steady 3V" \$\endgroup\$ – Brian Drummond May 22 at 11:15
  • \$\begingroup\$ @NomadAU V1 and V2 are 29V and 26V, but the voltage on the LM324 inputs is only 14.5V - this voltage is the important voltage for the performance of the chip. \$\endgroup\$ – Jasen May 23 at 10:45
  • \$\begingroup\$ Thanks to all who replied. I went back to a basics lesson in op amp behaviour (many on youtube) and now understand why my op amp is behaving as it does, and importantly, the relationship between the rail voltages and those presented on the inputs. \$\endgroup\$ – NomadAU May 25 at 7:33

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