Okay, so I want to do a little experiment involving measuring the potential across a component. So here's a schematic I drew in or-cad

Ideally, I was expecting the output of the omp amp to be 5 volts. However, here's the result I got after running a simulation

Can anyone explain what happened to the other .9 volts? Did the omp amp truncate the voltage because it was close to the saturation voltage?

 

Without trying to figure out what the circuit is doing, I can tell you that op amps do not like to output a voltage that is the same as the power supply voltage

Try raising your power supply inputs on the op amp to 9 volts and see what happens.

 

If the output load on an LM324 is >=10k, it can go nearly to ground, but won't get within about a volt of the positive rail; best to assume not within 1.5v. Like Kermit mentioned, raising the positive supply voltage should effect a cure.

 

The (+) and (-) op-amp inputs are backwards. The feedback goes to the inverting (-) input.

 

The (+) and (-) op-amp inputs are backwards. The feedback goes to the inverting (-) input.

Well spotted!

Along with the other useful comments, it might also be worth demonstrating the operation as a true difference amplifier - by inserting an additional resistor from the 5V source V4 positive terminal to the top of R6 and another from the bottom of R6 to the common rail. Still take the measurement across R6 as you have it. This will test the ability of the amplifier configuration to amplify only the potential difference across R6.

 

Try raising your power supply inputs on the op amp to 9 volts and see what happens.

The (+) and (-) op-amp inputs are backwards. The feedback goes to the inverting (-) input.

With these in mind I finally got it to properly measure 5 volts...well 4.998volts but that's close enough

Thanks everyone for the help!

Along with the other useful comments, it might also be worth demonstrating the operation as a true difference amplifier - by inserting an additional resistor from the 5V source V4 positive terminal to the top of R6 and another from the bottom of R6 to the common rail. Still take the measurement across R6 as you have it. This will test the ability of the amplifier configuration to amplify only the potential difference across R6.

I'll make sure to do that during my next or cad simulation with this schematic

 

Actually, this circuit is cheating, it isn't measuring the voltage on the resistor, it is measuring the +5V power supply! Try removing the R6 test resistor and see the voltage does not change.

Now to really see my point, put R6 back in along with another 2K resistor in the top leg where the "5.000V" marker is. You might expect to see approximately 2.5V out of the amp, but you will see much less.

 

Actually, this circuit is cheating, it isn't measuring the voltage on the resistor, it is measuring the +5V power supply! Try removing the R6 test resistor and see the voltage does not change.

I didn't remove r6, did a test with a 1/2vcc voltage divider. Here's what I got.