I would like to use a voltage reference along with an op amp to get dual polarity voltage references similar to figure 44 in the image below. In this figure, there is a 5kohm resistor (R3) connecting the positive input of the op amp to ground. From my (somewhat limited) understanding of op amps, the currents at the positive and negative inputs to an op amp are negligibly small, meaning the voltage across R3 is negligible as well. Hence the gain would just be -(R2/R1) using node voltage equations which makes sense why the output is -5V given R1=R2 and the input voltage is +5V. In this case I don't understand why R3 is included in this circuit at all. Can someone please explain why resistor R3 is needed in this circuit?
If you don't have it, the 1 nA input bias current would see 5k at the inverting input and create a 5 uV error times a noise gain
of two, so 10 uV at the output. The Vos is 60 uV, so you probably don't care. If the resistors are higher value, if the input bias current
is higher, (uA), and the noise gain is high, you can have tens of millivolts of error.
A more subtle reason. For some op amps, power supply sequencing is important. If the non-inverting input goes directly to ground,
you can have high current during turn on and blow the part up. So the resistor saves you.
Thanks Harry! Not an EE here, but I suppose my professor was over simplifying things when teaching us about op amps. When dealing with more precision in circuit design there are many more things to consider as I am learning.