No current flows in or out of the inputs. (Well actually a small bias current does, but its small)<\/p>\n\n\n\n
Output goes positive if the + input is more positive than the – input.<\/p>\n\n\n\n
Output goes negative if the + input is more negative than the – input.<\/p>\n\n\n\n
Output will do whatever is necessary to make the inputs equal.<\/p>\n\n\n\n
Rail to rail may mean the op amp can operate totally rail to rail, OR just the inputs can accept rail to rail but the output can’t OR the output can but the inputs can’t – watch out and always check the spec!<\/p>\n\n\n\n
VICMR defines a range of common-mode input voltages that result in proper operation of the op amp and describes how close the inputs can get to either supply rail.<\/p>\n\n\n\n
Another way to think of VICMR is that it describes a range which is defined as VICMR_MAX – VICMR_MIN <\/p>\n\n\n\n
If Vicmr is exceeded the normal linear operation of the op amp is no longer guaranteed. You may get clipping, the output jumping to a supply rail suddenly or other undefined behavior. This is a classic mistake often made in single supply op amp applications.<\/p>\n\n\n\n
Vicmr is quite different from op amp to op amp and may fall within or beyond the supply rails. Never assume that an op amp can receive a particular input signal range without verifying it in the datasheet specifications.<\/p>\n\n\n\n
Good resources:<\/p>\n\n\n\n
https:\/\/www.planetanalog.com\/are-you-violating-your-op-amps-input-common-mode-range\/<\/a><\/p>\n\n\n\n One of the golden rules of op amps says that no current flows into either input terminal. However, in reality, a small current flows into both inputs to bias the input transistors. Unfortunately, this bias current gets converted into a voltage by the circuit’s local resistors and amplified right along with the signal. The result is an output error in your circuit. <\/p>\n\n\n\n This can be an issue for:<\/p>\n\n\n\n For example, a LMV358 with an input 470K resistor to 2.5V outputed 2.87V in a circuit we worked on that had an op amp amplification of x17, so the bias current was adding 21.8mV to the input voltage. Reducing the pull resistor to 100K removed virtually all of the error, but also reduced the input sensitivity.<\/p>\n\n\n\n What can you do about it? <\/p>\n\n\n\n Opamps are very stable with temperature.<\/p>\n\n\n\n JFET (i.e. TL084) inputs can be bad for oscillations \/ noise as they are high impedance. Usually need to use with resistor and capacitor on input.<\/p>\n\n\n\n A typical opamp only has + and – power pins so you can use a dual rail op amp connected to a +V and 0V supply. The dual rail op amp design simply assumes GND it is half-way between its two power supplies. The main issue is typically circuitry around the opamp, but if you are constructing a fake GND that is half way between the supply rails, say a Vmid scenario where you use a potential divider, then using a dual rail opamp can be fine.<\/p>\n\n\n\n A voltage follower has the output connected to ‘-‘ and the input voltage connected to ‘+’.<\/p>\n\n\n\n Wire as a follower (output to ‘-‘) but with ‘+’ input tied to a mid voltage (0V for dual supply, between 2 resistors for single supply). If you have a single supply and don’t want to fit resistors then you can connect the ‘+’ input to 0V, but the op amp will consume more current and analog purists will berate you. Alternatively if you have a voltage rail sitting somewhere between the op amps +V & – V then you can use that.<\/p>\n\n\n\n Bad Approaches<\/p>\n\n\n\nInput Bias Current<\/h5>\n\n\n\n
Design Considerations<\/h4>\n\n\n\n
Dual Rail Opamps From A Single Rail Supply<\/h4>\n\n\n\n
Voltage Follower<\/h4>\n\n\n\n
Unused Inputs<\/h4>\n\n\n\n
Good resources<\/h5>\n\n\n\n