Last year I designed in my free time a precise e-bike battery capacity meter. In order to measure the battery charge or discharge current, I decided to make a low side voltage drop measurement over a 10 mΩ shunt resistor. For the 2.0 A end of scale, the voltage drop is going to be 20 mV, but in many other normal scenarios is going to be much lower. Voltage drop in the shunt resistor has to be low in order to provide small modification to the circuit and reduce its power dissipation.
An ideal operational amplifier has an output voltage which is the voltage difference between the positive and negative terminals multiplied by it open loop gain (Gol). However, in real devices appear another term: the offset voltage.
The offset voltage is the DC voltage that has to be between the two opamp inputs for the output voltage to be zero. We know is ideally zero, but in general, real things tends not to behave according to our ideal models!
Why should we care about offset voltage? Essentially because this offset is amplified by the stage gain and appear at the output as a DC bias. Typically no problem if the stage is AC coupled or gain is low, but can be a nightmare in systems that operate at DC and have large gain, as the example shown.
In such applications, the use of low offset operational amplifiers is a must.
In the market there is a large portfolio of low offset amplifiers. Needless to say, the best performance ones are more expensive that the others (despite cost increase is not so high). To illustrate it, let me show the offset voltage of various commonly used amplifiers. Beware of the units!
Across devices we can see an improvement of almost four orders of magnitude in offset voltage.
Between the first two devices there is just an improvement in the manufacturing technology. To reduce it further some other approaches are needed. This is the result of the work of very talented engineers.
Worth mentioning a legend in the reduction of input offset. Around 1975 PMI introduced OP07. By trimming the amplifier on the factory offset was guaranteed to be less than 150 μV, a giant step in performance at that time.
Next improvements (so called zero-drift) came by the hand of architectural improvements. Some of the zero-drift techniques are continuous time and others discrete time (the chopper amplifiers).
Chopper amplifiers not only reduce input offset to a negligible value but also removes the 1/f noise (a very low frequency noise) which can be a very limiting factor for many DC applications.
To learn more:
Zero-Drift Amplifiers: Features and Benefits: two pages application brief about zero-drift amplifiers. It includes a nice figure that shows why offset voltage cannot be calibrated, as it strongly depends on common mode voltage, which many times is the signal itself.
Opamp history - Analog Device's Walt Jung incredible work about the history of operational amplifiers. It will be a delight of the lovers of the history of electronics.