LM258: Can the tolerance of a unity gain buffer made with the LM258 be estimated?

Hi Christopher,

provided the input voltage is always within the recommended range and the output of LM258 isn't too heavily loaded or even short circuited, the deviation between input and output voltage is usually dominated by the input offset voltage. And if there are resistances mounted in series to the inputs of LM258, the voltage drop of the input bias currents across these resistances must also be taken into account. These voltage drops add to the input offset voltage of the LM258.

Kai

Hi Christopher,

By tolerance, I assume you mean the potential variance in expected versus actual voltages? Kai hit on the main point in his response, but I will flesh it out a bit more.

The typical, primary source of unexpected output variation is offset voltage (Vos). Offset voltage is the differential input voltage that would have to be applied to force the op-amp's output to zero volts. It is caused by the mismatch of transistors within the part during production, meaning it's basically unavoidable. Because of this mismatch, you'll typically see a deviation in your output. For an LM258 in unity buffer configuration, this offset will be on the order of mV.

Other factors can increase this deviation in expected performance. For example, the input bias current can contribute an error that can be modeled as a ±Vos shift. If your common mode changes, this can also lead to an output deviation that can be estimated by converting this effect to an effective Vos using the CMRR specification for the part. PSRR works similarly to CMRR. So depending on your conditions and use, the effects will vary.

For your sake, I would say the simplest way to estimate the maximum offset is to take the maximum input offset voltage from the datasheet and multiply it by the "noise gain" of the op amp configuration, which is one in your case. This will give you a quick idea of what worst case scenario is for your circuit. Again, factors such as PSRR, CMRR, noise, distortion, etc. can lead to a worse result. But, this will give you a good idea.

Without knowing anything about your setup other than the unity gain configuration and assuming other factors will be negligible, we can say that the expected tolerance for the part is 7 mV (from section 7.10 of the datasheet for details), based on the maximum input offset voltage over temperature.

Texas Instruments has a fantastic series of videos/slides on op-amps that discuss this topic in detail. Here's a link if you're curious: training.ti.com/ti-precision-labs-op-amps

You might find sections 2.1 and the first slide of 7.1 especially helpful.