Keep in mind that the two connections of a component such as a resistor create two equal, but opposite polarity thermoelectric voltages (assuming they are connected to the same metal, such as the copper trace on a PC board) which cancel each other assuming both are at exactly the same temperature. The accuracy of the measurement depends on the mechanical layout of the components and how they are placed on the PC board. Note that in the circuit additional resistors have been added to the non-inverting input in order to exactly match the thermocouple junctions in the inverting input path. The thermocouple voltage formed by temperature difference between two junctions can range from 2 μV/ oC to more than 40 μV/ oC. The largest potential source of error comes from parasitic thermocouple junctions formed where two different metals are joined. Measuring input offset voltage.Īs simple as it looks, this circuit may give inaccurate results. New processes, particularly developments in silicon gate technology, have overcome these problems and CMOS op-amps (Texas Instruments' LinCMOS range for instance) can achieve bipolar-level V OS figures with extremely good drift, 1−2 μV/☌ being quoted.įigure 4.2.2. This drift has to be added to the worst-case offset voltage when calculating offset effects and can be significant when operating over a wide temperature range.Įarly MOS-input op-amps suffered from poor offset voltage performance due to gate threshold voltage shifts with time, temperature, and applied gate voltage. A rule of thumb is 3.3 μV/☌ for each millivolt of initial offset. For bipolar inputs, the magnitude of drift is directly related to the initial offset at room temperature. Present technology for standard devices allows temperature coefficients of between 5 and 40 μV/☌, with 10 μV/☌ being typical. Most manufacturers will specify drift with temperature, but only those offering precision devices will specify drift over time. Offset voltage drift is closely related to initial offset voltage and is a measure of how V OS changes with temperature and time. Peter Wilson, in The Circuit Designer's Companion (Fourth Edition), 2017 Offset Drift

Inverting op amp offset 741