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. Unfortunately, FETs do suffer from larger input voltage variations due to transconductance curve mismatches.Īs mentioned in Chapter Two, the input current into the bases (or gates, in the case of an FET) of the first stage is called \(I_B\), the input bias current.Peter Wilson, in The Circuit Designer's Companion (Fourth Edition), 2017 Offset Drift
For field effect devices, current variation is much less of a problem as the magnitude of input current is very low to begin with. Base-emitter junction voltage variation is the major cause of input voltage deviation. Let's see what the causes are and how we can reduce or eliminate their effect.įor bipolar input sections the major cause of input current mismatch is the variation of beta. This will reduce their maximum volume and increase their distortion. Dynamic loudspeakers and headphones are two loads that should not be fed DC signals. In other applications, offsets can harm following stages or loads. It might also be 101 mV with -1mV offset. For example, if the circuit output measures 100 mV, the signal might be 99 mV with 1 mV of offset. For measurement applications, this offset creates uncertainty in readings. Because all op amps are slightly different, you never know what the exact output offset will be. This difference, or unbalance, is amplified by the remaining stages and will eventually produce a DC voltage at the output. Because of this, their DC bias points are slightly different.
One possible example is the fact that the transistors used for the differential amplifier stage will not have identical characteristics. Even though part matching is very close when ICs are made, the parts will not be identical. If op amps were perfect, there would be no such thing as an offset. Offsets are undesirable DC levels appearing at the output of a circuit.
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