Many precision op amps have pins available for optional offset null. Excluding the initial hour of operation, changes in the offset voltage of these devices during the first 30 days of operation are typically less than 2 μV.Īs a general rule of thumb, it is prudent to control amplifier offset voltage by device selection whenever possible, bus sometimes trim may be desired. This refers to a time period after the first 30 days of operation. Long-term stability of the OP177 and the AD707 is approximately 0.3 μV/month.
An aging rate of 1 μV/1000 hours becomes about 3 μV/year, not 9 μV/year. Since aging is a “drunkard's walk” phenomenon, it is proportional to the square root of the elapsed time. This aging effect is generally specified as long-term stability in μV/ month, or μV/1000 hours, but this is misleading. In addition to temperature related drift, the offset voltage of an amplifier changes as time passes. However, these units tend to generate quite a bit of airflow, which can be troublesome. Localized temperature cycling of the amplifier itself using a Thermostream-type heater/cooler may be an alternative. If cold testing is required, a dry nitrogen purge is recommended. Placing the printed circuit board containing the amplifier being tested in a small box or plastic bag with foam insulation prevents the temperature chamber air current from causing thermal gradients across the parasitic thermocouples. Measuring the offset voltage shift over temperature is an even more demanding challenge. The circuit should be placed flat on a surface so that convection currents flow up and off the top of the board, not across the components as would be the case if the board was mounted vertically. In some cases, the circuit should be placed in a small closed container to eliminate the effects of external air currents. Clean connections and short lead lengths help to minimize temperature gradients and increase the accuracy of the measurement.Īirflow should be minimal so that all the thermocouple junctions stabilize at the same temperature.
#Cmos op amp offset Pc#
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