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Input Offset Voltage Drift and Aging Effects

previous Offset Adjustment (Internal Method)
Input offset voltage varies with temperature, and its temperature coefficient is known as TCVOS, or more commonly, drift. As we have mentioned, offset drift is affected by offset adjustments to the op amp, but when it has been minimized, it may be as low as 0.1μV/°C (typical value for OP177F). More typical drift values for a range of general purpose precision op amps lie in the range 1-10μV/°C. Most op amps have a specified value of TCVOS, but some, instead, have a second value of maximum VOS that is guaranteed over the operating temperature range. Such a specification is less useful, because there is no guarantee that TCVOS is constant or monotonic.
The offset voltage also changes as time passes, or ages. Aging is generally specified in μV/month or μV/1000 hours, but this can be misleading. Since aging is a "drunkard's walk" phenomenon it is proportional to the square root of the elapsed time. An aging rate of 1μV/1000 hour therefore becomes about 3μV/year (not 9μV/year).
Long-term stability of the OP177F is approximately 0.3μV/month. This refers to a time period after the first 30 days of operation. 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 2μV.
Input Bias Current, IB
Ideally, no current flows into the input terminals of an op amp. In practice, there is always two input bias currents, IB+ and IB- (see Figure 1-43 below).
Op amp input bias current
Figure 1-43: Op amp input bias current
Values of IB range from 60fA (about one electron every three microseconds) in the AD549 electrometer, to tens of microamperes in some high speed op amps. Op amps with simple input structures using BJT or FET long-tailed pair have bias currents that flow in one direction. More complex input structures (bias-compensated and current feedback op amps) may have bias currents that are the difference between two or more internal current sources, and may flow in either direction.
Bias current is a problem to the op amp user because it flows in external impedances and produces voltages, which add to system errors. Consider a non-inverting unity gain buffer driven from a source impedance of 1MΩ. If IB is 10nA, it will introduce an additional 10mV of error. This degree of error is not trivial in any system.
Or, if the designer simply forgets about IB and uses capacitive coupling, the circuit won't work— at all! Or, if IB is low enough, it may work momentarily while the capacitor charges, giving even more misleading results. The moral here is not to neglect the effects of IB, in any op amp circuit. The same admonition goes for in-amp circuits.
Measuring input bias current
Figure 1-44: Measuring input bias current
Input bias current (or input offset voltage) may be measured using the test circuit of Figure 1-44 above. To measure IB, a large resistance, RS, is inserted in series with the input under test, creating an apparent additional offset voltage equal to IB×RS. If the actual VOS has previously been measured and recorded, the change in apparent VOS due to the change in RS can be determined, and IB is then easily computed. This yields values for IB+ and IB-. The rated value of IB is the average of the two currents, or IB = (IB+ + IB-)/2.
The input offset current, IOS, may also be calculated, by taking the difference between IB- and IB+, or IOS = IB+ − IB-. Typical useful RS values vary from 100kΩ for bipolar op amps to 1000MΩ for some FET input devices.
Note also that IOS is only meaningful where the two individual bias currents are fundamentally reasonably well-matched, to begin with. This is true for most VFB op amps. However, it wouldn't for example be meaningful to speak of IOS for a CFB op amp, as the currents are radically un-matched.
Extremely low input bias currents must be measured by integration techniques. The bias current in question is used to charge a capacitor, and the rate of voltage change is measured. If the capacitor and general circuit leakage is negligible (this is very difficult for currents under 10fA), the current may be calculated directly from the rate of change of the output of the test circuit. Figure 1-45 below illustrates the general concept. With one switch open and the opposite closed, either IB+ or IB- is measured.
Measuring very low bias currents
Figure 1-45: Measuring very low bias currents
It should be obvious that only a premium capacitor dielectric can be used for C, for example Teflon or polypropylene types.
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