11.6 Drift
Drift is a variation in the output offset voltage. Often, it is temperature induced. Even if a circuit has been manually nulled, an output offset can be produced if the temperature changes. This is because πππ and πΌππ are temperature sensitive. The only way around this is to keep the circuit in a constant temperature environment. This can be very costly. If the drift can be kept within an acceptable range, the added cost of cooling and heating equipment may be removed. As you might expect, the magnitude of the drift depends on the size of the temperature change. It also depends on the πΌππ and πππ sensitivities. These items are Ξπππ/Ξπ , the change in πππ with respect to temperature, and ΞπΌππ/Ξπ , the change in πΌππ with respect to temperature. Drift rates are specified in terms of change per centigrade degree. These parameters are usually specified as worst-case values and can produce either a positive or negative potential.
The development of the drift Equation pretty much follows that of the Equation for offsets. The only difference is that offset parameters are replaced by their temperature coefficients and the temperature change. The products of the coefficients and the change in temperature produce an input offset voltage and current.
![\[V_{drift} = \frac{\Delta V_{OS}}{\Delta T} \Delta T A_{noise} + \frac{\Delta I_{OS}}{\Delta T} \Delta T R_f \]](https://pressbooks.nscc.ca/app/uploads/quicklatex/quicklatex.com-e6f98ba07d723fc3f4a48a8dbb43622c_l3.png "Rendered by QuickLaTeX.com")
(5.21)
As with the offset calculation, the drift result may be either positive or negative. Also, because πΌππ is so small for FET input devices, ΞπΌππ/Ξπ is often not listed, as it is almost always small enough to ignore. For lowest drift, it is assumed that the op amp uses the offset compensation resistor π πππ , and that the circuit has been nulled.
Example 5.6.1
Determine the output drift for the circuit of Figure 5.5.3 for a target temperature of 80 β C. Assume that π πππ=909Ξ© and that the circuit has been nulled at 25 β C.
The parameters for the 5534 are Ξπππ/Ξπ=5ππ/πΆβ , ΞπΌππ/Ξπ=200ππ΄/πΆβ .
The noise gain was already determined to be 11 in Example 5.5.1. The total temperature change is from 25 β C to 80 β C, or 55C β .
![\[V_{drift} = 5 \mu V/C^{\circ}\times 55C^{\circ}\times 11+200 pA/C^{\circ}\times 55C^{\circ}\times 10 k \]](https://pressbooks.nscc.ca/app/uploads/quicklatex/quicklatex.com-f293b30fe892277f90135f7e9b56d489_l3.png "Rendered by QuickLaTeX.com")
![\[V_{drift} = 3.025mV+.11mV \]](https://pressbooks.nscc.ca/app/uploads/quicklatex/quicklatex.com-43c11b125a3708f13b20002efca59dac_l3.png "Rendered by QuickLaTeX.com")
![\[V_{drift} = 3.135mV \]](https://pressbooks.nscc.ca/app/uploads/quicklatex/quicklatex.com-f5dfc3f3cd9f6698f54f24880fe06882_l3.png "Rendered by QuickLaTeX.com")
Note that for this circuit the πππ drift is the major source of error. At 80 β C, the output of the circuit may have up to Β± 3.135 mV of DC error.
As with offsets, drift is partially a function of the circuit gain. Therefore high gain circuits often appear to have excessive drift. In order to compare different amplifiers, input referred drift is often used. To find input referred drift, just divide the output drift by the signal gain of the amplifier. Don’t use the noise gain! In this way, both inverting and noninverting amplifiers can be compared on an equal footing. For the circuit just examined, the input referred drift is
![\[V_{drift (input)} = \frac{V_{drift}}{A_v} \]](https://pressbooks.nscc.ca/app/uploads/quicklatex/quicklatex.com-3a18a74e087a427ef9342a8064fc0701_l3.png "Rendered by QuickLaTeX.com")
![\[V_{drift (input)} = \frac{3.135 mV}{11} \]](https://pressbooks.nscc.ca/app/uploads/quicklatex/quicklatex.com-4a5e716e830159488a4ec90df08e845b_l3.png "Rendered by QuickLaTeX.com")
![\[V_{drift (input)} = 285 \mu V \]](https://pressbooks.nscc.ca/app/uploads/quicklatex/quicklatex.com-fe6c4ec84c3a5e43425535eb5742e90c_l3.png "Rendered by QuickLaTeX.com")
For many applications, particularly those primarily concerned with AC performance, drift specification is not very important. A communications amplifier, for example, might use an output coupling capacitor to block any drift or offset from reaching the output if it had to. Drift is usually important for applications involving DC or very low frequencies where coupling capacitors are not practical.
