Page 414 - Op Amps Design, Applications, and Troubleshooting

390 NONiDEAL OP AMP CHARACTERISTICS

MC1741SC It consists of a 10-kilohm potentiometer connected between the off-
set null pins (1 and 5) of the op amp. The wiper arm of the potentiometer con-
nects to the negative supply voltage. The amplifier is connected for normal
operation (excluding any DC input signals), and the potentiometer is adjusted to
produce 0 volts at the output of the op amp. You should realize, however, that
this only cancels the output offset voltage at one particular operating point. With
temperature changes or simply over a period of time, the circuit may drift and
need to be readjusted. Nevertheless, it is an improvement over a circuit with no
compensation.

10.1.4 Drift
Drift is a general term that describes the change in DC operating characteristics
with time and/or temperature. The temperature drift for input offset current is
expressed in terms of nA/°C, and the drift for input offset voltage is expressed in
terms of /iV/°C. The temperature coefficient of each of these quantities varies over
the temperature range, and this variation may even include a change in polarity of
the temperature coefficient. The maximum drift may be provided in tabular form
by some manufacturers, but more meaningful data is available when the manufac-
turer provides a graph showing the response of input offset current and input off-
set voltage to changes in temperature. The data sheet in Appendix 1 is essentially a
compromise between these two methods. Here the manufacturer has provided the
values of input offset current and input offset voltage at room temperature and at
the extremes of the temperature range.
The usual way to reduce the effects of drift is to select an op amp that has a
low temperature coefficient for these parameters. Additionally, in certain critical
applications some success can be achieved by including a thermistor network as
part of the output offset voltage compensation network.

10.1.5 Input Resistance
An ideal op amp has an infinite input resistance. However, for practical op amps the
input resistance is lower but still very high. The errors caused by nonideal input
resistance in the op amp do not generally cause significant problems, and what
problems may be present can generally be minimized by ensuring that the follow-
ing conditions are satisfied:

1. The differential input resistance should be at least 10 times the value of
feedback resistor for inverting applications.
2. The differential input resistance should be at least 10 times the values of the
feedback and source resistances for noninverting applications.

In most cases, these requirements are easily met. In more demanding applications,
the designer may select a FET input op amp. The MC34001 op amp made by
Motorola, Inc., is an example of this type; it provides an input resistance of 10 12
ohms and is pin-compatible with the standard 741 device.