APPENDIX C --- REGULATOR HIGH-PRECISION DUAL-TRACKING
The L-17C requires good regulator dual-tracking over both time and temperature if it is to
exhibit its maximum precision and long-term stability. It is much less sensitive to
variations in the supply voltages if good dual-tracking is maintained. LM723 regulators
are sufficiently stable individually to provide adequate dual-tracking for optimal
performance of the L-17C when used with the desensitizer circuit described in section 5.2
(if adjusted as described in sub- section 5.3.6). Unfortunately, the LM723s require quite a
number of external trimming components, and we have only been able to obtain the
negative regulator in the form of a 14-pin DIP.
The LM117 - LM137 regulator pair are convenient to use, and are usually stable enough
to provide adequate dual-tracking (if used with a properly-adjusted desensitizer circuit) for
all but the most demanding applications. However, the stabilizing effects of the
desensitizer become increasingly imperfect if, e.g., the temperature- related shifts in dual-
tracking exceed 30 mV or so, which we have occasionally seen with these regulators.
Finally, there is always the question of regulator drift over long time-periods. In any case,
improved dual-tracking precision can also improve the linearity of any temperature-
induced baseline shifts, and, consequently, the rapidity and accuracy of the associated
compensation procedure.
We present here a simple method to greatly improve dual-tracking regulation over both
time and temperature using, in this instance, the LM117 - LM137 regulator pair. The
essence of the method is to feed the voltage from the balance-point of the desensitizer net
to the input of an operational amplifier, whose output is connected to the voltage-setting
resistor of one of the regulators. An example using the positive regulator is illustrated
below in Fig. C1
Fig. C1
Although in principle either regulator could be used, since it is the voltage difference
between them which is important, in this example we illustrate the method using a positive
(LM117) regulator.
-C1-
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