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fed from constant current and buffered

Fed from constant current and buffered


Chapter 11

Input bias current
Analog Electronics

In multichannel packages, some comparators may remain unused. Never leave unused inputs open, as that device could oscillate on its own, which would then be coupled into the other devices in the same package. If both inputs are grounded, the unpredictable offset voltage will mean that the output voltage, and hence unit supply current, will vary. The safest course is to ground one input and supply the other from another fi xed voltage within its differential and common mode limit (which might include the supply rail), so that the device is always saturated.

11.12.6 Comparator Sourcing

We have already discussed the operation of the basic zener diode. To produce a reference from a zener, it must be temperature compensated, fed from a constant current and buffered. Temperature compensation is achieved by selecting a low-tempco zener voltage in the range 5.5 � 7 V and mating it with a silicon diode so that the voltage tempcos cancel. The combination is driven from a constant current generator and buffered to give a constant output voltage regardless of load.

Since surface breakdown increases noise and degrades stability, a precision zener is usually fabricated below the surface of the IC which contains its support circuitry, but this gives a greater spread of tempco and absolute voltage. The overall reference must therefore allow adjustment of these parameters, normally by laser wafer trimming.

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