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ZN449D Datasheet(PDF) 9 Page - List of Unclassifed Manufacturers |
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ZN449D Datasheet(HTML) 9 Page - List of Unclassifed Manufacturers |
9 / 18 page ZN448/9 9 REFERENCE (a) Internal reference The internal reference is an active bandgap circuit which is equivalent to a 2.5V Zener diode with a very low slope impedance (Fig.11). A Resistor (R REF) should be connected between pins 8 and 10. The recommended value of 390 Ω will supply a nominal reference current of (5 - 2.5)/0.39=6.4mA. A stabilising/ decoupling capacitor, C REF (4µ7), is required between pins 8 and 9. For internal reference operation V REF OUT (pin 8) is connected to V REF IN (pin 7). UP to five ZN448/9's may be driven from one internal reference, there being no need to reduce R REF. This useful feature saves power and gives excellent gain tracking between the converters. Alternatively the internal reference can be used as the reference voltage for other external circuits and can source or sink up to 3mA. (b) External reference If required an external reference in the range +1.5 to +3.0V may be connected to V REF IN. The slope resistance of such a reference source should be less than 2.5 Ω, where n is the n number of converters supplied. GROUND (PIN 9) CREF (0.47µ) RREF (390) VREFOUT (PIN 8) VCC +5V (PIN 10) Fig.11 Internal voltage reference RATIOMETRIC OPERATION If the output from a transducer varies with its supply then an external reference for the ZN4448/9 should be derived from the same supply. The external reference can vary from +1.5 to +3.0V. The ZN448/9 will operate if V REF IN is less than +1.5V but reduced overdrive to the comparator will increase its delay and so the conversion time will need to be increased. COMPARATOR The ZN448/9 contains a fast comparator, the equivalent input circuit of which is shown in Fig.12. A negative supply voltage is required to supply the tail current of the comparator. However as this is only 25 to 150 µA and need not be well stabilised it can be supplied by a simple diode pump circuit driven from the BUSY output. |
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