3¾-Digit Dpm

Described is a digital panel meter- DPM-which is built around a special meter-IC, Type ADD3701, and may be used for the accurate measuring of voltage from a variety of sources. A highly stable reference voltage is pro- vided by an LM336. A ULN2003, IC4 is used to buffer the outputs of the ADD3701, so that the common-cathode displays can be driven direct. The ADD3701 multiplexes the displays so that the number of control lines is kept down. The current through the display segments is limited by resistors R8 to R is incl. The oscillator that determines the con- version rate of the analogue-to-digital converter in ICI requires an extern RC network (R7-C6). Because of the need of adequate suppression of the mains fre- quency, the oscillator frequency must be exactly 400 Hz (it is very nearly equal to 0.6R7C6). A preset potentiometer may be connected in series with R7 to adjust the frequency accurately. At this oscil- lator frequency, there are about 3 con- versions per second. Anoth...