Digital Electronics: Principles, Devices and Applications

518 Digital ElectronicsRefOut18GainADJ27Clock RateControl17ConvertCommand21BipolarOffset236.3KREFGatedClock191ClockOutLSB (12Bit)BufferOutBufferIn20V Span293025−+5K5KDACSAR3LSB (10Bit)12 MSB13 MSB10V Span24Comp IN22−32 Serial OutAnalogGND26+ComparatorAD ADC84AD ADC8420 Status15DigitalGND162814+5V +V s ShortCycle31−V sFigure 12.45Internal architecture/pin connection diagram of AD ADC-84/ADC-85/AD-5240.dual supplies of ±5 V. The following guidelines should be adhered to when selecting the values ofthe components for these circuits.1. The integrating resistor R 1 should be large enough to remain in the linear region over the inputvoltage range but small enough for undue leakage current requirements not to be placed on the PCboard. A value of 470 is the optimum for a2Vscale. For a 200 mV scale, 47 should be used.2. For a conversion rate of three readings per second (48 kHz clock), the nominal value of theintegrating capacitor C 7 is 0.22 F. A capacitor with low dielectric absorption should be usedto prevent rollover errors. Polypropylene or polycarbonate capacitors should be preferred. If theoscillator frequency is different, C 7 should be changed in inverse proportion in order to maintainthe same output swing.3. Capacitor C 8 , the auto zero capacitor, influences the noise of the system. For a 200 mV full scale,where the system noise is critical, a 0.47 F capacitor is recommended for C 8 . A smaller-valuecapacitor can be used on larger scales. For instance, 0.047 F would do for a2Vfull scale.A smaller auto zero capacitor has the additional advantage of a faster recovery from overloadcondition.