Interfacing the M68HC05 MCU to the MC145051 A/D Converter

Freescale Semiconductor, Inc.Application NoteAnalog InterfaceThe analog input consists of the converter's high and low voltagereference pins and all 11 analog input pins. The analog specificationsare listed in Table 1.Table 1. Analog SpecificationsSymbol Parameter Min MaxV REF DC Reference Voltage V AG +4.0 V DD +0.1V AG Analog Ground V SS –0.1 V REF –4.0nc...Freescale Semiconductor, IV AI Analog Input Voltage V AG V REFThe MC145051 will take the voltage it samples off its analog input pinand convert it to a number equivalent to the ratio of the input voltage andthe difference between the V REF and V AG . This number is the converter'sdigital representation of the sampled voltage input. Figure 4 illustratesthis ratio and describes an equation that predicts the ADC's conversionvalue. For example, if V AI = 2.34 volts, then the 10-bit representation ofthat voltage is 479 or $1DF.V REFV AIV AGV AIV REF –V AG=V AIFigure 4. A/D Conversion Ratio?102 –1x 1023 = ?V REF –V AGAN1228 — Rev. 2.06 MOTOROLAFor More Information On This Product,Go to: www.freescale.com