MSP430x5xx Family User's Guide
Contents
1. 2 bit resolution if Rs is 10 KQ tsampe Must be greater than 3 46 us SLAU208F June 2008 Revised March 2010 ADC12_A 437 Submit Documentation Feedback Copyright 2008 2010 Texas Instruments Incorporated2. I TEXAS INSTRUMENTS www ti com ADC12_A Operation Start Stop Start Conversion Sampling Sampling Conversion Complete M v y SHI SAMPCON 13 x ADC12CLK tampi gt lt tone gt P bane KE Figure 17 4 Pulse Sample Mode 17 2 5 3 Sample Timing Considerations When SAMPCON 0 all Ax inputs are high impedance When SAMPCON 1 the selected Ax input can be modeled as an RC low pass filter during the sampling time tyampie See Figure 17 5 An internal MUX on input resistance R maximum 1 8 kQ in series with capacitor C 25 pF maximum is seen by the source The capacitor C voltage V must be charged to within one half LSB of the source voltage V for an accurate n bit conversion where n is the bits of resolution required MSP430 s V Input voltage at pin Ax V External source voltage Rs V R R External source resistance Vs WY Ve R Internal MUX on input resistance C Input capacitance i E V Capacitance charging voltage NF Figure 17 5 Analog Input Equivalent Circuit The resistance of the source Rg and R affect tsampe The following equation can be used to calculate the minimum sampling time tsampie for a n bit conversion where n equals the bits of resolution trample gt Rg Ri x In 2 x C 800 ns Substituting the values for R and C given above the equation becomes trample gt Rg 1 8 KQ x In 2 x 25 pF 800 ns For example for 1
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