Datenblatt-Suchmaschine für elektronische Bauteile |
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MC145053D Datenblatt(PDF) 7 Page - LANSDALE Semiconductor Inc. |
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MC145053D Datenblatt(HTML) 7 Page - LANSDALE Semiconductor Inc. |
7 / 15 page www.lansdale.com Page 7 of 15 Issue A ML145053 LANSDALE Semiconductor, Inc. on the first four rising edges of SCLK, and the previous 10-bit conversion result is shifted out on the first nine falling edges of SCLK. After the fourth rising edge of SCLK, the new mux address is available; therefore, on the next edge of SCLK (the fourth falling edge), the analog input voltage on the selected mux input begins charging the RC DAC and continues to do so until the tenth falling edge of SCLK. After this tenth SCLK edge, the analog input voltage is disabled from the RC DAC and the RC DAC begins the “hold” portion of the A/D conver- sion sequence. Also upon this tenth SCLK edge, control of the internal circuitry is transferred to the internal clock oscillator which drives the successive approximation logic to complete the conversion. If 16 SCLK cycles are used during each trans- fer, then there is a constraint on the minimum SCLK frequen- cy. Specifically, there must be at least one rising edge on SCLK before the A/D conversion is complete. If the SCLK frequency is too low and a rising edge does not occur during the conversion, the chip is thrown out of sync with the proces- sor and CS needs to be toggled in order to restore proper oper- ation. If 10 SCLKs are used per transfer, then there is no lower frequency limit on SCLK. Also note that if the ADC is operat- ed such that CS is inactive high between transfers, then the number of SCLK cycles per transfer can be anything between 10 and 16 cycles, but the “rising edge” constraint is still in effect if more than 10 SCLKs are used. (If CS stays active low for multiple transfers, the number of SCLK cycles must be either 10 or 16.) EOC End-of-Conversion Output (Pin 1) EOC goes low on the tenth falling edge of SCLK. A low-to- high transition on EOC occurs when the A/D conversion is complete and the data is ready for transfer. ANALOG INPUTS AND TEST MODES AN0 through AN4 Analog Multiplexer Inputs (Pins 2 – 6) The input AN0 is addressed by loading $0 into the mux address register. AN1 is addressed by $1, AN2 by $2, AN3 by $3, and AN4 by $4. Table 2 shows the input format for a 16-bit stream. The mux features a break-before-make switching struc- ture to minimize noise injection into the analog inputs. The source resistance driving these inputs must be ≤ 1 k Ω. During normal operation, leakage currents through the analog mux from unselected channels to a selected channel and leakage currents through the ESD protection diodes on the selected channel occur. These leakage currents cause an offset voltage to appear across any series source resistance on the selected channel. Therefore, any source resistance greater than 1 k Ω (Lansdale test condition) may induce errors in excess of guar- anteed specifications.There are three tests available that verify the functionality of all the control logic as well as the succes- sive approximation comparator. These tests are performed by addressing $B, $C, or $D and they convert a voltage of (Vref + VAG)/2, VAG, or Vref, respectively. The voltages are obtained internally by sampling Vref or VAG onto the appropriate ele- ments of the RC DAC during the sample phase. Addressing $B, $C, or $D produces an output of $200 (half scale), $000, or $3FF (full scale), respectively, if the converter is functioning properly. However, deviation from these values occurs in the presence of sufficient system noise (external to the chip) onVDD, VSS, Vref, or VAG. POWER AND REFERENCE PINS VSS and VDD Device Supply Pins (Pins 7 and 14) VSS is normally connected to digital ground; VDD is con- nected to a positive digital supply voltage. Low frequency (VDD – VSS) variations over the range of 4.5 to 5.5 volts do not affect the A/D accuracy. (See the Operations Ranges Table for restrictions on Vref and VAG relative to VDD and VSS.) Excessive inductance in the VDD or VSS lines, as on automat- ic test equipment, may cause A/D offsets > ± 1 LSB. Use of a 0.1 µF bypass capacitor across these pins is recommended. VAG and Vref Analog Reference Voltage Pins (Pins 8 and 9) Analog reference voltage pins which determine the lower and upper boundary of the A/D conversion. Analog input volt- ages ≥ Vref produce a full scale output and input voltages ≤ VAG produce an output of zero. CAUTION: The analog input voltage must be ≥ VSS and ≤ VDD. The A/D conversion result is ratiometric to Vref – VAG. Vref and VAG must be as noise- free as possible to avoid degradation of the A/D conversion. Ideally, Vref and VAG should be single-point connected to the voltage supply driving the system's transducers. Use of a 0.22 µF bypass capacitor across these pins is strongly urged. |
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