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ZADCS147 Datasheet(PDF) 11 Page - Zentrum Mikroelektronik Dresden AG |
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ZADCS147 Datasheet(HTML) 11 Page - Zentrum Mikroelektronik Dresden AG |
11 / 19 page Datasheet ZADCS146 / ZADCS147 Copyright © 2008, ZMD AG, Rev. 1.1 All rights reserved. The material contained herein may not be reproduced, adapted, merged, translated, stored, or used without the prior written consent of the copyright owner. The Information furnished in this publication is preliminary and subject to changes without notice. 11/19 CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM IN- IN+ Converter SGL/DIF = HIGH See Table 3 & Table 4 for Coding Schemes Shown configuration A2 … A0 = 0x000 voltage at IN– to obtain codes unequal to 0x000. The entire 12 bit transfer characteristic is then covered by IN+ if IN+ ranges from IN– to (IN– +Vref). Any voltage on IN+ > (IN– + Vref) results in code 0xFFF. Code 0xFFF is not reached, if (IN– +Vref) > VDD + 0.2V because the input voltage is clamped at VDD + 0.2V by ESD protec- tion devices. The voltage at IN– can range from -0.2V … ½ VREF with- out limiting the Code Range, assuming the fore men- tioned VDD condition is true. See also Figure 5 for input voltage ranges in unipolar conversion mode. In bipolar mode, IN+ can range from (IN– - Vref/2) to (IN– + Vref/2) keeping the converter out of code saturation. For instance, if IN– is set to a constant DC voltage of Vref/2, then IN+ can vary from 0V to Vref to cover the entire code range. Lower or higher voltages of IN+ keep the output code at the minimum or maximum code value. Figure 6 shows the input voltage ranges in bipolar mode when IN– is set to a constant DC voltage. As explained before, ZADCS146 / ZADCS147 can also be used to convert fully differential input signals that change around a common mode input voltage. The bipolar mode is best used for such purposes since it allows the input signals to be positive or negative in rela- tion to each other. The common mode level of a differential input signal is calculated VCM = (V(IN+)+ V(IN–)) / 2. To avoid code clip- ping or over steering of the converter, the common mode level can change from ¼ Vref … ¾ Vref. Within this range the peak to peak amplitude of the differential input signal can be ± Vref/2. The average input current on the analog inputs depends on the conversion rate. The signal source must be capa- ble of charging the internal sampling capacitors (typically 16pF on each input of the converter: IN+ and IN–) within the acquisition time tACQ to the required accuracy. The equivalent input circuit in sampling mode is shown in Figure 7. The following equation provides a rough hand calculation for a source impedance RS that is required to settle out a DC input signal referenced to AGND with 12 bit accuracy in a given acquisition time Figure 4: Block diagram of input multiplexer 0V 1.5*VREF VREF VIN+ VIN- VDD-VREF Code Range 0xFFF 0x000 Figure 5: Input voltage range in unipolar mode 0.5*VREF 0V VREF ¾ VREF VCM VDIFF +VREF/2 VCM Range Figure 6: Input voltage range for fully differen- tial signals in bipolar mode ¼ VREF -VREF/2 0V RSW CHOLD+ 16pF CIN 4pF RSW AGND CHOLD- 16pF CIN 4pF AGND IN+ IN- CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM Channel Multiplexer VDC 3kΩ 3kΩ Figure 7: Equivalent input circuit during sampling |
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