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SP8538BN Datasheet(PDF) 7 Page - Sipex Corporation |
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SP8538BN Datasheet(HTML) 7 Page - Sipex Corporation |
7 / 12 page SP8538DS/01 SP8538 Micropower Sampling 12-Bit A/D Converter © Copyright 1999 Sipex Corporation 7 by the differential sampling of the SP8538. Layout Considerations To preserve the high resolution and linearity of the SP8538 attention must be given to circuit board layout, ground impedance and bypassing. A circuit board layout which includes separate analog and digital ground planes will prevent the coupling of noise into sensitive converter circuits and will help to preserve the dynamic performance of the device. In single ended mode, the analog input signal should be referenced to the ground pin of the converter. This prevents any voltage drops that occur in the power supply's common return from appearing in series with the input signal. In full differential mode, the high and low side board traces should run close to each other, with the same layout. This will insure that any noise coupling will be common mode, and cancelled by the converters (patent pending) full differential architecture. If separate analog and digital ground planes are not possible, care should be used to prevent coupling between analog and digital signals. If analog and digital lines must cross, they should CS CLK DIN DOUT CS CLK DIN DOUT X X POWER UP SAMPLE CONVERT 1 4 10 16 X START O/S S/D LSBFN HI-Z “0” D1 20 D0 D1 POWER DOWN 28 X D11 HI-Z X LSB FIRST CONVERSION D11 X X POWER UP SAMPLE CONVERT 1 4 10 16 X START O/S S/D MSBF HI-Z “0” D1 D0 POWER DOWN X HI-Z X D11 17 MSB FIRST CONVERSION do so at right angles. Parallel analog and digital lines should be separated by a circuit board trace which is connected to common. The SP8538 VCC pin is also the reference pin for the device. This means that noise on the VCC pin will be proportionally represented as noise in the converters output data. A noise signal of 1.22mV (at a 5V supply) will produce 1 LSB of error in the output data. The VCC pin should be bypassed to the ground pin with a parallel combination of a 6.8 µF tantalum and a 0.1µF ceramic capacitor. To maintain maximum system accuracy, the supply connected to the VCC pin should be well isolated from digital supplies and wide load variations. A separate conductor from the supply regulator to the A/D converter will limit the effects of digital switching elsewhere in the system. Power supply noise can degrade the converters performance. Especially corrupting are noise and spikes from a switching power supply. To avoid introducing distortion when driving the A/D converter input, the input signal source should be able to charge the SP8538's equivalent 20 pF of input capacitance from zero volts to the signal level in 1.5 clock periods. SP8538 Timing Diagram |
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