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SP8531JS Datasheet(PDF) 6 Page - Sipex Corporation |
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SP8531JS Datasheet(HTML) 6 Page - Sipex Corporation |
6 / 16 page SP8531DS/01 SP8531 12-Bit Sampling Serial Out Analog to Digital Converter © Copyright 1999 Sipex Corporation 6 Figure 1. Operating Circuit Input Impedance The input of the SP8531 can be modeled as a resistor in series with a ground referenced DC voltage source of 1.0 volt. Note that the input resistor is a switched capacitor resistor and so its value is inversely proportional to conversion rate. When the ADC is in free running mode with a 4 MHz clock applied (a conversion rate of 235 Ksps) the input resistance is nominally 600K. At a conversion rate of 117.5 Ksps the input resistor value would double to 1.2 megohms. In order to avoid introducing an un- adjusted gain error greater than 1 lsb, the device must be driven by a source whose resistance is 4096 times smaller than its input impedance. At the 4 MHz clock rate this would require a source whose resistance was less than 146 Ohms. Layout Considerations Because of the high resolution and linearity of the SP8531, system design considerations such as ground path impedance and contact resistance become very important. 116 GAIN ADJUST N.C. 215 REF OUT N.C. 314 V IN 413 OFFSET ADJ. AGND 512 V DA V SS 611 V DD SCLK 710 SD D OUT 89 CS STATUS SP8531 V IN SHUTDOWN CHIP SELECT STATUS OUT DATA OUT CLOCK IN 10kOhms * Optional filter capacitor is helpful in a noisy pc board application. 2kOhms 5kOhms N.C. +5V 0.1 µF 0.1 µF 6.8 µF + 6.8 µF To avoid introducing distortion when driving the analog inputs of these devices, the source resistance must be very low, or constant with signal level. Note that in the operating circuit there is no connection made between VDA (Pin 12) and the system power supply. This is because the analog supply pin (VDA) is connected internally to the digital supply pin (VDD) through a ten ohm resistor. This ten ohm resistor when combined with a parallel combination of 6.8 µF tantalum and 0.1 µF ceramic capacitor between VDA and analog ground, will provide some immunity to noise which resides on the system supply. To maintain maximum system accuracy, the supply connected to the VDD pin should be well isolated from digital supplies and wide load variations. To limit effects of digital switching elsewhere in a system, it often makes sense to run a separate +5V supply conductor from the supply |
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