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ADS7821PB Arkusz danych(PDF) 10 Page - Texas Instruments |
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ADS7821PB Arkusz danych(HTML) 10 Page - Texas Instruments |
10 / 11 page ® 10 ADS7821 LAYOUT POWER For optimum performance, tie the analog and digital power pins to the same +5V power supply and tie the analog and digital grounds together. As noted in the electrical specifica- tions, the ADS7821 uses 98% of its power for the analog circuitry. The ADS7821 should be considered as an analog component. The +5V power for the A/D should be separate from the +5V used for the system’s digital logic. Connecting VDIG (pin 28) directly to a digital supply can reduce converter performance due to switching noise from the digital logic. For best performance, the +5V supply can be produced from what- ever analog supply is used for the rest of the analog signal conditioning. If +12V or +15V supplies are present, a simple +5V regulator can be used. Although it is not suggested, if the digital supply must be used to power the converter, be sure to properly filter the supply. Either using a filtered digital supply or a regulated analog supply, both VDIG and V ANA should be tied to the same +5V source. GROUNDING Three ground pins are present on the ADS7821. DGND is the digital supply ground. AGND2 is the analog supply ground. AGND1 is the ground which all analog signals internal to the A/D are referenced. AGND1 is more suscep- tible to current induced voltage drops and must have the path of least resistance back to the power supply. All the ground pins of the A/D should be tied to the analog ground plane, separated from the system’s digital logic ground, to achieve optimum performance. Both analog and digital ground planes should be tied to the “system” ground as near to the power supplies as possible. This helps to prevent dynamic digital ground currents from modulating the analog ground through a common impedance to power ground. SIGNAL CONDITIONING The FET switches used for the sample hold on many CMOS A/D converters release a significant amount of charge injec- tion which can cause the driving op amp to oscillate. The FET switch on the ADS7821, compared to the FET switches on other CMOS A/D converters, releases 5%-10% of the charge. There is also a resistive front end which attenuates any charge which is released. Any op amp sufficient for the signal in an application should be sufficient to drive the ADS7821. INTERMEDIATE LATCHES The ADS7821 does have tri-state outputs for the parallel port, but intermediate latches should be used if the bus will be active during conversions. If the bus is not active during conversion, the tri-state outputs can be used to isolate the A/D from other peripherals on the same bus. Tri-state outputs can also be used when the A/D is the only peripheral on the data bus. Intermediate latches are beneficial on any monolithic A/D converter. The ADS7821 has an internal LSB size of 38 µV. Transients from fast switching signals on the parallel port, even when the A/D is tri-stated, can be coupled through the substrate to the analog circuitry causing degradation of converter performance. |
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