MMC2107 - Freescale Semiconductor

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Freescale Semiconductor, Inc. Queued Analog-to-Digital Converter (QADC) nc... Freescale Semiconductor, I Analog supplies should be isolated from digital supplies as much as possible. This necessity stems from the higher performance requirements often associated with analog circuits. Therefore, deriving an analog supply from a local digital supply is not recommended. However, if for economic reasons digital and analog power are derived from a common regulator, filtering of the analog power is recommended in addition to the bypassing of the supplies already mentioned. For example, an RC low pass filter could be used to isolate the digital and analog supplies when generated by a common regulator. If multiple high precision analog circuits are locally employed (for example, two A/D converters), the analog supplies should be isolated from each other as sharing supplies introduces the potential for interference between analog circuits. Grounding is the most important factor influencing analog circuit performance in mixed signal systems (or in standalone analog systems). Close attention must be paid not to introduce additional sources of noise into the analog circuitry. Common sources of noise include ground loops, inductive coupling, and combining digital and analog grounds together inappropriately. The problem of how and when to combine digital and analog grounds arises from the large transients which the digital ground must handle. If the digital ground is not able to handle the large transients, the current from the large transients can return to ground through the analog ground. It is the excess current overflowing into the analog ground which causes performance degradation by developing a differential voltage between the true analog ground and the microcontroller’s ground pin. The end result is that the ground observed by the analog circuit is no longer true ground and often ends in skewed results. Two similar approaches designed to improve or eliminate the problems associated with grounding excess transient currents involve star-point ground systems. One approach is to star-point the different grounds at the power supply origin, thus keeping the ground isolated. Refer to Figure 18-50. Another approach is to star-point the different grounds near the analog ground pin on the microcontroller by using small traces for connecting the non-analog grounds to the analog ground. The small traces are meant only to accommodate dc differences, not ac transients. Technical Data MMC2107 – Rev. 2.0 492 Queued Analog-to-Digital Converter (QADC) MOTOROLA For More Information On This Product, Go to: www.freescale.com
Freescale Semiconductor, Inc. Queued Analog-to-Digital Converter (QADC) Pin Connection Considerations NOTE: This star-point scheme still requires adequate grounding for digital and analog subsystems in addition to the star-point ground. ANALOG POWER SUPPLY DIGITAL POWER SUPPLY +5 V AGND +5 V PGND +5 V nc... Freescale Semiconductor, I PCB V RH V RL QADC V SSA V DDA V SS V DD Figure 18-50. Star-Ground at the Point of Power Supply Origin Other suggestions for PCB layout in which the QADC is employed include: • Analog ground must be low impedance to all analog ground points in the circuit. • Bypass capacitors should be as close to the power pins as possible. • The analog ground should be isolated from the digital ground. This can be done by cutting a separate ground plane for the analog ground. • Non-minimum traces should be utilized for connecting bypass capacitors and filters to their corresponding ground/power points. • Minimum distance for trace runs when possible. MMC2107 – Rev. 2.0 Technical Data MOTOROLA Queued Analog-to-Digital Converter (QADC) 493 For More Information On This Product, Go to: www.freescale.com
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