1. Analog (A/D) converters originate from analog paradigms, where much of the physical silicon remains analog in nature.
2. As new design topologies emerged, the paradigm shifted to emphasize digital components as the primary element in low-speed A/D converters.
3. Despite this transition from analog to digital prominence in A/D converters, the PCB wiring criteria have not changed.
4. When designers work on mixed-signal circuits, a strong understanding of critical wiring principles is essential for effective design.
5. This paper examines successive approximation and ∑-Δ type A/D converters to explore the necessary PCB routing strategies for these devices.
6. Successive approximation A/D converters are available in resolutions of 8-bit, 10-bit, 12-bit, 16-bit, and 18-bit.
7. Initially, these converters utilized bipolar processes with r-2R trapezoidal resistor networks.
8. Recently, however, they have been adapted to CMOS processes using capacitive charge distribution topologies.
9. Clearly, this transition does not alter the fundamental system routing strategies for these converters.
10. With the exception of higher-resolution devices, the basic wiring approach remains consistent.
11. For these devices, it is crucial to take precautions to avoid digital feedback from the serial or parallel output interfaces.
1. The converter utilizes a charge distribution created by an array of capacitors.
2. In this block diagram, the sampler/hold, comparator, most of the digital-to-analog converter (DAC), and the 12-bit successive approximation type A/D converter are analog, while the remainder of the circuit is digital.
3. Consequently, the majority of the energy and current needed for this converter is consumed by the internal analog circuits.
4. The device requires minimal digital current, with only a few switches operating alongside the D/A converter and digital interface.
5. Such converters may feature multiple ground and power connection pins.
6. Pin names can be misleading, as labels are often used to differentiate between analog and digital connections.
7. These labels do not reflect the system’s connection to the PCB but indicate how digital and analog currents flow from the chip.
8. Understanding this information, along with the fact that most resources used on a chip are analog, suggests that power and ground pins should be connected on the same plane, typically the analog plane.
9. For these devices, two ground pins are generally provided: AGND and DGND.
10. The power supply has a dedicated lead pin.
11. When implementing PCB wiring for these chips, AGND and DGND should be linked to the analog ground plane.
12. Both analog and digital power pins should connect to the analog power plane or at least the analog power rail, with appropriate bypass capacitance placed as close to each power pin as possible.
13. Devices like the MCP3201 may only have one ground pin and one positive power pin due to package pin limitations.
14. However, isolation increases the likelihood of good and repeatable converter performance.
15. For all these converters, the power strategy should connect all ground, positive, and negative power pins to the analog plane.
16. Additionally, the ‘COM’ or ‘IN’ pins related to the input signal should be connected as closely to the signal as possible.
17. For higher resolution successive approximation type A/D converters (16- and 18-bit), extra care is necessary to isolate digital noise from “quiet” analog converters and the power plane.
18. When these devices interface with a single-chip microcomputer, external digital buffers should be employed to ensure noiseless operation.
19. Although these successive approximation A/D converters typically include an internal dual buffer on the digital output side, an external buffer serves to further isolate the analog circuit from digital bus noise.
20. For high-resolution successive approximation type A/D converters, the converter’s power supply and ground should connect to the analog plane.
21. The digital output of the A/D converter should then be buffered using an external tri-state output buffer.
22. In addition to their high drive capability, these buffers effectively isolate the analog and digital sides.
23. The cabling strategy for high Σ-Δ type A/D converters emphasizes that their silicon area is primarily digital.
24. In the early days of these converters, users were encouraged to utilize PCB planes to separate digital noise from analog noise.
25. Similar to successive approximation A/D converters, these A/D converters can have multiple analog, digital, and power pins.
26. Generally, digital or analog design engineers prefer to separate these pins and connect them to different planes.
27. However, this tendency can be misguided, particularly when addressing serious noise issues in 16-bit to 24-bit devices.
28. For a high-resolution Σ-Δ type A/D converter operating at a 10Hz data rate, the clock (internal or external) can reach frequencies of 10MHz or 20MHz.
29. This high-frequency clock toggles the modulator and operates the oversampling engine.
30. In these circuits, AGND and DGND pins are interconnected on the same ground plane as with successive approximation A/D converters.
31. Furthermore, analog and digital power pins are also linked on the same plane.
32. The requirements for analog and digital power planes are consistent with those of high-resolution successive approximation A/D converters.
33. A floor plan is essential, necessitating at least two panels.
34. On this dual-panel layout, the floor plan should encompass at least 75% of the total area.
35. The ground plane layer aims to reduce grounding impedance and inductive reactance while shielding against electromagnetic interference (EMI) and radio frequency interference (RFI).
36. If internal wiring is necessary on the ground plane side of the board, it should be as short as possible and perpendicular to the ground current loop.
37. Conclusion: For low-resolution A/D converters, such as six-bit, eight-bit, or even ten-bit devices, it is acceptable to keep analog and digital pins unseparated.
38. However, as the choice of converters and resolutions increases, wiring requirements become more stringent.
39. High-resolution successive approximation A/D converters and Σ-Δ A/D converters must be directly connected to low-noise analog ground and the power plane.
2. As new design topologies emerged, the paradigm shifted to emphasize digital components as the primary element in low-speed A/D converters.
3. Despite this transition from analog to digital prominence in A/D converters, the PCB wiring criteria have not changed.
4. When designers work on mixed-signal circuits, a strong understanding of critical wiring principles is essential for effective design.
5. This paper examines successive approximation and ∑-Δ type A/D converters to explore the necessary PCB routing strategies for these devices.
6. Successive approximation A/D converters are available in resolutions of 8-bit, 10-bit, 12-bit, 16-bit, and 18-bit.
7. Initially, these converters utilized bipolar processes with r-2R trapezoidal resistor networks.
8. Recently, however, they have been adapted to CMOS processes using capacitive charge distribution topologies.
9. Clearly, this transition does not alter the fundamental system routing strategies for these converters.
10. With the exception of higher-resolution devices, the basic wiring approach remains consistent.
11. For these devices, it is crucial to take precautions to avoid digital feedback from the serial or parallel output interfaces.
1. The converter utilizes a charge distribution created by an array of capacitors.
2. In this block diagram, the sampler/hold, comparator, most of the digital-to-analog converter (DAC), and the 12-bit successive approximation type A/D converter are analog, while the remainder of the circuit is digital.
3. Consequently, the majority of the energy and current needed for this converter is consumed by the internal analog circuits.
4. The device requires minimal digital current, with only a few switches operating alongside the D/A converter and digital interface.
5. Such converters may feature multiple ground and power connection pins.
6. Pin names can be misleading, as labels are often used to differentiate between analog and digital connections.
7. These labels do not reflect the system’s connection to the PCB but indicate how digital and analog currents flow from the chip.
8. Understanding this information, along with the fact that most resources used on a chip are analog, suggests that power and ground pins should be connected on the same plane, typically the analog plane.
9. For these devices, two ground pins are generally provided: AGND and DGND.
10. The power supply has a dedicated lead pin.
11. When implementing PCB wiring for these chips, AGND and DGND should be linked to the analog ground plane.
12. Both analog and digital power pins should connect to the analog power plane or at least the analog power rail, with appropriate bypass capacitance placed as close to each power pin as possible.
13. Devices like the MCP3201 may only have one ground pin and one positive power pin due to package pin limitations.
14. However, isolation increases the likelihood of good and repeatable converter performance.
15. For all these converters, the power strategy should connect all ground, positive, and negative power pins to the analog plane.
16. Additionally, the ‘COM’ or ‘IN’ pins related to the input signal should be connected as closely to the signal as possible.
17. For higher resolution successive approximation type A/D converters (16- and 18-bit), extra care is necessary to isolate digital noise from “quiet” analog converters and the power plane.
18. When these devices interface with a single-chip microcomputer, external digital buffers should be employed to ensure noiseless operation.
19. Although these successive approximation A/D converters typically include an internal dual buffer on the digital output side, an external buffer serves to further isolate the analog circuit from digital bus noise.
20. For high-resolution successive approximation type A/D converters, the converter’s power supply and ground should connect to the analog plane.
21. The digital output of the A/D converter should then be buffered using an external tri-state output buffer.
22. In addition to their high drive capability, these buffers effectively isolate the analog and digital sides.
23. The cabling strategy for high Σ-Δ type A/D converters emphasizes that their silicon area is primarily digital.
24. In the early days of these converters, users were encouraged to utilize PCB planes to separate digital noise from analog noise.
25. Similar to successive approximation A/D converters, these A/D converters can have multiple analog, digital, and power pins.
26. Generally, digital or analog design engineers prefer to separate these pins and connect them to different planes.
27. However, this tendency can be misguided, particularly when addressing serious noise issues in 16-bit to 24-bit devices.
28. For a high-resolution Σ-Δ type A/D converter operating at a 10Hz data rate, the clock (internal or external) can reach frequencies of 10MHz or 20MHz.
29. This high-frequency clock toggles the modulator and operates the oversampling engine.
30. In these circuits, AGND and DGND pins are interconnected on the same ground plane as with successive approximation A/D converters.
31. Furthermore, analog and digital power pins are also linked on the same plane.
32. The requirements for analog and digital power planes are consistent with those of high-resolution successive approximation A/D converters.
33. A floor plan is essential, necessitating at least two panels.
34. On this dual-panel layout, the floor plan should encompass at least 75% of the total area.
35. The ground plane layer aims to reduce grounding impedance and inductive reactance while shielding against electromagnetic interference (EMI) and radio frequency interference (RFI).
36. If internal wiring is necessary on the ground plane side of the board, it should be as short as possible and perpendicular to the ground current loop.
37. Conclusion: For low-resolution A/D converters, such as six-bit, eight-bit, or even ten-bit devices, it is acceptable to keep analog and digital pins unseparated.
38. However, as the choice of converters and resolutions increases, wiring requirements become more stringent.
39. High-resolution successive approximation A/D converters and Σ-Δ A/D converters must be directly connected to low-noise analog ground and the power plane.
