Use best design practices when laying out a printed circuit board (PCB) for both analog and digital components. This practice typically means that the layout separates analog components from digital components.

Examples of analog components:

• ADCs
• Amplifiers
• References
• Digital-to-analog Converters (DACs)
• Analog MUXs

Examples of digital components:

• Microcontrollers
• Complex programmable logic devices (CPLDs)
• Field-programmable gate arrays (FPGAs)
• Radio frequency (RF) transceivers
• Universal serial bus (USB) transceivers
• Switching regulators

Figure 9-15 shows an example of good component placement. Although Figure 9-15 provides a good example of component placement, the best placement for each application is unique to the geometries, components, and PCB fabrication capabilities employed. No layout is a good fit for every design, so carefully consider the layout when designing with any analog component.

The use of split analog and digital ground planes is not necessary for improved noise performance (although for thermal isolation this option is a worthwhile consideration). However, the use of a solid ground plane or ground fill in PCB areas with no components is essential for optimum performance. If the system employs a split digital and analog ground plane, TI recommends connecting the ground planes as close to the device as possible. A two-layer board is possible using common grounds for both analog and digital grounds. Add layers to simplify PCB trace routing. Ground fill can also reduce EMI and RFI issues.

TI recommends that digital components, especially RF portions, be kept as far as practically possible from analog circuitry in a given system. Additionally, minimize the distance that digital control traces run through analog areas and avoid placing these traces near sensitive analog components. Digital return currents typically flow through a ground path that is as close to the digital path as possible. If a solid ground connection to a plane is not available, these currents can find paths back to the source that interfere with analog performance. The implications that layout has on the temperature-sensing functions are much more significant than for ADC functions.

Bypass Supply pins to a ground pin with a low-ESR ceramic capacitor. The optimum placement of the bypass capacitors is as close as possible to the supply pins. If the AVSS is connected to a negative supply, then connect an additional bypass capacitor from AVSS to AGND as well. The ground-side connections of the bypass capacitors must be low-impedance connections for optimum performance. The supply current flows through the bypass capacitor terminal first and then to the supply pin to make the bypassing most effective.

Analog inputs with differential connections must have a capacitor placed differentially across the inputs. The best input combinations for differential measurements are AIN0, AIN1 and AIN2, AIN3. Use high-quality differential capacitors. The best ceramic chip capacitors are C0G (NPO), which have stable properties and low-noise characteristics. Thermally isolate a copper region around the thermocouple input connections to create a thermally-stable cold junction. Obtaining acceptable performance with alternate layout schemes is possible when the design follows the above guidelines.