The analog power supply (VCCAUX_SMON) and ground (GND_SMON) inputs provide the power supply and ground reference for the analog circuitry in the SYSMON. A common mechanism for the coupling of noise into an analog circuit is from the power supply and ground connections. Excessive noise on the analog supply or ground reference affects the ADC measurement accuracy. For example, I/O switching activity can cause significant disturbance of the digital ground reference plane. Thus, it is not advisable to use the digital ground as an analog ground reference for SYSMON.
Similarly, for the digital supplies for the interconnect logic, high switching rates easily result in high-frequency voltage variations on the supply, even with decoupling. To mitigate these effects on ADC performance, a dedicated supply and ground reference is provided. The following figure illustrates how to use the 1.5V VCCAUX_PMC supply to power the analog circuitry. VCCAUX_PMC is filtered using a low-pass network. The filter design depends on the ripple and ripple frequency (if any) on the VCCAUX_PMC supply if, for example, a switching regulator is used. There is also a power-supply rejection specification for the external reference circuit to consider. The filtering should ensure no more than 1 LSB (1 mV) of noise on the reference output to minimize any impact on ADC accuracy at 10 bits. Depending on the ripple frequency of the supply, a 10–20 μH inductor might be better than a ferrite bead.
In mixed-signal designs it is common practice to use a separate analog ground plane for analog circuits to isolate the analog and digital ground return paths to the supply. Common ground impedance is a mechanism for noise coupling and needs to be carefully considered when designing the PCB. Although a separate analog ground plane is recommended for 10-bit operation, it is often not possible or practical to implement a separate analog ground plane in a design. For example, if only the on-chip sensors are used, one low-cost solution is to isolate VREFN and GND_SMON ground references (such as a trace) from the digital ground (plane) using a ferrite bead as shown in the following figure.
The ferrite bead behaves like a resistor at high frequencies and functions as a lossy inductor. The ferrite helps provide high frequency isolation between digital and analog grounds. Though it is recommended to use the on-chip reference, when using the external reference, an IC maintains a 1.024V difference between VREFP and VREFN. The ferrite offers little resistance to the analog DC return current. The reference inputs should be routed as a tightly coupled differential pair from the reference IC to the package pins. If routed on the same signal layer, the supply and analog ground traces (VCCAUX_SMON and GND_SMON) must be used to shield the reference inputs because they have a higher tolerance to any coupled noise.