7.2.1.2 Design Requirements

As with any high-resolution ADC, appropriate PCB layout and grounding techniques must be followed to eliminate ground loops, unwanted parasitic effects, and noise.

Ground loops are formed when return current from the ADC flows through paths that are common with other analog or digital circuitry. If care is not taken, this current can generate small unwanted offset voltages that can add to or subtract from the reference or input voltages of the ADC. The general guidelines in Section 7.1.1 combined with the connections shown in Figure 7-5 prevent this.

Quickly switching digital signals and noisy power supply lines can corrupt the conversion results, so keep the ADC input trace shielded from those digital and power supply lines. Putting the MCU in low-power mode during the ADC conversion improves the ADC performance in a noisy environment. If the device includes the analog power pair inputs (AVCC and AVSS), TI recommends a noise-free design using separate analog and digital ground planes with a single-point connection to achieve high accuracy.

Figure 7-5 shows the recommended decoupling circuit when an external voltage reference is used. The internal reference module has a maximum drive current as described in the sections ADC Pin Enable and 1.2-V Reference Settings of the MSP430FR4xx and MSP430FR2xx Family User's Guide.

The reference voltage must be a stable voltage for accurate measurements. The capacitor values that are selected in the general guidelines filter out the high- and low-frequency ripple before the reference voltage enters the device. In this case, the 10-µF capacitor buffers the reference pin and filters any low-frequency ripple. A bypass capacitor of 100 nF filters out any high-frequency noise.