SLAA890A December   2019  – August 2021 MSP430FR2000 , MSP430FR2032 , MSP430FR2033 , MSP430FR2100 , MSP430FR2110 , MSP430FR2111 , MSP430FR2153 , MSP430FR2155 , MSP430FR2310 , MSP430FR2311 , MSP430FR2353 , MSP430FR2355 , MSP430FR2422 , MSP430FR2433 , MSP430FR2475 , MSP430FR2476 , MSP430FR2512 , MSP430FR2522 , MSP430FR2532 , MSP430FR2533 , MSP430FR2632 , MSP430FR2633 , MSP430FR2672 , MSP430FR2673 , MSP430FR2675 , MSP430FR2676 , MSP430FR4131 , MSP430FR4132 , MSP430FR4133

 

  1.   Trademarks
  2. 1Overview of the MSP430FR4xx and MSP430FR2xx ADC Module
  3. 2Comparison Between the FR2xx/FR4xx ADC and ADC12_B
    1. 2.1 Outline of ADC12_B
    2. 2.2 Outline of FR2xx/FR4xx ADC
    3. 2.3 FR2xx/FR4xx ADC Pin Selection and Board Design
    4. 2.4 Key Parameters Comparison
  4. 3Tailoring the ADC and Reference Voltages to Your Application
    1. 3.1 Reference Voltages
    2. 3.2 Internal and External Reference Voltage
    3. 3.3 Signal Resolution
    4. 3.4 Selecting the Right Sampling and Conversion Time to Achieve the Target Conversion Rate
    5. 3.5 Clock Selection
  5. 4Using the Window Comparator to Monitor a Signal Without CPU Intervention
  6. 5Calibration of VREF and the Internal Temperature Sensor to Improve Performance
  7. 6FR2xx/FR4xx ADC Example Code and Resources
  8. 7References
  9. 8Revision History

3.2 Internal and External Reference Voltage

All MSP430™ ADCs have integrated internal reference voltages. The ADC also allows external references to be attached if the internal reference performance is not sufficient or if an alternate reference voltage is required.

Deciding between internal and external reference needs to be determined early in the design cycle by considering the various conditions of the application. If the reference voltage is used to source multiple devices or external sensors, consider the current load. The MSP430 reference voltage output can only handle a maximum load as stated in the data sheet.

The MSP430FR215x, MSP430FR235x, MSP430FR247x, MSP430FR267x internal shared reference in the ADC module can generate three selectable 1.5 V, 2.0 V, or 2.5 V. Other FR2xx/FR4xx devices include a fixed-voltage reference. AVCC is also a selectable reference voltage. Using AVCC as a reference achieves lower power. however, it measures analog voltages radiometric to a potential imprecise and changing AVCC voltage. For some applications, this may be exactly what is desired (for example, measuring full-bridge sensors that are also powered by AVCC).

Another factor that could affect the reference voltage is the dependence on temperature. The reference voltage temperature drift is located in the device-specific data sheet. If an application is in an environment with large temperature swings and accuracy is required over the entire temperature range, evaluate if the internal reference temperature coefficient is good enough. If not, consider using one of TI's external reference solutions with a low temperature coefficient and connect use the VeREF+ input on the MSP430 device.

Another consideration between the internal and the external reference is the analog reference voltage offset. All internal references have a voltage offset that needs to be taken into consideration and TLV calibration data can be used to reduce this. However, using an external reference voltage for the ADC would have a smaller voltage offset relative to the internal reference. The external reference may also be calibrated to obtain the closest reference voltage target. Provide some headroom when selecting the right reference voltage for the application so that the signal is not saturated during conversion or the signal attenuated.