SSZTCU4 may   2016


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    2.     Reference Options
    3.     Single-ended or Differential Input
    4.     Programmable Number of Bits
    5.     Keep Reading! Check Out the Rest of the Posts in This Series about  Integrated Precision ADC:
    6.     Additional Resources

Christy She

Have you wondered why MSP microcontrollers (MCU) offer flexibility in its integrated analog-to-digital converters (ADCs) such as programmable resolution or power modes? This degree of flexibility is typically not offered in standalone ADCs. Developers can use MSP MCUs for multiple applications to leverage flexibility to optimize performance, ease of use and power consumption for a variety of applications. Recently, we explored increasing ADC performance by oversampling the precision ADC integrated into the MSP432™ MCU on Analog Wire.

Today, I will focus on a few key performance features of the MSP432P401R MCU’s precision ADC which offers the flexibility to customize for your application:

  • Performance features:
    • Reference options
    • Select single ended or differential input per channel
    • Programmable number of bits


Reference Options

Selectable reference options for the Precision ADC allows for flexibility to have the best reference voltage for different applications. The reference voltage must be larger than the maximum input signal but the closer to the maximum input signal value the better resolution the ADC will have because the step size will be smaller. The internal reference can be chosen with ADC14VRSEL bits and the voltage selected as 1.2V, 1.45V, or 2.5V with REFVSEL bits. The internal reference can even be outputted externally (with REFOUT bit) to power the sensor for ratiometric measurements or vice versa by using the AVCC supply to source the ADC reference voltage and the sensor. If the internal reference and AVCC supply don’t offer the required voltage, then pins for an external reference voltage can be selected.

Here is an example showing the increased resolution by selecting the optimal reference voltage. Input signal is precision ADC 1V and mode is selected for this example:

With a 2.5V reference it results in a precision ADC resolution of 153uV per code

With a 1.2V reference it results in a precision ADC resolution of 73uV per code

In this case using a 1.2V reference with a precision ADC provides better resolution than a 15-bit ADC with a 2.5V reference. Thus, a lot can be gained from choosing a lower reference voltage greater than the maximum input signal.

Single-ended or Differential Input

Single-ended or differential input can be selected per conversion with the memory control registers ADC14MCTLx. This allows true differential mode support, i.e., 0 - VREF common-mode when needed to simplify on-board signal conditioning circuit, thus reducing cost and system power. The ability to select differential input for one required input channel and single-ended for the rest maximizes the pins usage of the device as differential input requires two input pins vs the single-ended only requires one pin.


Programmable Number of Bits

The Precision ADC offers a programmable number of bits 8, 10, 12, or 14 with ADC14RES bits. There is less clock cycles required to complete a conversion as you reduce the number of bits, so select the minimum number of bits required to both maximize sample rate as well as minimize energy. This allows applications which prioritize speed such as fault detection to select a lower number of bits and applications where speed is not critical, such as temperature measurements, to prioritize resolution. As the number of bits is programmable, it can even be changed between conversions based on requirements of different parts of the application code that may require different ADC resolutions.

To get started, order our easy-to-use MSP432 MCU LaunchPad™ development kit.

If leveraging the precision ADCs flexibility to leverage ease of use for your application is interesting – stay tuned for the next blog in this series where I will discuss ease of use features for Precision ADC on MSP432 MCU.

For those of you developing on an MSP430™ microcontroller, the "ADC12B" inside the MSP430FR5x/MSP430FR6x MCUs have similar features.

Additional Resources