Are you utilizing all the features of your integrated analog-to-digital converter (ADC) inside your microcontroller (MCU) to lower the power consumption of your design? This blog will walk you through how an integrated ADC can help you achieve lower power consumption.
For this discussion, we will use the integrated precision ADC inside the MSP432P401R MCU as an example. The Precision ADC was designed with low power applications in mind and turn on times reduced with duty cycled applications. However, each application is different so to reach the lowest power consumption possible, the knobs or programmability of the Precision ADC must be selected with care.
This post focuses on a few key features of the MSP432™ MCUs, which allow you to customize the power and performance of the Precision ADC:
The selectable reference lets users select the minimum current for their performance. Use the supply as the reference for ultimate low power if it is a stable supply. Using the supply as the reference means no current is needed for the internal reference and there is no startup time for the reference.
Fast startup time
Precision ADC has been designed with fast startup times to enhance the low power for duty cycled applications. The ADC and clock (MODOSC or SYSOSC) turn on time is fast. Also, the internal reference, which is low power, turns on first before its buffers which settle fast (see device data sheet for specific values) turn on. The fast buffer settle time is enabled because an external capacitor is not required which would take time to charge. This minimizes the time the buffer is on to just when it is being used, verses a longer time to charge an external capacitor.
Selectable clock source
A system level power budget needs to be considered when looking at clock choice. A faster clock that finishes the conversion sooner can save energy in some cases. A duty cycled application may benefit from MODCLK that has a fast startup time. The user must consider that the increased current from a different clock source could minimize the time the ADC is on and result in a net power save.
Power modes (ADC14PWRMD bit) adjust the current consumption based on the maximum sample rate, primarily by adjusting the buffers used when internal reference is selected. If you are using a slower clock for the Precision ADC, consider using the low power mode (ADC14PWRMD = 2), as is the case with SYSOSC, as the clock source (See device specific data sheet for specific clock requirements).
When external reference is used, the delta in energy per conversion between ADC14PWRMD settings is small as the reference buffer is not used. In this case, the slower clock reduces the ADC current consumption but it takes longer to finish.
When internal reference is used, the minimum energy power mode depends on your application. Factors such as any power savings from going to a lower power mode when ADC in not active, sample time, number of conversions, clock or reference is used elsewhere, clock frequency, number of conversions, etc. needs to be considered on a per application basis. Note for applications with long sample times, the ADC sample time current is less than the conversion current so you will see numbers less than what is in the datasheet. You may want to do some bench testing to see what the ADC current consumption is in your application.
Using the internal reference with minimum sample time and considering energy of MODOSC/SYSOSC, the low power mode of a single ADC conversion is the minimum energy. But with five or more conversions back to back, the conversion speed starts to dominate and the regular power mode with the faster clock offers the minimum energy. See Figure 1 for a comparison of the energy required for the two power modes for different number of conversions in 12-bit mode.
To help you optimize for your system, two example current profiles are shown below in Figure 2 for Precision ADC with internal reference in regular and low power modes.
Low minimum supply
Precision ADC supports a best-in-class minimum supply voltage of 1.62 V when ADC14PWRMD = 2 (200ksps max) or 1.8 V min for full speed operation. For battery operation, this can extend the battery life if the low power mode can be used and still sample the signal adequately. For regulated supplies, using a buck converter for lower voltage can dramatically increase efficiency for all the current sources and lower the current pull from the supply.
Ability to use integrated DC/DC to power core voltage
MSP432 MCUs offer an integrated DC/DC converter to increase efficiency on the core supply which includes the Precision ADC digital logic. The DC/DC reduces the current draw from the supply for the digital portion of the Precision ADC current. For differential input, there is negligible difference in performance when DC/DC is used. For single-ended input mode, there is a small affect 70 dB v. 73 dB typical SINAD (signal-to-noise and distortion ration). See the device specific datasheet for full details to ensure Precision ADC with DC/DC converter will work for your application.
Auto power down
Auto power down is a part of the Precision ADC that helps it achieve low power without the user doing anything. When the Precision ADC is not actively converting, the core is automatically disabled and automatically re-enabled when needed. The clock source, MODOSC or SYSOSC, is also automatically enabled to provide MODCLK or SYSCLK to the Precision ADC when needed and disabled when not needed for the Precision ADC or for the rest of the device. The Precision ADC MODOSC/SYS OSC turns on in parallel with the internal reference so there is no penalty for having the clock automatically powered off.
The internal reference can also be automatically powered down between not in sample or convert phase by setting ADC14REFBURST bit and having REFON bit set to 0.
Internal temperature sensor
The internal temperature sensor was designed to require a shorter sample time than previous MSP devices to minimize energy used to measure temperature.
The last four items on the list were covered in more detail in blog two and three of this series:
How many of the above knobs can you leverage to lower the power consumption of MSP432 MCU’s ADC for your application?
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