ADCs in SoCs – Just a placeholder or a vital subsection? -

ADCs in SoCs – Just a placeholder or a vital subsection?


Analog to Digital Convertors (ADCs) are omnipresent in mostembedded applications. This fact has lead manufacturers of microcontrollers andSystem on Chip (SoC) architectures to integrate one or more ADCs into their productofferings. In many applications, these integrated ADCs are good enough toreplace a dedicated ADC used in the application. Careful selection of a SoCthat has an ADC with specifications closely matching the application yields acompact system at a lower cost.

There are many specifications to be taken into considerationwhile selecting an ADC – resolution, sample rate, noise performance, and powerconsumption, to name a few. However, in controllers that integrate an ADC,there are other specifications that are worth checking as well. These include CPUoverhead, the option to multiplex multiple signals, and flexibility in pinassignment. Thus, evaluating an integrated ADC in an SoC can be a little bittrickier than evaluating an external ADC.

Why a device withintegrated ADC?
Compact system:
An integrated ADC helps in reducing the board size and BOM (Bill-of-material).Based on my experience, most system designers aim to reduce the PCB size. SomeSoCs also have integrated op-amps or programmable gain amplifiers (PGA) thatfurther reduce component count on the PCB (Figure1 ). An SoC that offers flexibility in assigning various pins for analogfunctionality can help reduce layout complexity as well.

Figure 1: An MCU with an integrated ADC likethe PSoC 4 from Cypress can help reduce board size and system cost as canintegration of op-amps and programmable gain amplifiers (PGA). SoCs that offerflexibility in assigning various pins for analog functionality canreduce layout complexity as well.

Lower Powerconsumption: Most SoCs provide an option to power down various blocks whennot in use. For battery-powered applications or other power criticalapplications, this means integrated ADCs can help reduce average powerconsumption by being able to power down the ADC when it is not needed. Thereare various applications where analog-to-digital conversion is not needed allthe time. For example, a weather monitoring system needs to measure a physicalquantity just once a minute and report to a host system. In this application, thecontroller can take a sample, power down the ADC, transmit the result to thehost system, and then go to sleep. Some devices support various power modesthat automatically disable all the resources when entering a low-power mode andthen power them up when device enters into a higher-power mode. This helps reducefirmware overhead as well speeds implementation.

Lower system cost : Generally, the cost of a MCU+ADC is more compared to a device with an integratedADC with similar specifications.

There may be other issues worth considering while selectinga device. As integrated ADCs bring analog and digital counterparts of thesystem together on same silicon, this may introduce noise from the digitalsubsystem into the analog subsystem. Be sure to check noise specificationscarefully. Some manufacturers place a proper guard band around the analogsection of chip to keep noise to a minimum. Also, noise may vary based on what otherresources are being used on the chip. Thus, noise performance analysis shouldbe performed when all required resources are active on the chip.

Another challenge can be routing of traces carrying analogresources. No doubt, a device that supports analog functionality on all pins makesit is easier to pick a pin based on the placement of the sensor or the signalsource. However, most SoCs limit analog functionality to a select subset of pins.Limiting routing options in this way may force analog traces to travel alongsidetraces carrying digital signals, which may add significant noise to the signal.This issue can be solved by some PCB layoutbest practices like keeping the analog and digital lines isolated as far aspossible, surrounding analog signals with GND plane, among others, to reducethe capacitive coupling between analog and digital traces.

Sometimes assigning a digital pin next to an analog pin canalso degrade the overall noise performance due to pin-to-pin leakage. Ifpossible, leaving a pin unused between analog and digital pins can help in improvingthe performance.

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