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

Sachin Gupta, M. Ganesh Raja, Cypress Semiconductor

November 16, 2013

Sachin Gupta, M. Ganesh Raja, Cypress SemiconductorNovember 16, 2013

Analog to Digital Convertors (ADCs) are omnipresent in most embedded applications. This fact has lead manufacturers of microcontrollers and System on Chip (SoC) architectures to integrate one or more ADCs into their product offerings. In many applications, these integrated ADCs are good enough to replace a dedicated ADC used in the application. Careful selection of a SoC that has an ADC with specifications closely matching the application yields a compact system at a lower cost.

There are many specifications to be taken into consideration while selecting an ADC - resolution, sample rate, noise performance, and power consumption, to name a few. However, in controllers that integrate an ADC, there are other specifications that are worth checking as well. These include CPU overhead, the option to multiplex multiple signals, and flexibility in pin assignment. Thus, evaluating an integrated ADC in an SoC can be a little bit trickier than evaluating an external ADC.

Why a device with integrated 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. Some SoCs also have integrated op-amps or programmable gain amplifiers (PGA) that further reduce component count on the PCB (Figure 1). An SoC that offers flexibility in assigning various pins for analog functionality can help reduce layout complexity as well.

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

Lower Power consumption: Most SoCs provide an option to power down various blocks when not in use. For battery-powered applications or other power critical applications, this means integrated ADCs can help reduce average power consumption by being able to power down the ADC when it is not needed. There are various applications where analog-to-digital conversion is not needed all the time. For example, a weather monitoring system needs to measure a physical quantity just once a minute and report to a host system. In this application, the controller can take a sample, power down the ADC, transmit the result to the host system, and then go to sleep. Some devices support various power modes that automatically disable all the resources when entering a low-power mode and then power them up when device enters into a higher-power mode. This helps reduce firmware overhead as well speeds implementation.

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

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

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

Sometimes assigning a digital pin next to an analog pin can also degrade the overall noise performance due to pin-to-pin leakage. If possible, leaving a pin unused between analog and digital pins can help in improving the performance.

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