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Is ADC in SoC essential or just a placeholder?

Posted: 31 Jan 2014     Print Version  Bookmark and Share

Keywords:Analogue-to-digital converters  ADCs  SoC  CPU  PGA 

Reference: The overall accuracy of an ADC depends on the accuracy of the ADC reference. Some SoCs may use an inaccurate reference, whereas some may use a very precise reference. Also, some SoCs provide an option to use the device supply as the reference. This can be useful in ratiometric measurements.

Additionally, some SoCs have internal dividers for references that indirectly provide additional gain to the input signal by reducing the step size per bit (equation 1).

Another option provided in some SoCs is to connect the analogue reference to a pin so that a bypass capacitor can be connected to filter the noise. A careful examination of reference options and specifications can help in realising precise and accurate analogue-to-digital conversion.

Input Buffer: Some ADCs are based on switched capacitor analogue and have low input impedance. However, this will introduce an error due to attenuation while measuring signals with high output impedance. For such ADCs, an input buffer is required in between the signal source and the ADC. Some SoCs offer an internal op amp that can be configured as a buffer or a Programmable Gain Amplifier. However this does not apply to integrated ADC's that have high input impedance.

Based on the application and type of ADC, it may be good to select a device that has an internal buffer. Also, these internal buffers/amplifiers can help in providing a gain stage to small input signals without the need for an external amplifier.

If the complete signal chain (amplifier + ADC) is inside the same device, this allows inputs to be multiplexed so that advanced signal processing techniques like correlated double sampling can be employed. In correlated double sampling, the input is shorted to GND and the ADC measurement is stored as an offset. Then the input is connected to the signal and the previously measured offset is subtracted to get the offset compensated result. This dynamic offset compensation is useful for getting rid of low frequency 1/f noise and also offset drift due to temperature.

Multiplexing: In many applications, ADC inputs need to be multiplexed so that various analogue signals can be measured. Some SoCs may provide flexible multiplexers with flexible pin choices, whereas others may have limited options by restricting the number of analogue input pins.

When inputs need to be multiplexed, another thing worth checking is the time required to switch from one channel to other. Also, ADC throughput reduction due to multiplexing has to be considered. Pipelined ADCs like Delta Sigma ADCs require a few samples to be dropped after switching channels. This greatly reduces the throughput of the ADCs.

Some SoCs have an ADC with on-chip op-amps and hardware sequencer. The hardware sequencer allows input channels to be multiplexed without any firmware involvement and help in reducing the latency. Figure 2 shows the high level architecture of SAR ADC block in such a devices.

Figure 2: The high-level architecture of the SAR ADC block in PSoC 4 devices allows ADC inputs need to be multiplexed without any firmware involvement while helping to reduce latency.

Power Consumption: Integrated ADCs in SoCs can help in achieving lower average power if the SoC provides the ability to power down the ADC when it not in use. Some SoCs support various power modes for ADCs. However, as the power of the ADC is reduced, this may result in reduced performance in terms of sample rate, noise, offset, and gain errors, etc. The optimum power may be selected by balancing the required analogue performance and power budget.

SoCs with DMA (Direct Memory Access) can further help in improving system efficiently when ADC samples needs to be stored in RAM and processed later or just need to be filtered using some digital filter that takes input using DMA.

In a nutshell, an ADC is a useful functional block on a SoC. It can help in yielding a compact system at lower cost. Various low-power options provided by SoCs can help in designing an application that consumes a lower power. To make it usable for a given application, specifications of integrated ADC must be examined carefully.

About the authors
Sachin Gupta is working with Cypress Semiconductor as Applications Engineer Staff in Programmable Systems Division. He holds diploma in Electronics and Communications from Vaish Technical Institute and Bachelors in Electronics and Communications from Guru Gobind Singh Indarprastha University, Delhi.

M. Ganesh Raja completed his Diploma in Electronics and Communications Engineering in 1992. He works as a Senior Principal Applications Engineer at Cypress Semiconductors.

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