How to choose between discrete and on-board ADCs

by Franck Vermeulen, Senior FAE, Future Electronics (France)

READ THIS TO FIND OUT ABOUT: The relative merits of discrete and on-board ADCs Which applications benefit from the different types of ADC

Integration has always been one of the main driving factors in the semiconductor industry, however, one of the hardest tasks in semiconductor integration is to combine analogue and digital functions onto a single chip. Even when it can be achieved, by using high-volume ASICs for example, it often results in design and performance compromises. Franck Vermeulen, Senior FAE, Future Electronics (France), explains.

This compromise appears most obviously in the device that stands at the borderline of the digital and analogue worlds: the Analogue-to-Digital Converter (ADC). ADCs are often integrated into ICs such as microcontrollers. However, in contrast to the general rule about increasing integration, discrete (off-chip) ADCs are forecast to enjoy strong growth.

So for standard conversion applications, what are the important differences between an on-chip and off-chip ADC? What should a designer know when deciding between them?

The issues for the designer fall into three broad categories:

• board-design constraints
• conversion performance
• requirements of the design cycle

 

Board-design constraints

A microcontroller with an on-chip ADC offers the benefits of reduced cost and part count compared to a circuit using a discrete ADC. However, if the signal to be captured (i.e. the sensor) is relatively remote from the place in which it is to be used (i.e. the data-processing, control, display or storage components), then the risk of noise corrupting the signal must be mitigated.

It may, for example, be possible for the converter and the microcontroller to be relocated next to the sensor. However, this might be impossible if the application cannot provide enough space around the sensor to carry the microcontroller and its associated peripheral components, or if other parts of the complete system prevent the relocation of the microcontroller. Also, the environment in which the sensor is located might not be suitable for the microcontroller if, for instance, the microcontroller could be exposed to extremes of temperature or hazardous conditions. For these reasons, relocation of the microcontroller is rarely practical.

The alternative is to adopt circuit-design measures that protect the integrity of the signal over a long trace. If the range of the analogue input signals is large enough, the noise on the signal path might have negligible impact.

Noise can affect not just the integrity of the signal on the signal path, but also compromise the accuracy of the analogue-to-digital conversion. Microcontrollers that offer a fully-differential ADC, such as the MCF52211 ColdFire v2 MCU from Freescale Semiconductor, help provide immunity from noise.

At their most extreme, ground glitches can reach 200mV in certain conditions. It is often impossible to split the noisy ground areas from the analogue signal path when using an integrated ADC, as the signal path and ground are very close together (see Figures 2 and 3). The result will be output errors from the ADC. So, in applications with high levels of ground noise, there is a clear argument in favour of using an external ADC. This allows complete freedom to optimise the design of the PCB traces for noise reduction.

Of course, changes to the power-supply design might also reduce the noise rejected to the ground. The power supply is not the only source of noise on the ground plane: the power profile of the microcontroller itself will spill noise to its ground, and hence to the ADC’s input signal. This noise will bear more relation to CPU usage, than it would to the sampling activity in the on-board ADC.

The very high frequencies at which today’s microcontrollers operate generate steep current changes on the power-supply pins of the processor. Such high di/dt generates voltage spikes in the parasitic inductances on the circuit. These conditions can, in turn, temporarily change the voltage reference of the processor, and hence change the reference voltage (V_ref) of their integrated ADCs. This introduces errors into the values captured by them. The isolated power supply of an off-chip ADC is immune from such noise, and its performance can often be markedly better than that of an on-chip ADC.

However, the fundamentals of good board design cannot be ignored: splitting the power circuits, digital circuits and analogue or small-signal circuits is often the only way to achieve acceptable input-signal integrity. This mandates the use of a discrete ADC.

 

 

Conversion performance and specifications

On-chip ADCs have a performance ceiling that is lower than that of specialised discrete ADCs. System architects must therefore consider the required sampling rate and precision.

To account for the required level of precision, consider this example: signal range is 0mV to 40 mV and required precision is less than 0.4 mV (see Figure 1). A typical microcontroller is used, offering an on-chip 12-bit ADC with a V_ref of 3V. The design employed uses a standard pressure sensor such as the MPX2050 from Freescale. In this example, the microcontroller’s on-chip ADC would not be suitable, as it only provides a 0.73mV precision value.

To match the requirements of this application with an integrated ADC, a complete change of microcontroller is needed, to a device capable of providing an on-chip 14-bit ADC with a V_ref of 3V, giving a precision value of 0.18mV.

Standard microcontrollers with 14-bit ADCs are difficult to find, especially within a coherent and flexible family. One kind of microcontroller device that offers high-quality integrated analogue functionality is the PSoC family from Cypress Semiconductor. PSoC parts offer the DelSig or ADCINC14 programmable ADCs with 14-bit precision. These devices can provide a flexible, reconfigurable combination of analogue functions, but have a small 8-bit CPU that gives limited controller functionality.

It may, therefore, be possible to provide the necessary functionality with an integrated converter. To be sure, study some typical microcontroller datasheets: how many specify the compute bandwidth used by the converter? The answer is very few. A notable exception is the PSoC family. See, for instance, AN2239 on www.cypress.com.

Other built-in ADC parameters might also be missing or partly missing from a standard microcontroller datasheet. For instance:

• What is the Effective Number Of Bits (ENOB)?
• What happens if the signal falls outside the specified input range?
• What is the power consumption at different sampling rates?
• What is the maximum control jitter?

Of course, all of these issues will be clear from the data provided with any high-precision discrete ADC. Such high-performance ADCs are available in many variants from suppliers such as National Semiconductor and austriamicrosystems. These stand-alone ADCs offers additional advantages:

• Versatile input configurations
• Signal type optimisation
• Support for high-impedance sensors

Finally, the latest generation of stand-alone ADCs offers a way to gain high precision without the need for a complex signal-conditioning circuit, or for an expensive sensor with built-in signal conditioning. These flexible ADCs can zoom and pan to the relevant portion of the voltage range before they sample it at high resolution. A good example of this type of flexible ADC is the SX8724 with ZoomingADC™ technology from Semtech Corporation.

 

 

The requirements of the design cycle

At the outset of the design process it can be hard to know whether the application might eventually need a faster or more precise ADC than initially forecast. In this instance, an off-chip device would appear to offer the flexibility to change the ADC without requiring a replacement microcontroller.

Indeed, ADC manufacturers often produce families of devices that make it easy to upgrade or downgrade. National Semiconductor’s general-purpose ADCs, for instance, offer pin- and code-compatibility across the whole ADCxx1Sxx1 family.

The PSoC device is another way to build flexibility into the design cycle. PSoC implements ADC functionality via a set of user-configurable digital and analogue blocks. These blocks can be easily reconfigured at any time using Cypress’ PSoC Express or PSoC Designer design tools, without affecting board layout.

A similar argument applies to products with variants offering different levels of performance. In this case, again, it is often best to maintain one basic hardware architecture with one microcontroller, and change the discrete ADC to meet the requirements of each product variant.

 

 

Conclusion

This article has described a number of factors that might push a designer towards choosing a discrete ADC in preference to a microcontroller’s on-board ADC. These factors include the requirement for flexibility in the design process, the need for high analogue performance at reasonable cost, and the constraints of low-noise board design.

The ability of microcontrollers to meet the needs of mixed-signal designers looks set to become even more constrained in future. The main market driver for microcontroller manufacturers is the race to offer improved processor performance and digital feature sets at ever lower cost. This forces them to migrate constantly to smaller process geometries to drive down manufacturing cost. State-of-the-art microcontrollers such as the LPC2468 ARM7 device or the LPC3180 ARM9 device from NXP Semiconductors are being manufactured on 0.14 micron or 0.09 micron processes.

By contrast, the best discrete ADCs on the market are manufactured on processes no smaller than 0.6 microns. So the incompatibility between the processes for the digital microcontroller circuitry and on-board ADCs is continuing to grow. This suggests that the discrete ADC has, as the market data shows, a healthy future ahead of it.