Cost Reduction Through Precision Analog - Embedded.com

Cost Reduction Through Precision Analog

This “Product How-To” article focuses how to use a certain product in an embedded system and is written by a company representative.

In order to meet increasingly complex design challenges, such as adding new features while reducing board size, designers are more frequently utilizing mixed signal microcontrollers.

These devices allow for an increased level of analog integration while increasing the signal integrity with high performance analog, which allow for a reduction in the overall system cost. System cost is reduced by allowing for a reduction in the BOM and board space savings while gaining the ability to protect analog IP and promoting flexibility through the design process.

Previous generations of mixed signal microcontrollers allowed for an increased level of integration in a design, but often those featured analog components did not have enough precision for many designs. Low-end analog designs benefited from the integration, but mid- and high-end designs still required external analog components.

Newer generations of mixed signal microcontrollers have improved their analog components, and therefore are allowing mid- and some high-end designs to take advantage of the benefits of a signal device incorporating both digital and analog signals. These new mixed signal microcontrollers have upwards of 20-bit ADCs, low offset voltage amplifiers, and precision 0.1% voltage references.

The more capable the mixed signal microcontroller is, the more likely the design can be a true system on a chip. By incorporating ADCs, DACs, comparators, mixers, amplifiers, filters, and voltage references, a single mixed signal microcontroller can serve as the complete analog front-end of a design as well as the control system.

For example, a design could accept inputs from two sensors, amplify and condition the signals, then quantify them to be displayed on an LCD that is directly driven by the device. An example of this would be a temperature compensated gas meter. By using a single device and removing as many external components as possible, overall system cost can be reduced.

One of the main benefits of using a mixed signal microcontroller is analog IP protection. A complex analog design which uses individual components can be reversed engineered by competition. The list of components used can easily be determined, and the signals can be read by an oscilloscope effectively turning the analog front-end into a convenient reference design.

When a design effectively uses a mixed signal microcontroller to condition analog signals, the design is rendered as a black box. A competitor trying to determine how an analog signal is being handled only sees the input into the device and has no insight into the components used, how they are interconnected, or their settings.

Good designs utilize novel methods to solve a problem; smart designs employ novel means to protect their IP. And while IP protection does not directly save on the cost of the design, it can enable the final product to maintain a healthy ASP since a competitor's barrier to entry is increased.

Another benefit of certain mixed signal microcontrollers is that they simplify the routing of signals. When laying out traces on a PCB, designers need to take careful consideration of noise-inducing signals while properly shielding sensitive signals.

The software tools used to program these devices automatically route all the internal signals to provide the optimal integrity for analog signals, so a system designer has more time to focus on other portions of the project saving design costs.

Overall, mixed signal microcontrollers allow for cost savings in a design. The integration of ADCs, DACs, comparators, amplifiers, mixers, voltage references, analog MUXs, etc. reduce the overall bill of materials (BOM) for a design. As a device incorporates more commonly used analog components, the external components are obviously not needed and the overall system cost is reduced.

In the same respect, as these components are no longer on the board, the PCB can be reduced in size which further reduces system cost. And since there are fewer components on the board, routing is simplified, allowing a designer to spend less time dealing with trace layout noise issues.

As mixed signal microcontrollers increase the precision and accuracy of the included analog components, more designs can utilize the money savings from using these devices as systems on chips.

A good example of how new high precision mixed signal microcontrollers can provide a concise solution over previous generations is through a digital cooking thermometer. In this example, a type K thermocouple will be used for the temperature probe, which provides an output of ~40 μV/o C.

Since the output is small for the range needed, a very precise reference is needed to accurately capture the signal. The thermocouple output is an absolute measurement so a cold junction reading is required as well, which is performed by a thermistor (the thermistor provides a ratiometric measurement).

Previous generations of mixed signal microcontrollers required the use of some external components to properly measure a thermocouple. Because the internal voltage reference inside of older mixed signal microcontrollers are accurate to 3% on average, an external precision reference, typically with an 0.1% accuracy, is commonly used for this purpose.

The voltage reference is used as a scale reference point and is fed into the controller's ADC as well as the thermocouple input. The ADC then alternates between reading the thermocouple and the voltage reference to provide a proper reading.

Because the thermocouple's output is small, an amplifier may be used to increase the signal, depending on the resolution of the ADC. The output of the thermistor is read as a thermal reference and the output of the thermocouple is added to this measurement. Figure 1 below shows this setup.

Figure 1: Block diagram of a thermocouple reading based on less precise mixed signal microcontrollers.

For the new mixed signal microcontrollers the set up for measuring a thermocouple's output is far more simple. Since some of these mixed signal microcontrollers have highly accurate voltage reference – such as Cypress' PSoC3 with a 0.1% voltage reference – an external reference is not needed.

Figure 2: Block diagram of a thermocouple reader implemented on a high-precision mixed signal microcontroller.

The ADC can use the internal reference for measurement so the designer does not need to worry about accounting for the additional set up to accurately read a thermocouple as shown in Figure 2 above . In Figure 3 below is the same thermocouple reader as implemented in PSoC Creator.

Figure 3: Thermocouple reader as implemented in PSoC Creator for the PSoC 3

If the device being used also has a high resolution ADC, then the amplification stage can be removed since the conversion will provide enough granularity of the signal. For example, the availability of a 20-bit Delta Sigma ADC enables measurement of a 1.0V range signal down to 1 μV.

New generations of mixed signal microcontrollers allow designers to simplify their designs, protect their IP, reduce the need for external components, and increase the amount of value they are receiving from a device resulting in cost savings. The increased quality of the analog components in these devices provide designers with more choices in how a design can be implemented for even more simplified designs.

Aaron GL Podbelski is a Product Manager at Cypress Semiconductor in San Jose, CA. He is responsible for PSoC marketing, focusing on analog applications. His previous experience at Cypress involves USB and RF products. Aaron received his BE in Computer Engineering from Marquette University.

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