Mobile-portable, green energy-saving is the development trend of future sensors

Industry analysts agree that the future trend of the system is mobile portable, "green" energy saving, and integration of more sensors in the terminal equipment. This trend requires analog-to-digital (ADC) and digital-to-analog (DAC) converters to have more channels, higher speeds and performance, while also requiring lower power budgets, smaller sizes, and Lower cost.

Major data converter vendors respond positively to these demands by making more data converters that integrate other circuit components. Despite the large number of peripherals surrounding many microprocessor cores, some performance requirements are driving the development of many special analog front ends or other analog "matching" chips that work with a single processor.

For example, TI recently introduced the ADS1298, a complete electrocardiogram (ECG) system front-end. It packs eight 24-bit ADCs with programmable gain amplifiers and numerous auxiliary circuits into a single BGA or TQFP package. Since data converters are part of a single-package integrated system, they tend to become more practical; the product specifications of the ADS1298 cover many specific functions and terminology, and some manufacturers outside the field of ECG devices may not be familiar with this. Does this mean that you can only use ADS1298 for ECG applications?

If you want to study these integrated devices and understand how they can benefit your system, simply split them and see how they implement the so-called signal chain, as shown in Figure 1.

Figure 1 Signal Chain Structure Diagram The structure diagram shown in Figure 1 can represent all systems for signal processing. If it is a measurement or data acquisition system, the signal chain starts at the sensor, goes through a signal conditioning circuit, enters an ADCr, and ends at the processor. If it is a control system, an audio processing system, or a software-defined radio device, there may be some processor output that must be returned to the analog signal; this situation is shown on the right-hand side of the structure diagram by a factor of two.

Regardless of the type of system you are designing, there is a better way to determine the components that implement your signal chain. In general, the processor is the first component to be selected first. This choice is generally based on familiarity with the device (which is the processor your company has used in previous designs), or on certain peripherals and the features they have. Therefore, you start at the center of the structure diagram shown in Figure 1, and then develop outwards.

This means that the next choice is the data converter, and starting with analog circuits is logical. Assuming we are designing a measurement system, we only need to deal with an ADC. Determining how high resolution your measurement requires and how fast it takes to measure is an important decision. Of course, there are many other aspects to consider, but two important aspects are speed and resolution. Please note that I haven't said how many bits of data converters there are—it's just a matter of how high resolution your measurements require. It's a number of physical parameters. Therefore, it is best to say that your measurement requires a resolution of at least 250 ppm, rather than choosing a 12-bit converter.

If our design process is really from the inside out, the next one is signal conditioning, but its purpose is to use all the signals provided by the sensor and then match it to the input range of the data converter. Therefore, we must first understand what kind of signal the sensor provides to us. We assume that the sensor can output 2V at maximum, then 2*250ppm=0.5mV is the result that you want to measure in the sensor.

Now you can consider how to measure a 0.5mV change. One way to solve this problem is to use an amplifier to boost the signal to match the full-scale range of your converter—we assume 5V. After a gain of 2.5, the 0.5mV of the sensor becomes 1.25mV, so the converter needs to resolve 1.25mV from 5V, that is, 1/4000. So, a 12-bit converter can do the job. Another way is to use a higher resolution converter that can measure 0.5mV directly without the need for signal conditioning. Which method is used depends on how much power and size are saved after the high-resolution converters are removed and how much the cost is saved. There is another case where the impedance of the sensor is so high that it cannot directly enter the converter, so removing the amplifier is not an option.

Understanding the system signal chain and understanding the needs of each module can help you determine whether these highly integrated converters really have a converter that helps your design. You can of course use the ADS1298 for systems other than ECG, but the many benefits it brings are attractive only if your signal chain requires all the internal modules of the device.

In future articles, we will introduce some basic knowledge of accurately acquiring signals and expressing them in the digital domain. Many of the rules of thumb or suggestions we take for granted need specific analysis. The purpose is to understand the reasons for giving these suggestions so as to help you understand how to apply these rules and suggestions given the requirements of specific systems.