How to achieve energy-saving analog-to-digital conversion for high-precision measurements

A typical application in electrical engineering is to record physical quantities through sensors and forward them to a microcontroller for further processing . This process requires the use of an ADC to convert the analog sensor output signal into a digital signal. In this article, ADI introduces a low-power analog-to-digital converter (ADC) solution for high-precision measurement applications, namely the SAR-ADC or Σ-Δ ADC. Because in low-power applications, every milliwatt saved will be useful.

Signal conversion using Σ-Δ ADC

Σ-Δ ADCs have some advantages over SAR-ADCs. First, they usually have higher resolution. In addition, they are usually integrated with programmable gain amplifiers (PGAs) and general-purpose input/outputs (GPIOs). Therefore, Σ-Δ ADCs are well suited for DC and low-frequency high-precision signal conditioning and measurement applications. However, due to the high fixed oversampling rate, Σ-Δ ADCs usually consume more power, which can result in a shorter service life in battery-powered applications.

If the input voltage is small (i.e. in the millivolt range), it must first be amplified so that it can be more easily managed by the ADC. A PGA analog front end (AFE) is required to interface voltages less than 10mV output. For example, in order to interface the small voltage of a bridge circuit to a sigma-delta ADC with a 2.5V input range, the PGA must have a gain of 250. However, this results in a larger noise at the ADC input, since the noise voltage is also amplified. The effective resolution of a 24-bit sigma-delta ADC is thus significantly reduced to 12 bits. However, in some cases, it is not necessary to use all the code values ​​in the ADC, and sometimes further amplification does not improve the dynamic range any more. Another disadvantage of sigma-delta ADCs is that they are generally more expensive due to their internal complexity.

Benefits of Combining a SAR-ADC with an Instrumentation Amplifier

An equally accurate but more economical and efficient alternative is to combine a SAR-ADC with an instrumentation amplifier, as shown in Figure 1.

Figure 1. Schematic showing a simplified bridge measurement circuit combined with an instrumentation amplifier and SAR-ADC

The functionality of a SAR-ADC can be divided into two phases: the data acquisition phase and the conversion phase. Basically, during the data acquisition phase, the current consumption is low. Most SAR-ADCs are even powered down between conversions. The conversion phase draws the most current. The power consumption depends on the conversion rate and scales linearly with the sampling rate. For power-saving applications targeting slow-response measurements (i.e., measurements where the measured quantity changes slowly, such as temperature measurements), a low conversion rate should be used to keep the current draw low and thus the losses low. Figure 2 shows the power loss of the AD4003 at different sampling rates. At 1kSPS, the power loss is about 10µW; at 1 MSPS, this has increased to 10mW.

Figure 2. Power loss in the AD4003 as a function of sampling rate.

Compared to this slow measurement, a Σ-Δ ADC has the advantage of oversampling while using an internal oscillator frequency that is much higher than the output rate. This allows the designer to optimize sampling to faster speed with worse noise performance, or slower speed with better filtering, noise shaping (moving noise into a frequency band outside the measurement area of ​​interest), and better noise performance. However, this means that the power consumption of the Σ-Δ ADC is much higher than that of the SAR-ADC. Many Σ-Δ ADCs have their effective resolution and noise-free resolution stated in their data sheets, so it is easy to compare the trade-offs.

in conclusion

The combination of Σ-Δ ADC with PGA and the combination of SAR-ADC with instrumentation amplifier are both suitable for signal conversion in high-precision measurement applications. The accuracy of these two solutions is similar. However, for energy-saving or battery-powered measurement applications, the combination of SAR-ADC with instrumentation amplifier is better, which consumes less power and costs less than the solution composed of PGA and Σ-Δ ADC. In addition, PGA with high gain usually limits the performance because the noise is also amplified. This article only introduces one possible solution for SAR-ADC. There are more integrated solutions, such as Σ-Δ ADC with integrated PGA such as AD7124-4/AD7124-8.

About Analog Devices

Analog Devices, Inc. (NASDAQ: ADI) is a leading global semiconductor company dedicated to building a bridge between the real world and the digital world to achieve breakthrough innovations in the field of intelligent edge. ADI provides solutions that combine analog, digital and software technologies to promote the sustainable development of digital factories, automobiles and digital healthcare, meet the challenges of climate change, and establish reliable connections between people and everything in the world. ADI's revenue in fiscal year 2022 exceeded US$12 billion and it has more than 24,000 employees worldwide. Together with 125,000 customers around the world, ADI helps innovators continue to surpass all possibilities.

About the Author

Thomas Brand joined Analog Devices in Munich, Germany in 2015 while still completing his master's degree. After graduation, he joined Analog Devices as a trainee. In 2017, he became a field applications engineer. Thomas supports large industrial customers in Central Europe and specializes in the field of Industrial Ethernet. He graduated in electrical engineering at the University of Cooperative Education in Mosbach, Germany, and then obtained a master's degree in international sales at the University of Applied Sciences in Constance, Germany.