DESIGN
T&M: SIGNAL CONVERSION
A similarly accurate but more cost effective and more efficient alternative is to utilise a SAR-ADC in combination with an instrumentation amplifier (in-amp), as shown in Figure 1. The function of a SAR-ADC can be divided into two phases: the data acquisition phase and the conversion phase. Basically, in the data acquisition phase, the current consumption is low. Most SAR-ADCs even power down between conversions. So, the
Figure 2. Power loss in the AD4003 as a function of sampling rate.
conversion phase draws the most current. The power consumption is dependent on the conversion rate and linearly scales with the sample rate. For power-saving applications for slow-response measurements - that is, measurements in which the measured quantities change slowly (for example, temperature measurements) - a low conversion rate should be used to keep the current draw and thus the losses low. Figure 2 shows the power losses in the AD4003 at various sampling rates as an example. At 1kSPS, the power loss is approximately 10µW; at 1MSPS, it has already risen to 10mW. In contrast to such slow measurements, sigma-delta ADCs have the strengths of oversampling, while using a much higher internal oscillator frequency than the output rate. This allows designers to optimise sampling for higher speeds with worse noise performance or for lower speeds with more filtering, noise shaping (pushing the noise into the frequency band outside the area of measurement interest), and better noise performance. However, that means a lot more power consumption with sigma-delta ADCs compared to SAR-ADCs. The effective resolution and noise-free resolution of many sigma-delta ADCs are mentioned in their data sheets, making it easy to compare the trade-offs. Conclusion Both sigma-delta ADCs in combination
with PGAs and SAR-ADCs with an in-amp are suitable for signal conversion in high precision measurement applications. The solutions both have a similar accuracy. For power-saving or battery-operated measurement applications, however, the combination of the SAR-ADC and the in-amp is better because it offers a lower power consumption and lower costs compared with the solution consisting of the PGA and the sigma-delta ADC. In addition, a PGA with a high gain often limits the performance because the noise is also amplified. This article just covers one possible solution for a SAR-ADC. There are more integrated solutions available such as a sigma-delta converter like the AD7124-4/ AD7124-8 with an integrated PGA. About the author Thomas Brand began his career at Analog Devices in Munich in 2015 as part of his master’s thesis. After graduating, he was part of a trainee programme at ADI. In 2017, he became a field applications engineer. Thomas supports large industrial customers in Central Europe and also specialises in the field of Industrial Ethernet. He studied electrical engineering at the University of Cooperative Education in Mosbach before completing his postgraduate studies in international sales with a master’s degree at the University of Applied Sciences in Constance.
21 ELECTRONICSPECIFIER.COM
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