In this series, I have written eleven articles (see reference section) about the AC performance of systems with SAR-ADCs and delta-sigma ADCs. The target applications were sensor-based that, historically, have been evaluated in their DC performance areas. But the focus for this series was to view this application corner in terms of a multiplexed system that demanded the additional dimension of timing. The center of attention and conclusion for these articles was to define and compare four multiplexed systems that simply had 5V supply voltage systems and able to produce gain changes from 1 to 128.
For quick reference, Figures 1 – 4 are of the four systems in Table 1.
All four systems in Table 1 are able to achieve 12-bit accuracy, which meets the basic system requirements. At this level of evaluation, the system in column A has the lowest noise performance. A delta-sigma converter made this exceptional noise performance achievable. This great performance comes at the expense of power, conversion speed, and cost. But don’t forget to ask yourself, “How much is enough?” In other words, is it critical that the converter noise is so pristine that the only noise source is from your front-end sensors?
The system in column D is the lowest power and lowest cost solution. Although the operational amplifiers (op amps) dominated the power-on and power-off values, the lower power-off value was made possible by using the SAR-ADC converter. For battery-operated applications, this is a good advantage. At a circuit level, the complexity of Figures 2 to 4 surpasses the complexity in Figure 1, where a delta-sigma converter is the main attraction. This converter absorbs the task of gaining the sensor input signal. Additionally, the anti-aliasing filter is absorbed easily with a passive R/C network.
However, if speed is an issue with your system, I recommend you choose the circuit in column C. The SAR-ADC in this system is capable of converting while accepting multiplexor settings for the next conversion.
This concludes this series of Planet Analog’s ADC Basics. There were a lot of things to take away from these articles throughout this series. We took a bird’s eye view and examined the sensor market place versus appropriate ADCs. The perspectives in the beginning of this series included an evaluation of the appropriate bandwidths of different sensors and a suggested map to the appropriate ADCs for those sensors.
From there we delved into the components in the signal chain: SAR-ADCs, delta-sigma converters, and operational amplifiers. We identified the block diagrams for the basic signal chains using these two types of converters. From there we went deeper and looked at each component in these signal chains. These investigations were atypical excursions.
If I wanted to talk about a typical component investigation, I would discuss the standard specifications, such as offset voltage, gain error, integral non-linearity, and so on. The emphases during these component deep dives delved into the key operation and specifications of these devices as they related to 12-bit multiplexed systems. Time, noise, and accuracy were primary areas under consideration as we went forward into the system level.
To finish this article series, we used this knowledge to evaluate four different 12-bit systems. You can see the final evaluation of these four systems in the beginning of the article. From reading this article, my thinking is that you have decided there was no real final decision; and that is correct.
So, what conclusions came out of this in-depth investigation? The analog world is challenging. During design, the end application needs to guide you to your final solution. Once you perform this upfront system definition, the designer needs to consider solutions from a strategy perspective and engage is a complete investigation, similar to what you see in this article series.
I may return to this topic in the future, but for now, it is time to move on. In my next series articles, we will investigate the other side of the data converter coin: DAC Basics. We will start out by defining our digital-to-analog (DAC) system and finding the right converter for the job.
See you next month!
- ADC Basics, Part 1: Does Your ADC Work in the Real World?
- ADC Basics, Part 2: SAR & Delta-Sigma ADC Signal Path
- ADC Basics, Part 3: Using Successive-Approximation Register ADC in Designs
- ADC Basics, Part 4: Using Delta-Sigma ADCs in Your Design
- ADC Basics, Part 5: Key ADC Specifications for System Analysis
- ADC Basics, Part 6: Key Op-Amp DC & AC Specifications
- ADC Basics, Part 7: Key Op Amp Frequency & Timing Specifications
- ADC Basics, Part 8: A 4-System Matrix With PGA + 12-bit SAR
- ADC Basics, Part 9: PGA Embedded in an 8-Channel, 12-Bit SAR
- ADC Basics, Part 10A: 16-Bit Converter Gives a Gainable 12-Bit System
- ADC Basics, Part 10B: 16-Bit Converter Gives a Gainable 12-Bit System
- ADC Basics, Part 11A: Take the 24-Bit Leap
- ADC Basics, Part 11B: Take the 24-Bit Leap