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ADC Basics, Part 12: An ADC Design Matrix to Fit Your Needs

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.

Table 1. Ten system specifications for: a) Figure 1 circuit 12,13, b) Figure 2 circuit 10,11, c) Figure 3 circuit 9, and, d) Figure 4 circuit8.

Table 1. Ten system specifications for: a) Figure 1 circuit 12,13 , b) Figure 2 circuit 10,11 , c) Figure 3 circuit 9 , and, d) Figure 4 circuit8 .

For quick reference, Figures 1 – 4 are of the four systems in Table 1.

Figure 1. A 24-bit delta-sigma ADC with a 16-channel multiplexer input. Circuit design for specification in Table 1, column A.

Figure 1. A 24-bit delta-sigma ADC with a 16-channel multiplexer input. Circuit design for specification in Table 1, column A.

Figure 2. Multiplexed, 12-bit ADC-SAR with an embedded PGA gain cell. Circuit design for specification in Table 1, column B.

Figure 2. Multiplexed, 12-bit ADC-SAR with an embedded PGA gain cell. Circuit design for specification in Table 1, column B.

Figure 3. Multiplexer / gain cell followed by a 16-bit ADC. Circuit design for specification in Table 1, column C.

Figure 3. Multiplexer / gain cell followed by a 16-bit ADC. Circuit design for specification in Table 1, column C.

Figure 4. PGA followed by an operational amplifier that drives a 12-bit ADC. Circuit design for specification in Table 1, column D.

Figure 4. PGA followed by an operational amplifier that drives a 12-bit ADC. Circuit design for specification in Table 1, column D.

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.

Conclusion
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!

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6 comments on “ADC Basics, Part 12: An ADC Design Matrix to Fit Your Needs

  1. RedDerek
    March 23, 2014

    Bonne,

    Nice wrap-up. Best part is the basic discussion on the comparison and how one would compare the four options and why. Granted, no one component can do everything an engineer likes, thus the trade-off discussion when looking at a chart such as this is helpful.

  2. Netcrawl
    March 24, 2014

    Great post Bonne! You provide everything designers need. The  is a variety of ADC archiectectures on the market today, we got SAR, sigma-delta and pipeline. SARS converter is a general-purpose converter, its boast high dc and ac accuracyand often used in low-power applications. 

  3. Netcrawl
    March 24, 2014

    Great post Bonne! You provide everything designers need. The  is a variety of ADC archiectectures on the market today, we got SAR, sigma-delta and pipeline. SARS converter is a general-purpose converter, its boast high dc and ac accuracy and often used in low-power applications. 

  4. geek
    March 25, 2014

    @RedDerek: I agree with you. It was well-summarized and made easy to understand even for people who're not into hardcore electronics. The comparison part was also interesting and well-elaborated with the diagram.

  5. geek
    March 25, 2014

    “Once you perform this upfront system definition, the designer needs to consider solutions from a strategy perspective and engage is a complete investigation,”

    @Bonne: I think that's a very important point which not a lot of designers are careful about. The idea of looking at the big picture and making decisions based on the strategy is not very common at lower levels. This often leads to inconsistent designs and reduces their interoperatibility and usefulness.

  6. Bonnie Baker
    June 1, 2014

    Tzubair, Netcrawl, and RedDerek,

    Thank you for reading and commenting on this last article of the ADC Baisics series. I have had a hard time getting back to you because of a little Planet Analog web snafu. But all is good now.

    When I started to write this series I immediately came to terms with the fact that one ADC does not solve all your problems. I then wrestled with determining exactly what I wanted to accomplish with a design and what process would I go through to reach a first level decision; knowing full well that in the end I may change my initial selected product to work with. With all this in mind, I went through a complete, comprehensive examination of all the devices and in the end tryied to compare them. 

    I seems that all three of you picked up on this and I thank you.

     

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