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Signal Chain Basics #80: Optimizing Power vs. Performance for a SAR-ADC Drive Amplifier

One of the most common challenges that board- and system-level engineers face in designing an amplifier driver for a precision successive approximation register (SAR) is optimizing the effective number of bits (ENOB) to achieve lowest power dissipation. In this article we examine the relationships among ENOB, distortion, and noise, as well as the key tradeoffs in achieving the lowest power and best ENOB using three different SAR driver topologies.

Since most modern SAR converters linearly scale analog power dissipation with sampling rate, the drive amplifier quickly can become the dominant source of power when optimizing an application for ENOB. Figure 1 shows a fast Fourier transform (FFT) with the fundamental input signal (VINPUT_RMS ) and the other performance figures of merit (SNR, SINAD, THD) that are key to calculating overall system ENOB.

Figure 1

Fast Fourier Transform of ADC figures of merit

Fast Fourier Transform of ADC figures of merit

Table 1 shows how each performance figure of merit is calculated based on amplifier noise (en_AMP_RMS ), analog-to-digital converter (ADC) noise (en_ADC_RMS ), and harmonics in the FFT (en_HARM_RMSx ). Also, to calculate power dissipation, the quiescent current (IQ ), dynamic output current (ΔIOUT ), and analog supply (VS ) are used:

Table 1

Figures of merit of a SAR ADC data acquisition system For a larger image of this Table, click here.

Figures of merit of a SAR ADC data acquisition system
For a larger image of this Table, click here.

Note that the calculation for ENOB uses the SINAD term because it takes into account distortion introduced by passing an AC signal through the drive amplifier, as shown in equation 1:

Equation 1

Figure 2 shows the quantitative relationship among THD, noise, and ENOB for a 16-bit data acquisition system.

Figure 2

Plot of THD vs. ENOB for a given noise floor (SNR) for a 16-bit system

Plot of THD vs. ENOB for a given noise floor (SNR) for a 16-bit system

Design topologies for the input drive amplifier
Figure 3 shows a fully differential amplifier-I/O driver (FDA), which is very common in applications where distortion is a primary optimization target. Distortion effects can be introduced when there is a large signal change on the inputs that result in AC CMRR errors. These errors show up in the FFT as harmonics that degrade the THD. This driver topology does not suffer from this effect because the inputs are driven in the inverting configuration and the common-mode remains at fixed voltage, typically midscale.

Figure 3

Fully differential input and output driver configuration

Fully differential input and output driver configuration

There are two potential shortcomings to the FDA topology shown in Figure 3.

  1. Noise and settling at higher input bandwidths: The feedback resistors inherently add thermal noise (for example, en =[4kRTBW]0.5 ) which can be filtered (fc =1/[2πRf Cf ]) at the expense of settling time. Balancing the design between noise filtering and recharging CF can become a challenge.
  2. Power dissipation:
      a. Lower broadband and flicker noise comes at the cost of power.
      b. Lowering the values of the feedback resistors (all shown with the same value R) increases the current driven by the output.
      c. The ratio of R in the feedback network may need gain for stability, which means the inherent noise will be gained up to the output.

Alternatively, Figure 4 shows a driver configuration that can be easily optimized for low noise and/or power because there are no feedback resistors to contribute thermal noise. This is especially useful in applications where a sensor output is differential and does not require the common-mode level-shift offered by the topology in Figure 3.

Figure 4

Differential buffer configuration

Differential buffer configuration

Figure 5 shows a “best of both worlds” topology because it separates the common-mode level-shift by the FDA from the function of driving the ADC. This is important because the values of R can be made much larger to reduce power consumption. Additionally, fC1 can be dramatically reduced to filter the added thermal noise, leaving the buffer amplifier to do the job of recharging CF in between ADC conversions.

Figure 5

FDA + buffer configuration

FDA + buffer configuration

Summary
Ultimately, to minimize distortion effects and therefore maximize ENOB, the following two rules-of-thumb criteria must be observed:

Equation 2

Equation 3

To summarize, SNR, THD, and SINAD are all important parameters that need to be considered together in one figure of merit (ENOB) so that the power required to achieve a desired performance target is not ignored and the appropriate driver topology may be properly selected.

Join us next time when we will cover a basic overview of OFDM and PAPR for non-RF engineers.

References

About the author
Matthew Hann, SAR ADC Product Line Manager in the Precision Analog business unit at TI, has 15 years of product expertise, which includes development and applications support in operational amplifiers, 4-20mA transmitters, temperature sensors, difference amplifiers, instrumentation amplifiers, programmable gain amplifiers, and power amplifiers. Through his past role as an applications engineer, Matt developed a focused expertise on the design of analog front-ends for medical applications such as ECG, EEG, EMG, blood glucose monitoring, and pulse oximetry. He received his BSEE from the University of Arizona, Tucson. He can be reached at .

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5 comments on “Signal Chain Basics #80: Optimizing Power vs. Performance for a SAR-ADC Drive Amplifier

  1. Davidled
    August 9, 2013

    This is good information. I wonder which the existing component provides the less power and the better performance in the market. Unless I am designing IP Block inside chip, I might dig the existing components in the market. Datasheet might provide this kind of information for engineer.

  2. goafrit2
    August 9, 2013

    For you to dig into the market, it means you are not going for full integration. Full integration means everything has to be monolithic and completely integrated.

  3. Davidled
    August 9, 2013

    In some case, it may not true. For example, application engineer might look some type ADC driver to meet these requirements to integrate engineering circuit. it does not mean that engineer does not dig full integration. There are so much components regarding on ADC which are very common in the market now. Engineer gets on pressure to release product in the market on time. Sometime experience engineer knows the right product line every application. I knew that searching the right component and integratng circuit are not easy one. This is only my view.

  4. samicksha
    August 12, 2013

    Any Idea on Doherty amplifiers, i guess its getting more and more attention in cellular base station transmitters for GHz frequencies.

  5. fasmicro
    August 14, 2013

    >> There are so much components regarding on ADC which are very common in the market now

    The novelty in the industry is not about designing away from the common units but making them tighter and very unique. In other words, you can have all the ADCs to look common,but the real deal is the spec that comes from the individual blocks.

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