Balancing ADC Size, Power, Resolution, and Bandwidth for Precision Data Acquisition Systems

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Balancing ADC Size, Power, Resolution, and Bandwidth for Precision Data Acquisition Systems [Copy link]

The trend toward smaller industrial products is creating new challenges for precision data acquisition systems. Designers must balance the solution size and power consumption of the entire system while achieving more accurate signal measurements at higher bandwidths and making trade-offs in the process.

This article will discuss these challenges in detail, focusing on the role of analog-to-digital converters (ADCs) in industrial systems.

ADC Package Size

Just as the demand for consumer electronics continues to grow, the need to reduce the size and power consumption of industrial equipment is also increasing. As long as the function and performance of the product are not compromised, users will prefer smaller and lighter portable or semi-portable data acquisition devices because such devices are easier to move around the lab or on the go. Miniaturized programmable logic controller plug-in modules take up less space in the control panel on the factory floor, and secondly, less shelf space is required for equipment inventory and spare parts standby inventory.

Of course, small product design is directly related to the size of the internal electronics. Figure 1 shows the layout of a data acquisition system that uses TI’s THS4551 fully differential amplifier with a fourth-order low-pass filter, the REF6041 voltage reference with an integrated buffer, and the ADS127L11 wideband ADC. Given the advancement of new technologies, it is worth noting that the converter is no longer the largest component in the design.

Figure 1: Typical analog front-end printed circuit board (PCB) layout

ADC Power Consumption

Minimizing power consumption is important for extending the battery run time of portable devices, and achieving low power consumption also means that smaller, lighter devices can be designed with the lowest possible cost. For example, reducing the number of batteries in parallel from four to three.

Off-line powered devices can also benefit from reduced power consumption. Low power dissipation reduces temperature rise within the enclosure, thereby extending product life by reducing the average junction temperature of the integrated circuit (IC) (and in some cases, reducing or eliminating forced air cooling). In contrast, while removing ventilation slots from product enclosures or control panels can reduce dust and vapor buildup on the surface of printed circuit boards, it can cause problems for field devices if exposed to harsh environments for extended periods of time.

Lower power consumption also means that the overall size of the power supply magnetics can be smaller. Of course, this size reduction also means that a smaller enclosure can be selected.

ADC resolution

Sources that generate noise can limit measurement resolution in data acquisition systems (from reference voltages and input signal conditioning circuits), but many desirable optional components can also help minimize the effects of noise sources. Arguably, the primary factor affecting system resolution in any industrial equipment measuring ac signals (such as vibration/acoustic monitoring and general data acquisition) comes down to the converter. The converter should not have tones and other spurious frequencies that limit measurement resolution, but should have low broadband noise (to resolve small signal levels) and low distortion for good spectral performance.

An example of good spectral performance of a precision data acquisition system is shown in Figure 2. For the data presented in this example, the system components used were also the THS4551, REF6041, and ADS127L11.

Figure 2: ADC spectral performance

ADC bandwidth

During the accurate acquisition of AC signals, the converter should have nearly ideal frequency characteristics: low ripple flat passband, steep transition band (to save as much bandwidth as possible), and a stopband that is completely effective at the Nyquist frequency (to minimize signal aliasing). Once signal aliasing occurs, it is impossible to correct the signal through post-processing, so it is very important to attenuate the out-of-band signal as economically as possible.

Wideband delta-sigma ADCs provide these filter characteristics, including the critical function of anti-aliasing. Wideband or brick-wall filters are based on digital filters with the passband, transition band, and stopband performance described above. The filter itself can only be realized through the concept of oversampling and is often used in conjunction with a delta-sigma ADC to achieve the desired power and resolution specifications. Figure 3 shows the frequency response of a typical wideband ADC.

Figure 3: Wideband ADC filter response

The wideband filter provides stop-band attenuation, eliminating the need for an external antialiasing filter that would normally require a successive approximation register ADC. Both the wideband filter and the successive approximation register ADC provide signal attenuation at the Nyquist frequency. The equivalent order of an external antialiasing filter would be very high and expensive to implement. Avoiding the use of an external filter saves design and component cost, and avoids significant in-band phase shift.

Figure 4: Typical piezoelectric accelerometer with response peak at resonance

The disadvantage of integrating wideband filters into the converter is that many logic gates are required to implement the filter and occupy silicon area. IC designers of ADCs can take advantage of small transistor size and associated low threshold voltage to reduce power consumption, but at the same time need analog-friendly transistors to achieve excellent analog noise and linearity performance. TI has developed IC processes that meet both criteria.

Small transistor geometries reduce the stray capacitance (C) associated with the logic gates, thereby reducing internal power losses. Equation 1 expresses the power loss (P) operating at a clock frequency (f) and operating voltage (V):

P = V ² × f × C (1)

Lowering the threshold voltage reduces the power loss associated with the V2 ^supply term. Another advantage is the reduction in peak switching currents through the small transistor sizes used in the digital portion of the ADC, which reduces the noise coupled from the digital switches into the analog portion.

Conclusion

With the ADS127L11, TI has designed a wideband ADC that reduces package size by 50%, reduces power by 50%, improves resolution by 3dB, and increases signal bandwidth by 50% compared to existing wideband converters. The ADS127L11 balances size and power factors without sacrificing resolution or bandwidth.

When selecting a precision wideband ADC, designers no longer have to choose between optimizing power consumption, package size, resolution, and measurement bandwidth.

Kevin Chen1