“At present, industrial products tend to be miniaturized, and this trend brings new challenges to 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.
Author: Texas Instruments
At present, industrial products tend to be miniaturized, and this trend brings new challenges to 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 people’s growing demand for consumer Electronic products, people’s demand for reducing the size and power consumption of industrial equipment is also increasing. As long as it does not affect the function and performance of the product, users will give priority to smaller and lighter portable or semi-portable data acquisition equipment, because such equipment is easier to use in the laboratory or outside. Miniaturized programmable logic controller plug-in modules take up less space in the control panel of the factory floor. Secondly, equipment inventory and spare parts backup inventory require less shelf space.
Of course, the small product design is directly related to the size of the internal electronic equipment. Figure 1 shows the layout of the data acquisition system that uses TI’s THS4551 fully differential amplifier with a fourth-order low-pass filter, REF6041 voltage reference with integrated buffer, and ADS127L11 broadband ADC. In view of the advancement of new technology, 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
Reducing power consumption to a greater extent is important for extending the battery runtime of portable devices. In addition, achieving low power consumption also means that smaller and lighter devices can be designed, and costs can be reduced as much as possible. For example, reduce four batteries in parallel to three.
Reduced power consumption can also benefit offline-powered devices. Low power dissipation reduces the temperature rise inside the enclosure, thereby extending product life by lowering the average junction temperature of the integrated circuit (IC) (in some cases, reducing or eliminating forced air cooling). In comparison, although removing the ventilation slots on the product casing or control panel can reduce the dust and steam accumulated on the surface of the printed circuit board, if it is exposed to harsh environments for a long time, it may cause problems with the field equipment.
Reducing power consumption also means that the overall size of the power supply’s magnetic components is smaller. Of course, this size reduction also means that a smaller housing can be selected.
The source of the noise will limit the measurement resolution in the data acquisition system (from the reference voltage and input signal conditioning circuit), but many satisfactory optional components can also help minimize the influence of the noise source. It can be said that the main factor affecting the resolution of any industrial equipment system that measures 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 the measurement resolution, but should have low broadband noise (which can resolve small signal levels) and low distortion to achieve good spectral performance.
Figure 2 is an example of good spectral performance of a precision data acquisition system. For the data shown in this example, the system components used are also THS4551, REF6041 and ADS127L11.
Figure 2: ADC spectrum performance
During the accurate acquisition of AC signals, the converter should have nearly ideal frequency characteristics: low ripple, flat passband, steep transition band (which can save as much bandwidth as possible), and fully effective stopband at the Nyquist frequency ( Can greatly reduce signal aliasing). Once signal aliasing occurs, post-processing cannot be used to correct the signal, so it is very important to attenuate out-of-band signals as economically as possible.
Wideband delta-sigma ADCs provide these filter characteristics, including key anti-aliasing functions. Broadband or brick wall filters are based on digital filters with the above-mentioned passband, transition band, and stopband performance. The filter itself can only be realized through the concept of oversampling, and in order to obtain the ideal power and resolution index, it is usually combined with a delta-sigma ADC. Figure 3 shows the frequency response of a typical wideband ADC.
Figure 3: Broadband ADC filter response
Wideband filters have stop-band attenuation, and do not need to use external anti-aliasing filters and usually require a successive approximation register ADC. Both the wideband filter and the successive approximation register ADC can provide signal attenuation at the Nyquist frequency. The equivalent order of the external anti-aliasing filter will be very high and costly to implement. Avoiding the use of external filters saves design and component costs, and avoids a large amount of in-band phase shift.
The anti-aliasing feature of the converter’s wideband filter can suppress the out-of-band signal generated by the piezoelectric sensor. For example, the resonance frequency of the piezoelectric accelerometer sensor commonly used in vibration monitoring systems is at most +20dB higher than the normal signal level. This resonant frequency appears before the sensor response roll-off. If the resonance is excited, the resonance peak (and other frequencies that appear higher than Nyquist) may alias into the passband, resulting in incorrect frequency analysis of the signal. Figure 4 shows the frequency response of a piezoelectric accelerometer with a typical high-frequency resonance peak.
Figure 4: Typical piezoelectric accelerometer with peak response under resonance conditions
The disadvantage of integrating a wideband filter into the converter is that many logic gates in the filter implementation require silicon area. ADC IC designers can use the small transistor size and associated low threshold voltage to reduce power consumption, but at the same time they need analog-friendly transistors to achieve excellent analog part noise and linearity performance. TI has developed IC processes that meet these two standards.
The small transistor geometry reduces the stray capacitance (C) associated with logic gates, thereby reducing internal power loss. Equation 1 expresses the power loss (P) operating at the clock frequency (f) and operating voltage (V):
P = V2 × f × C (1)
Lowering the threshold voltage can reduce the power loss associated with the V2 power supply term. Another advantage is that the small transistor size used in the digital part of the ADC reduces the peak switching current, thereby reducing the noise coupled to the analog part of the digital switch.
With the help of ADS127L11, TI designed a wideband ADC. Compared with the existing wideband converter, its package size is reduced by 50%, power consumption is reduced by 50%, resolution is increased by 3dB, and signal bandwidth is increased by 50%. ADS127L11 balances size and power factor without sacrificing resolution or bandwidth.
When choosing a precision broadband ADC, designers no longer need to choose between optimizing power consumption, package size, resolution, and measurement bandwidth. TI can meet the demands for smaller and smaller form factors, lower power consumption and higher resolution, allowing you to easily select converters for next-generation data acquisition equipment.