By Heribert Geib, VP of Technology & Systems Engineering at Xignal Technologies.
The author looks at the new design opportunities created by the introduction of continuous time delta sigma analog-to-digital converters, citing medical ultrasound as one of many applications that could benefits from the latest components to implement the technology.
Wherever high-speed, high-resolution conversion of analog signals into the digital domain is required, the power consumed by analog-to-digital converters (ADCs) is an important consideration in the overall system design. This applies equally to sensors in industrial systems, in test and measuring equipment, in communications systems and in medical electronic devices. Medical ultrasound scanning, Figure 1, is a typical example of a mutli-channel sensing application where conventional pipeline ADC technologies—the most frequently adopted—consume so much power and create so much waste heat that it becomes necessary to physically separate the sensor head from the digital conversion and processing parts of the system. In such systems, the sensor head otherwise becomes too hot to touch.
In these scanners, a transducer transmits ultrasound waves that are reflected by the target object and received again by the transducer. For scanning a larger area and focusing onto a target at a certain distance, multiple transmit/receive elements are arranged in a one- or two-dimensional array in order to form a beam. Focus and direction of the beam is controlled electronically.
The transducer is connected via a flexible cable to the data processing unit (PU) that processes the data. Each transducer element is connected to the PU through its own data-channel, or multiplexing circuits are used to reduce the number of cables. High-end systems are equipped with up to 512 channels, mid-level performance systems with up to 256 channels and portable systems up to 128.
Depending on the distance from the sensor head and properties of the object, a wide range of analog signal amplitudes are received and transmitted over the cable. Therefore the cable consists of a number of low-loss coaxial cores. The cable is one of the most expensive components of an ultrasound system but cable losses and the losses that occur at the transducer interface create demand for very high performance, and relatively expensive, receivers.
Signal integrity would be improved if the analog-to-digital conversion could be done close to the transducer. Integration of the analog front-end with the ADC and placing the device directly into the transducer would reduce the receiver’s performance requirement, and the transmission of digital rather than analog signals to the PU would be more robust and less expensive. However, analog front-ends using pipeline ADCs consume up to 0.5W for each channel. That's 64W for a mid-range (128 channel) system with enough heat being generated to affect the performance of the transducer head and cause significant discomfort to both patient and doctor. By contrast, utilizing a newly implemented ADC architecture known as continuous time delta sigma (CT?S), more details of which are explained below, the same system would consume just 8.75W. Total consumption can be reduced even further by using a multi-channel ADC that shares resources, such as the PLL, across multiple channels. With an 8-channel 12-bit ADC, a power dissipation of 40mW/channel or 5.12W for 128 channels can be achieved.
As demand grows for portable systems that shrink the size of an ultrasound scanner from a small rack to the size of a notebook or even smaller, ADC power dissipation is an important design parameter in realizing a compact and low-cost system that needs minimal cooling, whether the conversion takes place in the transducer head or the PU. New systems may also be battery operated, so minimizing power consumption is even more critical.
Research is in progress to replace analog beam formers in continuous wave Doppler applications with digital beam formers and to process all data from the several ultrasound nodes through the same digital processing path. The increasing power consumption in the digital domain can be lowered by moving to advanced CMOS technologies with lower supply voltages, 1.2V or lower. Conventional ADC technologies cannot achieve the required performance with such low voltages, therefore on a system level a number of supply voltages would have to be available, adding further cost and complexity. Continuous time delta sigma technology provides the required performance with a 1.2V supply and will further reduce in size and power consumption as CMOS process technologies develop.
The architecture of an ultrasound system with analog-to-digital conversion using CTDS ADCs in the transducer head is shown in Figure 2. In addition to the ADCs, the active transducer houses low-power variable gain amplifiers, serializers and a digital interface, with many fewer cables needed to connect with the main processor unit.
CT?S ADC Performance and Architecture
The continuous time delta sigma (CT?S) ADC parts developed by Xignal Technologies operate at 40 MSPS (equivalent to 50-60 MSPS in pipeline parts) with12- or 14-bits of resolution. Functional integration includes an accurate on-chip clock source and total power consumption is 70 mW. An added advantage of the technology is a resistive input stage that’s easy to drive without resorting to power-hungry buffer amplifiers.
Figure 3 shows a complete analog-to-digital conversion system. The left hand side shows the pipeline converter with the five external circuit elements that are needed for a complete system: a programmable gain amplifier with the gain controlled via a separate digital-to-analog converter (DAC), anti-alias filters to remove noise, and input driver to buffer the capacitive input of the ADC itself, and a high performance clock and phase locked loop to provide an accurate timing reference. By contrast, the continuous time delta sigma implementation removes the need for anti-alias filtering and the input driver, and Xignal’s implementation of the technology integrates all of the other functions on-chip.
The generic benefits of CT?S conversion are faster and simpler system design, lower power consumption, and no compromise in dynamic range or speed. In multi-channel applications these benefits are multiplied and can enable designers to adopt new and beneficial system architectures that were not previously possible.
Company: XIGNAL TECHNOLOGIES AG
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