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High-Performance Data-Acquisition System Enhances Images for Digital X-Ray and MRI
Figure 1. Digital X-ray signal chain.
Today, manufacturers of digital X-ray detectors typically use indirect conversion. Amorphous silicon flat-panel detectors or photodiode arrays with more than one million pixels capture the photon energy, multiplexing the outputs into one or two dozen ADCs. This technology offers effective X-ray photon absorption and a high signal-to-noise ratio to obtain dynamic high-resolution images in real time with a 50% lower X-ray dose. The sampling rate of each pixel is low, from a few hertz for bones and teeth, to a maximum of 120 Hz for capturing images of a baby’s heart, which is the fastest organ in the body.
The performance of a digital radiography detector is measured by its image quality, so accurate acquisition and precise processing of the X-ray beam is essential. Digital radiography’s increased dynamic range, fast acquisition speed and frame rate, and uniformity using special image processing techniques allows it to display an enhanced image.
Medical imaging systems must provide enhanced images for accurate diagnoses and shorter scanning times for decreased patient exposure to X-ray dosages. High-end radiography systems (dynamic acquisition) are typically used in surgical centers and operating rooms, whereas basic systems are used for emergencies, in small hospitals, or in doctors’ offices. Industrial imaging systems must be rugged, as they have long lifetimes and are subject to high radiation dosages in harsh environments. Security or baggage inspection applications can use low X-ray dosages, as the X-ray source remains on for long periods of time.
MRI Gradient Control
Control systems for MRI specify tight tolerances, thus requiring high-performance components. In MRI systems, a large coil is used to create the main magnetic field of 1.5 T to 3 T. A high voltage—up to 1000 V—is applied to the coil to develop the required current of up to 1000 A. MRI systems use gradient control to linearly vary the main magnetic field by changing the current in special coils. These gradient coils are modulated rapidly and precisely, altering the main magnetic field to target very small locations within the body. The gradient control energizes a thin cross section of the body tissue using RF energy to generate the x-, y-, and z-axis images. MRI demands fast response time, with its gradient precisely controlled to within
Figure 2. MRI system.
High-Performance Data-Acquisition Signal Chain
Figure 3. Precision, fast-settling signal chain using AD7960, ADA4899, AD8031, and ADR4550.
Figure 4. AD7960 typical FFT and linearity performance.
This type of high-speed, multichannel, data-acquisition system could be used in CT, DXR, and other medical imaging applications that require higher sampling rates without sacrificing accuracy. Its 18-bit linearity and low noise provide enhanced image quality, and its 5-MSPS throughput allows a shorter scanning period (more frames per second) and decreased exposure to the X-ray dosage for accurate physician diagnostics and a better patient experience. Multiplexing multiple channels creates higher-resolution images for full analysis of organs such as the heart, and achieves affordable diagnosis while minimizing power dissipation. Accuracy, cost, power dissipation, size, complexity, and reliability are of paramount importance for medical equipment manufacturers.
In CT scanners, the pixel current is captured continuously using one track-and-hold per channel, with outputs multiplexed to a high-speed ADC. A high throughput rate allows many pixels to be multiplexed to a single ADC, saving cost, space, and power. Low noise and good linearity provide a high-quality image. High-resolution infrared cameras could benefit from this solution.
Oversampling is the process of sampling the input signal at a much higher rate than the Nyquist frequency. Oversampling is used for spectroscopy, MRI, gas chromatography, blood analysis, and other medical instruments that require a wide dynamic range to accurately monitor and measure both small and large signals from multiple channels. High resolution and accuracy, low noise, fast refresh rates, and very low output drift can significantly simplify the design, reducing development cost and risk for MRI systems.
One of the key requirements for MRI systems is measurement repeatability and stability over long periods of time in a hospital or doctor’s office. For enhanced image quality, these systems also demand tight linearity and high dynamic range (DR) from dc to tens of kilohertz. As a guideline, oversampling the ADC by a factor of four provides one additional bit of resolution, or a 6-dB increase in DR. The DR improvement due to oversampling is ΔDR = log2 (OSR) × 3 dB. In many cases, oversampling is implemented well in Σ-Δ ADCs, but these are limited when fast switching between channels and accurate dc measurements are required. Oversampling with a successive-approximation (SAR) ADC also improves antialiasing and reduces noise.
State-of-the-Art ADC Architecture
Figure 5. AD7960 functional block diagram.
The capacitive DAC, shown in Figure 6, consists of a differential 18-bit binary weighted capacitor array—which is also used as the sampling capacitor that acquires the analog input signal—a comparator, and control logic. When the acquisition phase is complete, the conversion control input (CNV±) goes high, the differential voltage between inputs IN+ and IN− is captured, and the conversion phase begins. Each element of the capacitor array is successively switched between GND and REF, charge is redistributed, the input is compared to the DAC value, and the bit is kept or dropped depending upon the result. The control logic generates the ADC output code at the completion of this process. The AD7960 returns to acquisition mode about 100 ns after the start of conversion. The acquisition time is approximately 50% of the total cycle time, making the AD7960 easy to drive and relaxing the required settling time of the ADC driver.
Figure 6. AD7960 simplified internal schematic.
The AD7960 series operates from 1.8-V and 5-V supplies, dissipating only 39 mW at 5 MSPS when converting in self-clocked mode. The power dissipation scales linearly with sample rate, as shown in Figure 7.
Figure 7. AD7960 power consumption vs. throughput rate.
The power dissipation at very slow sample rates is dominated by the LVDS static power. The AD7960 is twice as fast, dissipates 70% less power, and occupies a 50% smaller footprint than the industry’s next fastest 18-bit SAR ADC.
The AD7960 allows three external reference options: 2.048 V, 4.096 V, and 5 V. An on-chip buffer doubles the 2.048-V reference voltage, so the conversions are referred to 4.096 V or 5 V.
The digital interface uses low-voltage differential signaling (LVDS), offering self-clocked and echoed-clock modes to enable high-speed data transfer (up to 300 MHz) between the ADC and the host processor. The LVDS interface reduces the number of digital signals and eases signal routing, as multiple devices can share a common clock. This also reduces power dissipation, which is especially useful in multiplexed applications. The self-clocked mode simplifies the interface with the host processor, allowing simple timing with a header that synchronizes the data from each conversion. A header is required to allow the digital host to acquire the data output because there is no clock output synchronous to the data. The echoed-clock mode provides robust timing at the expense of an extra differential pair. The AD7960 achieves over 120-dB typical dynamic range at output data rates below
Figure 8. AD7960 dynamic range vs. output data rate.
Table 1. AD7960 ADC Driver Selection Benchmark
Notes: N = 18, tacq = 100 ns, Vrms_in2 = 52/2 = 12.5 V2, en_amp = 2 nV/√Hz, f–3dB_ADC = 28 MHz.
The op amp’s data sheet usually provides the settling time specification as a combination of the time for linear settling and slewing; the formulas given are first-order approximations assuming 50% for linear settling and 50% for slewing (multiplexed application) using a 5-V single-ended input.
The ADA4899-1 rail-to-rail amplifier features 600-MHz bandwidth, –117-dBc distortion @ 1 MHz, and 1-nV/√Hz noise, as shown in Figure 9. It settles to 0.1% within 50 ns when configured as a unity-gain buffer driving the inputs of the AD7960 with a 5-V differential signal.
Figure 9. ADA4899 noise spectral density.
Reference and Buffers
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