In digital X-Ray (DXR) systems, film detectors are replaced with solid state sensors, including flat panel and line scan detectors. Flat panel detectors use either direct and indirect conversion. With direct conversion, a selenium array forms capacitive elements that convert high frequency photons directly into an electronic current.
With indirect conversion, a caesium iodide scintillator converts the photons into visible light, then a silicon photodiode array converts that light into current. Each photodiode represents a pixel. A low noise analogue front end transforms the small current from each pixel into a large voltage, which is then converted into digital data and passed the image processors.
A typical DXR system (see fig 1) multiplexes many channels at high sampling rates into one a/d converter without sacrificing accuracy.
Manufacturers of DXR detectors typically use indirect conversion systems, in which flat panel detectors or photodiode arrays with more than 1million pixels capture the photon energy, multiplexing the outputs into one or two dozen a/d converters. This offers effective X-Ray photon absorption and a high signal to noise ratio, allowing dynamic high resolution images to be captured in real time with a 50% lower X-Ray dose. The sampling rate of each pixel is low, from few hertz for bones and teeth, to a maximum of 120Hz for capturing images of a baby's heart.
Medical imaging systems must provide enhanced images for accurate diagnoses and shorter scanning times for decreased patient exposure to X-Ray dosages.
High performance data acquisition
Fig 2 shows a high precision, low noise, 18bit data acquisition signal chain that features ±0.8 LSB integral nonlinearity (INL), ±0.5 LSB differential nonlinearity (DNL) and a 99dB signal to noise ratio.
This type of data acquisition system could be used in CT, DXR and similar medical imaging applications that require higher sampling rates without sacrificing accuracy. Its 18bit linearity and low noise provide enhanced image quality and its 5Msample/s throughput allows a shorter scanning period and decreased exposure to X-Ray dosage.
Multiplexing multiple channels creates higher resolution images for full analysis of organs such as the heart, and achieves affordable diagnosis while minimising power dissipation. Accuracy, cost, power dissipation, size, complexity and reliability are of paramount importance.
In CT scanners, the pixel current is captured continuously using one track and hold per channel, with outputs multiplexed to a high speed a/d converter. A high throughput rate allows many pixels to be multiplexed to a single converter, 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 from dc to tens of kilohertz. As a guideline, oversampling the a/d converter by a factor of four provides one additional bit of resolution, or a 6dB increase in DR. The DR improvement due to oversampling is ?DR = log2 (OSR) x 3dB. In many cases, oversampling is implemented well in sigma-delta a/d converters, but these are limited when fast switching between channels and accurate dc measurements are required. Oversampling with a successive approximation converter also improves antialiasing and reduces noise.
Precision high speed data acquisition systems used in imaging systems and other oversampled applications, require a state of the art a/d converter. The AD7960 18bit, 5Msample/s PulSAR differential a/d converter (see fig 3) uses a capacitive d/a converter to provide noise and linearity without latency or pipeline delay. It provides the bandwidth, high accuracy (100dB DR) and fast sampling (200ns) required for medical imaging applications, while reducing power dissipation and cost in multichannel applications.
The capacitive d/a converter consists of a differential 18bit binary weighted capacitor array – also used as the sampling capacitor that acquires the analogue 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 d/a converter value and the bit is kept or dropped, depending upon the result. The control logic generates the a/d converter output code at the completion of this process. The AD7960 returns to acquisition mode about 100ns 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 converter driver.
The AD7960 series operates from 1.8V and 5V supplies, dissipating 39mW at 5Msample/s when converting in self clocked mode. The AD7960 allows three external reference options – 2.048V, 4.096V and 5V – while an on chip buffer doubles the 2.048V reference voltage, so conversions are referred to 4.096V or 5V.
The digital interface uses LVDS, offering self clocked and echoed clock modes to enable data transfer between the converter and host processor at up to 300MHz. 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 synchronises 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.
A/D converter driver
The converter's acquisition time determines the settling time requirements for the a/d converter driver. 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, using a 5V single ended input.
The AD8031 rail to rail amplifier buffers the 5V output from the ADR4550 voltage reference. A second AD8031 buffers the a/d converter's 2.5V common mode output voltage. Its low output impedance maintains a stable reference voltage independent of the converter's input voltage to minimise INL. Stable for large capacitive loads, the AD8031 can drive the decoupling capacitors required to minimise spikes caused by transient currents and is ideal for a range of applications that demand low power dissipation.
Maithil Pachchigar is with Analog Devices' precision converters business unit.