POWER AND LINEAR IC SOLUTIONS

High Performance, Low Power ADCs for Medical Imaging

Diagnostic imaging systems such as Magnetic Resonance Imaging (MRI) and digital x-ray imaging continue to achieve significant imaging advancements to improve diagnostic accuracy for the radiologist. High speed Analogue/Digital Converters (ADCs) such as Linear Technology Corporation’s 16-bit low-power devices lie at the heart of these imaging systems and their performance dictates the maximum image quality and contrast resolution achievable.

For such applications, Linear Technology Corporation’s 16-bit 25 Mega-Sample-per-Second (MSPS) ADC, the LTC2203, offers unparalleled 81.6dB Signal-to-Noise Ratio (SNR) and 100dB Spurious Free Dynamic Range (SFDR) performance with power dissipation of just 220mW. The pin-compatible 16-bit 10MSPS LTC2202 delivers the same levels of performance while consuming only 140mW.



The Linear Technology Corporation 16-bit high performance family extends to sampling rates of up to 160MSPS. The devices include an internal transparent dither circuit, for improving SFDR performance with low-level input signals, and a digital output randomiser for reducing digital feedback caused by large voltage transitions on the digital output bus.

The dither circuit improves the ADC’s SFDR response well beyond 100dBc for low-level input signals, by randomising the location of the sampled signal on the ADC transfer function curve. By integrating this function, the LTC2203 eliminates the complexity required by external dither circuits that consume valuable ADC bandwidth and headroom.

Unwanted digital feedback from the ADC outputs may occur due to capacitive coupling, ground currents or inductive coupling. One solution is to reduce the digital output voltage swing, thus reducing the digital noise coupled into the analogue circuitry. For this reason, the LTC2203’s output drivers are designed to operate on supply voltages as low as 0.5V without any speed penalty.



The LTC2203’s optional digital output randomiser produces further improvements by encoding the data using the least significant bit (LSB) of the 16-bit digital output word. Each of the data bits is exclusive-OR’ed with the LSB for randomisation before transmission. Such a data-encode scheme reduces by 10dB to 15dB the residual tones caused by digital feedback. The randomiser will provide a noticeable benefit to the output when averaging multiple frames in a medical imaging system.

All Linear Technology’s high speed ADCs achieve the smallest solution size by integrating bypass capacitance on-chip while being supplied in space-saving QFN packages.

Features

• Sample rate up to 25MSPS
• 81.6dB SNR and 100dB SFDR (2.5V range)
• SFDR >83dB at 70MHz (1.667V peak to peak input range)
• 380MHz full power bandwidth sample/hold
• Optional internal dither
• Optional data output randomizer
• Single 3.3V supply
• Power dissipation: 220mW (25MSPS); 140mW (10MSPS)
• Clock duty cycle stabilizer
• Pin compatible 25MSPS/10MSPS variants
• 48-pin QFN package

Applications

• X-ray systems
• MRI scanners
• CT scanners
• Wireless basestations
• Spectrum analysis
• Telecommunications

For further information, including the Linear Technology LTC2202/2203 Datasheet, please click here