A new quad 14-bit, 125Msps analogue-to-digital converter (ADC) has been introduced by Linear Technology. The company’s LTM9012 integrates fixed gain drivers, passive filtering and bypass capacitance. The integrated µModule converters offer a considerable reduction in board space for high channel-count applications as diverse as medical imaging systems and MIMO (multiple input multiple output) 4G base stations.
The high level of integration enables smaller boards with greater density, while eliminating costly layout iterations often required to optimise the driver-to-ADC interface. This results in a significant reduction in design and debug time and faster time to market. Equivalent implementations require five devices and dozens of passive components compared to the LTM9012 in an easy-to-use 15mm by 11.25mm BGA µModule package.
Integrating the driver with the ADC simplifies high-speed design, whether the signal originates in an image sensor with CMOS logic outputs or an RF signal chain. CMOS image sensors are used in high-end medical instruments, such as blood cell or molecular microscope cameras, and industrial imaging equipment used for detecting voids in metal objects. Depending on the sensor, many channels of high-speed digitisers are required along with filtering and amplification to translate the sensor output to the ADC input.
Filters and amplifiers often consume more than twice the circuit board area as the ADC itself and constitute the key challenge in achieving maximum camera resolution. The amplifiers easily accommodate the single-ended CMOS sensor outputs and translate these levels to differential for the high performance ADC to maximise dynamic range.
This device is ideal for digitising high-speed digital signals for imaging applications and excels in baseband receiver applications such as cellular infrastructure. Two receiver architectures dominate base station designs: direct conversion and IF-sampling. With the capability of supporting baseband frequencies up to about 90MHz, It is suitable for both types. Direct conversion demodulates the RF signal and downconverts to DC; therefore a 20MHz lowpass filter supports 40MHz signal bandwidth.
Low IF-sampling below 90MHz is also possible with four channels for next generation small base station designs. With 20dB of gain, the converter achieves 68.3dB signal to noise ratio (SNR) and 78dB spurious-free dynamic range (SFDR).
The converter offers many enhancements; it has twice the density – two channels versus four in the same package size. It requires only a third the number of digital data lines since it utilises serial LVDS. It consumes about one fourth of the power, just 318mW per channel, operating on 1.8V for the ADC core and 3.3V for the amplifiers.
The space-saving 15mm by 11.25mm BGA package, utilises a multilayer substrate that shields sensitive analogue lines from the digital traces to minimise digital feedback.
Supply and reference bypass capacitance is placed inside the module, tightly coupled to the die, providing a space, cost and, performance advantage over traditional packaging.
Linear Technology
