UNUSUAL FEATURES
Designed for portable medical imaging and ultrasound, portable test and instrumentation, software-defined radios, and 3G/4G Long-Term Evolution (LTE) and WiMAX basestations, Linear’s low-power, 14-bit, 150-Msample/s LTC2262 ADC dissipates 149 mW. That’s less than one-third the power of competitive solutions, according to the company.
What’s unusual is that in addition to low power consumption, the LTC2262 integrates two unique features for reducing digital feedback where even good layout practice may fail. Such feedback occurs when energy from ADC outputs couples back into the analog section, causing interaction that appears as odd shaping in the noise floor and spurs in the ADC output spectrum.
The worst case is when the analog input is at midscale, and all outputs are changing from ones to zeroes, or vice versa, generating large ground currents that couple back into the input. To combat this, the LTC2262’s designers created a proprietary alternate bit polarity (ABP) mode that inverts all of the odd bits ahead of the output buffers.
This equalizes the number of ones and zeroes switching and effectively cancels the large ground-plane currents that contribute to digital feedback. In addition to the alternate bit polarity mode, an optional data output randomizer decorrelates the digital output, reducing the likelihood of repetitive code patterns.
Both digital feedback reduction techniques improve spurious free dynamic range (SFDR) by 10 to15 dB, Linear says. Actual SFDR is 88 dB at baseband, according to the datasheet. Other key specs include SNR of 72.8 dB and jitter of 0.17 psRMS. A 12-bit version is also available.
Finally, a veritable electronic Swiss Army knife, Maxim’s MAX1329 low-power data-acquisition system (DAS) integrates a 12/16-bit, 300-ksample/s ADC, dual 12-bit force-sense DACs, and voltage references. The 12-bit ADC’s natural 12-bit resolution can be extended to 16 bits using internal dithering and digital signal processing.
For sensor excitation, the device’s high-accuracy, internal 12-bit DACs exhibit ±8-LSB integral nonlinarity and ±1 differential nonlinearity, along with 10-µs (maximum) settling times. An intelligent microcontroller interface completes the kind of closedlooped system used in many battery-powered/portable devices and data-acquisition systems.
Other integral analog support blocks include operational amplifiers, a 16:1 input multiplexer, a 1-V/V to 8-V/V PGA, and lowleakage single-pole/double-throw (SPDT) and single-pole/singlethrow (SPST) solid-state switches. An integrated 3.6864-MHz oscillator provides a master clock source. Other support features include dual voltage monitors, internal and external temperature sensors, and user-programmable general-purpose I/Os that can be used to output system interrupts, control switches, and drive shutdown.
The MAX1329 operates on digital supply voltages from +1.8 to +3.6 V and analog supplies from +2.7 to +5.5 V. An on-chip charge pump provides a +5-V supply up to 25 mA to power other circuitry. With all of the analog blocks enabled, the MAX1329 has a typical quiescent supply current of 3.75 mA. This drops to 0.5 µA in shutdown mode.