ISSCC Session 10 Analog
February 18, 2013, ISSCC, San Francisco—Session 10 offered some interesting implementations of various analog building-block functions. Contributors included university of Twente and Delft University of Technology in the Netherlands, Toyohashi University of Technology and Electronics Inspired Interdisciplinary Research Institute in Japan, NXP Semiconductor, Broadcom, Oregon State, MIT, TI, Katholic University at Leuven Belgium, Pohang University of Science and Technology in Korea and the University of Michigan.
Qinwen Fan from Delft described a multiple path chopper-stabilized, capacitively-coupled amplifier. The challenge in many systems is to level shift and amplify a signal for further processing. Unfortunately, this process usually requires trading off low noise, low power, and a high common-mode range. By coupling the input through capacitors and chopping the signal, any offset is changed to ripple. The problem is that any signals at the chopping frequency are blocked. In most architectures, this results in a ringing and overshoot response to a step input.
In this work, they added an additional DC path with some high frequency peaking and a transconductance current summing point to address the frequency response. The high frequencies go through a 2-stage AC coupled path and the low frequencies go through the DC path. The resulting design has been implemented in a 0.7 micron CMOS process. The multi-feedback, high-complexity design has good DC gain and high-frequency response with reduced overshoot and ringing. The common mode voltage range is 20 V with a 5 volt supply. The chip draws 40 microwatts and has less than 3 microvolts of offset.
Another design from NXP is a 2.3 W class-D amplifier with a DC-DC boost converter and a DSP for sound modification and speaker protection. The desire for higher output volume from portable devices must also address the need for monitoring voice coil temperature and membrane excursion to prevent damage to the output device.
The vice coil temperature is estimated by detecting DC impedance variations with a 35 mV RMS 50 Hz signal. Current sense resistors in series with the low-side drivers enablethe indirect temperature measurement. In conjunction with the DSP, the speaker is protected from excessive excursions and high temperatures. Although all of the building blocks are standard functions, this combination of features takes advantage of inherent speaker characteristics and affords both amplification and speaker protection.
