ISSCC – Sensors and Displays
February 20, 2013, ISSCC, San Francisco—This session looked at sensors and displays in various fields. Three of the papers from KAIST in Korea addressed sensors as a part of a touch panel. The other papers covered gravity, sound, current, temperature, and light.
Delft University in The Netherlands and Infineon produced a micropower battery current sensor with +/- 0.03 percent (3 sigma) inaccuracy over the -40 to + 85 ° C range. Masdar Institute in UAE made a heart-rate sensor with a non-uniform quantizer light-to digital converter that operates on less than 4 uW on a 0.5 V supply.
A differential accelerometer from Robert Bosch is intended for vehicle stability control. The differential sensing, a charged-balanced capacitance-voltage converter, and pseudo-random chopping all contribute to noise and EMI reduction. One issue they had to address was the need to minimize charge leakage, as any imbalance creates electrostatic forces that can be an order of magnitude larger than the desired measurements.
To solve this problem, they used a self-balanced bridge approach to cancel the leakage currents. The outputs from the integrator go to summing amplifiers and are fed to a sigma-delta converter. The fully differential signal path uses two proof masses split into electrically isolated parts, and a chopper alternates the sensor signals to maintain zero average DC voltage. The pseudo-random chopping spreads the noise over a wide band, so only a small fraction of the noise and EMI appears in the signal band.
NXP and Delft made an AC-biased MEMS microphone. The problems with DC bias are extreme load resistor values and EMI and ESD leakage sensitivity. In addition, the parasitic capacitance in MEMS microphone readouts lowers signal and adds flicker noise. Their solution is to put the mike on the inverting input of the amplifier and a fixed capacitor on the other lead. The DC bias is modulated and switched between the two inputs resulting in low in-band flicker and shot noise, and reduced EMI and leakage sensitivity. Carrier cancellation is accomplished by cross–coupling two more capacitors across the mike and the compensating capacitor. The resulting mike produces a 58 dBA SNR at 1 Pa input pressure. The noise is below 100 dBV from 10-20k Hz.
CEA-LETI-MINATEC from France presented a IR bolometer imager that doesn’t require an external cooler. The imager achieves less than 50 mKelvin noise-equivalent temperature difference (NETD) on a 4 V supply. For infrared light between 8 and 12 microns, a bolometer—a temperature dependent resistor—is the principle detector. The challenges of offsets and noise being as much as two orders of magnitude larger than the desired signal has been a major stumbling block to shrinking the detector.
The design uses a shielded reference pixel per row as one half of a differential input. The reference pixel compensates for biasing, self-heating, focal plane temperature, and scene temperature. To increase accuracy, the design uses offset pre-correction that reads 4 bit pre-readout data to manage saturation and improve centering and tightening of the raw image histograms.
Stanford, TI, and NC State produced a 3-d ultrasound imager using capacitive micromacined ultrasonic transducers (CMUT). The transducers are emerging alternatives to piezoelectric transducers that benefit from standard IC processing, wide bandwidth, and easy integration with other electronics. A large 2-D array of transducer offers improved noise performance, better image quality, reduced parasitics, smaller size, and reduced I/Os.
The basic CMUT array is a 16×16 array with a 5 MHz center frequency and 250 um pitch. This design uses TSVs for flip-chip bonding. The accompanying IC transmits on all channels and receives on the diagonal elements for faster response and a reduction in output cables. The diagonal receive is about the same resolution as the full array. A set of 4 CMUTs is attached to an interposer and the driver-receiver ICs are attached to the other side of the interposer. The design achieves real-time volumetric imaging in the 3-D imager assembly. Total power consumption is 600 mW.
