University College London revealed a 30MHz impedance spectroscopy IC for analysing biological samples at ISSCC in San Francisco this week.

ISSCC25 paper 35.3 IEEE UcollegeLondon
The chip exhibits a 92.7dB signal to noise ration and 99.6% accuracy, while consuming only 3.18mW. At 30MHz, triple the bandwidth of most bio-impedance amplifiers according to the IC’s designers, magnitude and phase errors are 0.39% and 0.57°.

The device uses four electrodes, injecting a sinewave current through two and measuring the injected current plus the voltage across the other two electrodes.

Time-to-digital demodulation is used, usually requiring a clock at tens of GHz for 30MHz bandwidth.

To save power, a programmable picosecond resolution delay-locked clock generator is instead used, driven by a nine-phase 240MHz clock, which also drives a 240MHz 10-phased DLL.

The least significant delay step between these output phases is 46.3ps (21.6GHz equivalent), which provides the short sample time. The trade-off is that it takes far longer to get a complete set of samples that it would with a real 21.6GHz clock – although the result is easily fast enough for the application.

A look-up table with 24 over-sampling rates and 77 unit steps is used for sinewave generation, and the output current generator can deliver from 15.6 to 500μA and from 10kHz to 30MHz, with 0.39% THD at 3MHz and 0.82% at 10MHz.

The IC occupies 1.6mm2 in 65nm CMOS.

ISSCC25 paper35.8 UcalBerkley IEEEIn the same bio-IC session at ISSCC, the University of California, Berkeley, presented an implantable network of tiny sensors, linked by time-division multiplexed ultrasound.

The scheme has a single external reader which projects data-modulated ultrasound into the body.

Eight implants, each with their own time-slot, take power and data from the incident ultrasound, and reply by back-scattering and modulating (16-level amplitude) that incident ultrasound.

Within US medical ultrasound power limits, the system can work at up to 90mm depth – far deeper than RF or optically-powered implants – and transfer 400kbit/s on a 2MHz carrier.

Each implant transmits at 50kbit/s and consumes 6μW (average), transmitting fewer symbols when buried deeply to maintain error rate.

The custom 28nm cmos IC measures 0.43mm2 and shares space on the 1 x 2.3mm implant substrate with a 0.34mm3 PZT piezoelectric resonator that receives power and handles data, and a 10nF energy storage capacitor that maintains operation between ultrasound bursts.

ISSCC 2025

Paper 35.3 University College London
A 30MHz wideband 92.7dB SNR 99.6% accuracy bioimpedance spectroscopy IC using time-to-digital
demodulation with co-prime delay locked sampling

Paper 35.8 University of California, Berkeley
DustNet: A network of time-division multiplexed ultrasonic implants with 16-level ASK backscatter modulation

Editor’s note: all eight papers in ISSCC 20025 Session 35 on medical related devices were fascinating and a tribute to their authors – if only thee was time to cover them all.