LOW-POWER TELEMETRY SYSTEM USING SC VOLTAGE-TO-FREQUENCY CONVERTER

J.B. BEGUERET, F. RODES, M. ZHANG and J.P. DOM
Laboratoire IXL - ENSERB - Université Bordeaux I
351 cours de la Libération
33405 TALENCE - FRANCE

INTRODUCTION - The design constraints of an intracorporal telemetry system are a minimum size, a minimum power consumption and a maximum noise rejection. This paper presents a new strain gage telemetry system based on an optimized switched-capacitor (SC) voltage-to-frequency converter, which is well suited for long term monitoring of orthopedic implants used for the treatment of bone fractures.

METHODS - The improvement of the noise rejection is performed with the use of a voltage-to-frequency converter which integrates the input signal. The SC technique minimizes the size circuitry, integrating the precision integrator time constant into the ASIC chip. The topology of this system is the following : the strain gage is included into a Wheaststone bridge which generates an input signal V_IN and a reference voltage V_REF. These bridge output voltages are applied to a SC integrator and then coded with a pulse position modulation (PPM). This PPM coded signal modulates a 180 MHz radiofrequency transmitter. The conversion principle is performed with two successive integrations of V_IN and V_REF. The originality of this system is to realize, at the receiver end, the ratio between this two integration times. Then the transfer function only depends of the strain gage resistance. Indeed no external components are needed. Furthermore, this transfer function is independent of the supply voltage, which is very attractive for intracorporal circuit where very often the supply voltage can fluctuate.

RESULTS - We realized an ASIC chip in a 2 µm BiCMOS technology and an hybrid circuit measuring 16 x 12 mm². The overall circuit has been successfully tested and the measurements mainly exhibit a 0.1% accuracy, a 12 mW power consumption under 4 V, a low drift characteristic and zero off-chip component.

DISCUSSION - We successfully designed and tested an intracorporal telemetry system dedicated for long term monitoring of stainless steel orthopedic implants. The retained topology proved to be an optimized version in terms of size minimization, low power consumption and input noise rejection. Therefore, this PPM converter can find applications in other low power telemetry systems.