**3. The transducer circuit**

The transducer circuit is an inductively coupled, load modulated design similar in concept to Radio Frequency IDentification tags used for inventory and security. The 125 kHz magnetic field sourced by the interrogating reader induces a voltage in the LC tank of the implant. The LC tank (1 cm diameter, 600 turns) is resonant at 125 kHz. The AC voltage is then rectified, filtered, and regulated using a low quiescent power regulator. A supply of 3VDC, 28 μA is required to power the oscillator circuit described above and a frequency divider circuit composed of flip-flops. The oscillator frequency is divided to less than 1/20 the carrier frequency so that detection is simplified. The frequency divider also buffers the oscillator signal so that it can drive the gate of a MOSFET placed across the LC tank. The MOSFET acts a load that modulates the 125 kHz tank output at the divider output frequency. A diagram that shows the functionality can be seen in Fig. 6(a).

As it is functioned as an oscillator in Fig. 6(a), a capacitance to frequency (c-f) converter is used to convert the strain signal to a signal that can be transfered wirelessly. The c-f circuit is comprised of a pair of CMOS inverters (Lancaster, 1997) and can be seen in detail in Fig. 6(b). The oscillator produces a periodical voltage due to the charging and discharging of the RC across a threshold of an inverter input. The frequency of the RC oscillator is expressed as F=2.3RtC, where C is the variable capacitor, or a capacitive strain sensor, and Rt is the matched resistance of the oscillator. The resistor RI is to unload the RC network from clamping effect of protection diodes in the inverter. It will also result in a nearly square duty cycle and make the frequency less dependent of power supply variations. RI is normally set about 10 times as higher as Rt to minimize the effect of the protection diodes. The oscillator oscillates empirically at 20 kHz with 20 pF capacitor and 100 kOhm resistor.

Fig. 6. The block diagram of the (a) transducer circuit and (b) oscillator circuit.
