Hip Implant for Temperature Measurements Figure 3. External equipment. The power coil with GMR sensor are fixed near the patient’s hip and connected to the external device TELETEMP. doi:10.1371/journal.pone.0043489.g003 external induction coil (n = 210, D = 25 cm, L = 7.85 mH, C = 0.22 mF) are therefore fixed by a common, massive plastic housing (Figure 3). Within 5 cm from this housing the coil windings are furthermore inflexible. The sensor position is finally optimized by precisely adjusting the sensor board inside the housing. The sensor signal is first high-pass-filtered (fc = 109 Hz, first- order) to remove the 50-Hz content. After a first amplification (AD8230, Analog Devices) a low-pass filter (1 kHz, 10th order, LTC1569-6, Linear Technology Corp.) eliminates the remaining influence of the 4-kHz powering field. The gain of a second amplifier (AD8042, Analog Devices) can be set by a digital Figure 4. Measuring program. Screen shot from test measurements. Left: pulse signal from implant. Marked peak values and time points for counting temperature-dependent pulse intervals. Right: sudden temperature increase in a water bath. doi:10.1371/journal.pone.0043489.g004 PLOS ONE | www.plosone.org 4 August 2012 | Volume 7 | Issue 8 | e43489