New circuit design techniques for implementing very high-valued resistors are presented, significantly improving power and area efficiency of analog front-end signal processing in ultra-low power (ULP) biomedical systems. Ranging in value from few hundreds of MOhm to few hundreds of GOhm, the proposed floating resistors occupy a very small area, and produce accurately tunable characteristics. Using this approach, a low-pass MOSFET-C filter with tunable cutoff frequency (fC=20 Hz to 184 kHz) has been implemented in a conventional 0.18 um CMOS technology. Occupying 0.045 mm2/pole, the power consumption of this filter is 540 pW/Hz/pole with a measured IMFDR of 70 dB.