An analog front-end (AFE) for a multifunction environmental sensor is proposed. The AFE is designed to be able to sense 1 mu V, 1 fC, and 1 pA level signals in a low-frequency band around DC, and consists of an instrumentation amplifier (IA), a charge amplifier (CA), and a continuous time delta-sigma (CT-Delta Sigma) ADC. To reduce the low-frequency noise of operational amplifier (OPAMP) which is used for signal path, this paper proposes a weak-inversion biasing technique. The designed IA achieved 26 nVrms input-referred noise (IRN) in the band of 0.1-10 Hz with 0.22 mm(2) die area. The designed CA achieved 25 aCrms IRN in charge sensing mode and 19 fArms IRN in current sensing mode in the band of 0.01-1 Hz, respectively, and occupies 0.12 mm(2) die area. The AFE has been fabricated in a 0.18 mu m CMOS process with 4.5 mm(2) die area. The current consumption at 2.56 kHz output data rate (ODR) is 2.5 mA, reducing to 2 mu A at 0.01 Hz ODR.