Miniature mm3-sized sensor nodes have a very tight power budget, in particular, when a long operational lifetime is required, which is the case, e.g., for implantable devices or unobtrusive IoT nodes. This paper presents a fully integrated signal acquisition IC for these emerging applications. It integrates an amplifier with 32 dB gain and 370 Hz bandwidth that includes positive feedback to enhance input impedance and dc offset compensation. The IC includes also a 10 bit 1 kS/s SAR ADC as well as a clock generator and voltage and current biasing circuits. The overall system achieves an input noise of 27 μVrms, consumes 3 nW from a 0.6 V supply, occupies 0.20 mm2 in 65 nm CMOS, and has a single-wire data interface. The amplifier achieves an noise-efficiency factor (NEF) of 2.1 and the ADC has a figure-of-merit (FoM) of 1.5 fJ/conversion-step. Measurements confirm reliable operation for supplies from 0.50 to 0.70 V and temperatures in the range of 0-85 °C. As an application example, an ECG recording is successfully performed with the system while a 0.69 mm2 photodiode array provides its power supply in indoor lighting conditions.