Electrochemical noise limits of femtoampere-sensing, CMOS-integrated transimpedance amplifiers

Low-noise operational amplifiers are an important tool in the life sciences. Biosensor measurements typically rely on low-noise transimpedance amplifiers to record biological signals. Two different techniques were used to leverage the advantages of low-noise circuitry for bioelectronics. A CMOS-integrated system for measuring redox-active substrates using electrochemical read-out at very low noise levels is presented. The system incorporates 112 amplifier channels capable of current sensing with noise levels below 1 fArms in a 3.5-Hz bandwidth. The amplifier is externally connected to a gold microelectrode with a radius of 15 µm. The amplifier enables measurement of redox-couples such as potassium ferrocyanide/ferricyanide with concentrations down to 10 nM at current levels of only 300 fA. The electrochemical noise that sets the limits of detection is also measured and analyzed based on redox mass transfer equation and electrochemical impedance spectroscopy. Secondly, CMOS-integrated low noise junction field-effect transistors (JFETs) were developed in a standard 0.18-µm CMOS process. These JFETs reduce input referred flicker noise power by more than a factor of 10 when compared with equally sized n-channel MOS devices by eliminating oxide interfaces in contact with the channel. We show that this improvement in device performance translates into a factor-of-10 reduction in the input-referred noise of integrated CMOS operational amplifiers when JFET devices are used at the input.