A Low-Power Spiking Chemical Pixel Sensor

This letter introduces an ISFET-based integrate and fire neuron forming the front-end of a chemical pixel sensor. With the sense data being encoded in the spike domain, i.e. asynchronous/continuous time and discrete value, it is directly compatible with asynchronous communication hardware, e.g. Address Event Representation. The circuit is shown to be tunable to yield a linear relation with both pH and actual hydrogen ion concentration with a peak power consumption of 35μW. Furthermore, its compact pixel footprint of under 6500μm, makes it ideal for use in arrayed architectures with application in biochemical imaging. Introduction: ISFET (Ion Sensitive Field Effect Transistor) based chemical sensors pioneered by Bergveld [1], have gained considerable interest due to their ability to sense hydrogen ions, fast temporal response, compact size and prospect of monolithic integration [2]. By patterning a matrix of sensors on a single substrate, robustness and accuracy can be vastly improved through redundancy. Furthermore, by embedding local processing throughout the sensor array, added functionality can be realised in chemical sense applications, such as spatiotemporal chemical filtering, multi-chemical sensing and action potential detection from neural ensembles. Presented here is a monolithic chemical sensing chip with integrated signal conditioning and pre-processing circuitry. Fabricated in standard CMOS technology, the sensor includes a modified ISFET device, bias, drive and signal conversion circuitry. The processing circuitry adopted within the chemical sensor pixel is a low power integrate and fire (I&F) neuron circuit, initially proposed by Indiveri [3], encoding the sensor information in the spike domain. In the case of a chemical image sensor, multiple sensor fusion and off-chip transmission is facilitated using a standard AER (Address Event Representation) [4] protocol. Circuit Description: Fig. 1 shows the schematic of the chemical pixel sensor (excluding the in-pixel AER handshake switches). The circuit comprises of a custom ISFET device, the H-Cell module [5] and a leaky I&F neuron. The ISFET device sinks an output current which is dependant on the pH of the solution. This is due to the binding of hydrogen ions to the silicon nitride surface (CMOS passivation layer) above the gate forming a capacitive coupling with the Ag/AgCl reference electrode [1]. The ISFET is biased in the weak inversion region of operation to minimise power consumption and exploit the exponential device characteristic. The output current relation to hydrogen ion concentration is given by Eqn. 1 and to pH by Eqn. 2, where Kchem is a pH independent constant, n the sub-threshold slope factor, Ut is the thermal voltage and a is a dimensionless sensitivity parameter relating the ISFET’s sensitivity to the Nernstian relationship [5]. The function of the H-cell is to linearise the drain current to hydrogen ion concentration relationship, as analysed in [5].