Linear low-frequency filter using on-chip giga-ohm resistance

Integrated grounded resistors of very large value are essential circuit elements for the design of compact filters with very low cut-off frequencies. A typical application is the rejection of DC voltages in amplifier circuits in physiological recording systems exhibiting electrode offset and low-frequency drift. This paper describes the implementation of a giga-ohm resistance using a fixed-gain OTA and a cascade of current scalers. The circuit yields low complexity, small power and area consumption as well as high linearity. Measured results from a test filter integrated in 0.35 μm CMOS technology on an active area of 0.011 mm² are presented. It yields 41 Hz cut-off frequency, 1 V input range and less than -52 dB harmonic distortion.

I. Introduction

Integrated circuits for the recording of physiological signals have gained significance in a wide range of medical applications. Examples include the use of nerve signals to control functional electrical stimulation (FES) prostheses [1][3] and the detection and localization of brain activity (electroencephalogram, EEG) [4]. The recording of neural signals (ENG) has considerable impact on neuroprosthetics when used as a control or feedback signal [5]. Amplification with a relatively high gain is required early in the signal processing chain to ensure that the signal-to-noise ratio (SNR) can be maintained. Due to chemical effects at the electrode-tissue interface DC offsets and slow moving drift components are common across differential recording electrodes [6]. When amplified, these signals can saturate the gain stage. Therefore, the offset removal stage is an important building block in the recording chain. AC-coupling using RC- filters is a typical solution yielding low power consumption and simplicity. Filter cut-off frequencies of a few Hertz are required to separate interference from the signal, which often contains energy down to very low frequencies. Since the capacitance is limited to a few pico-farad in an integrated circuit, a physical resistor of the required magnitude would consume excessive chip area. Filters based on the OTA-C technique or the use of MOS transistors as pseudo-resistors have been reported [7][10]. However, these techniques often yield poor linearity which reduces their useful input range, provide insufficient resistance, or are rather complex circuits requiring a high design effort to implement. A giga-ohm active resistor is presented in this paper which realizes a filter having a low cut-off frequency of 41 Hz. The circuit yields low power consumption and high linearity, making it suitable for physiological signal recording. The simplicity of the implementation ensures a short design time and its easy reusability also for other applications. First-order filters are generally sufficient for the suppression of DC electrode offset and the removal of low-frequency interference to a level that can be handled by cascading circuit stages. To realize the very large resistance required a transconductance stage (OTA) connected in negative feedback configuration can be used. The stage yields an incremental resistance at the feedback node which is inversely proportional to the transconductance of the stage. In general, the resistance of the ideal OT A with feedback gain is given by: $${r_{OTA}} = {(\beta {g_m})^{-1}}\eqno{\hbox{(1)}}$$