Fully-differential Circuit Research Articles

A design method for active high-CMRR fully-differential circuits

A simple method for designing instrumentation fully-differential (FD) circuits based on standard single-ended (SE) operational amplifiers (OAs) is presented. It departs from a SE prototype that verifies the desired differential-mode transfer function, thereby leading to FD versions of the circuit. These circuits have a high common mode rejection ratio (CMRR), independent of component imbalances, and a unity common-mode gain. The proposed method does not allow the design of common-mode response, but it does verify common-mode stability, thus ensuring stable FD circuits. It is intended for instrumentation applications in which high CMRRs are required. The proposed approach makes it possible to design and implement inverter and non-inverter topologies as well. Design examples and experimental data are presented. Using general-purpose OAs and 5%-tolerance components, the CMRR of these circuits easily exceeds 90 to 100 dB.

A design method for active high-CMRR fully-differential circuits

A simple method for designing instrumentation fully-differential (FD) circuits based on standard single-ended (SE) operational amplifiers (OAs) is presented. It departs from a SE prototype that verifies the desired differential-mode transfer function, thereby leading to FD versions of the circuit. These circuits have a high common mode rejection ratio (CMRR), independent of component imbalances, and a unity common-mode gain. The proposed method does not allow the design of common-mode response, but it does verify common-mode stability, thus ensuring stable FD circuits. It is intended for instrumentation applications in which high CMRRs are required. The proposed approach makes it possible to design and implement inverter and non-inverter topologies as well. Design examples and experimental data are presented. Using general-purpose OAs and 5%-tolerance components, the CMRR of these circuits easily exceeds 90 to 100 dB.

Selection of the common-mode feedback network connection of fully differential Gm-C filters

It is well known that fully differential (FD) structures have many advantages over their single-ended counterparts. However, the main disadvantage of FD circuit is the potential instability caused by the inherent common-mode (CM) positive feedback loop. To solve such an instability problem, extra common-mode feedback (CMFB) and/or common-mode feedforward (CMFF) circuitries are often incorporated to stabilise the circuits. We present a systematic method for checking the stability condition and identifying the most suitable CMFB network connection for a particular FD Gm-C filter. It has been demonstrated that the CMFB network connection that provides highest CM rejection is not necessarily the conventional arrangement commonly used in FD Gm-C design.

High-accuracy circuits for on-chip capacitive ratio testing and sensor readout

This paper presents novel CMOS switched-capacitor circuits for high-accuracy, on-chip capacitive-ratio testing and sensor readout. Using sigma-delta and correlated-double-sampling (CDS) techniques, these circuits provide accurate digitized capacitive-ratio readout. Both single-ended and fully differential circuits are presented. Simulation results show that the resolution can be as fine as 100 aF for 10 pF capacitors. Single-ended circuit and fully-differential circuits were implemented and tested. The measured standard deviation was below 20 aF when 10 pF capacitors were tested.