CMRR enhancement techniques for current-mode instrumentation amplifiers

The current-mode instrumentation amplifier based on second-generation curre nt conveyors (CCII) offers many advantages over conventional instrumentatio n amplifier architectures. It does not need any matched components to reach high CMRR and its bandwidth is not gain-bandwidth product limited, However , improving the CMRR of this amplifier is not as straightforward as dth vol tage-mode topologies. Therefore, in this paper the matrix representation of the current conveyor is extended to include all linear nonidealities and b ased an this an enhanced current-conveyor macromodel is derived. Using this model the common-mode behavior of the current-mode instrumentation is disc ussed in detail. Furthermore, different CMRR enhancement techniques of curr ent-mode instrumentation amplifiers are discussed and compared. As an examp le, a new current-mode instrumentation amplifier topology is realized with AD844 current-feedback operational amplifiers, The new instrumentation ampl ifier provides approximately 40 dB of improvement on the high-frequency CMR R up to the -3-dB corner frequency and even higher, without any matched com ponents. Furthermore, composite conveyor techniques improving the instrumen tation amplifier performance are discussed. Also, other conveyor applicatio ns affected by similar nonidealities are briefly reviewed.