CMRR enhancement techniques for current-mode instrumentation amplifiers

The current-mode instrumentation amplifier based on second-generation current conveyors (CCII) offers many advantages over conventional instrumentation 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 with voltage-mode topologies. Therefore, in this paper the matrix representation of the current conveyor is extended to include all linear nonidealities and based on this an enhanced current-conveyor macromodel is derived. Using this model the common-mode behavior of the current-mode instrumentation is discussed in detail. Furthermore, different CMRR enhancement techniques of current-mode instrumentation amplifiers are discussed and compared. As an example, a new current-mode instrumentation amplifier topology is realized with AD844 current-feedback operational amplifiers. The new instrumentation amplifier provides approximately 40 dB of improvement on the high-frequency CMRR up to the -3-dB corner frequency and even higher, without any matched components. Furthermore, composite conveyor techniques improving the instrumentation amplifier performance are discussed. Also, other conveyor applications affected by similar nonidealities are briefly reviewed.