A Practical Guide to Minimising the Effect of Common-Mode Noise Interference in Low-Temperature Applications

J. R. Lindemuth and E. A. Codecido, Lake Shore Cryotronics

Introduction

The MeasureReady™ M81-SSM Synchronous Source Measure System provides a true balanced current source module, the BCS-10. Most other commercial current sources are single-ended type sources. The low connection (current return) of these types of current sources is at measure ground by design. The measurement instrument attached to the current source sees different impedances looking back into each side of the current source. This will generate common-mode signals that are converted to differential signals in the preamp of the measurement modules and result in measurement error.

The BCS-10 module uses technology based on Lake Shore’s expertise with balanced current sources used in the Model 370 and 372 AC resistance bridges.

When coupled with the common-mode rejection (CMR) feature, the balanced current source provides additional accuracy and noise reduction in a wide variety of applications. This application note will explain the advantages and best practice usage of the CMR feature of the M81-SSM BCS-10 module.

An external CMR input connection is a new feature of the BCS-10 module not present on the Model 372 bridge. The external CMR input of the BCS-10 is typically used in conjunction with a VM-10 voltage measurement module to improve the accuracy and lower the noise of the VM indication. We begin with a simple review of ideal differential amplifier behavior and how common-mode signals affect practical differential amplifiers.