Safeguarding low-level signals

摘要

Accurately measuring very low-level signals requires more than just good instrumentation. Special care is needed throughout the measurement system because the very small size of low-level signals exposes error sources that usually can be ignored with larger signals. Differential circuitry often is used with low-level signals because it minimizes noise pick-up problems. Ideally, external noise will couple equally to both sides of a differential signal and can be removed by a circuit's common-mode rejection. To ensure equal coupling, the two conductors needed for each differential signal must be identically routed on a PCB. Twisted-pair wiring is used between assemblies. Differential circuitry also avoids the single-ended signal corruption caused when imperfect PCB layouts allow ground current mixing.