What are common mode voltage limits and how do they affect the operation of the PC board?

What are common mode voltage limits and how do they affect the operation of the PC board?

Common Mode Voltage (CMV) is the voltage appearing between the PC board's input terminals and its ground. It is an in-phase noise signal that appears simultaneously on both input terminals of the board. CMV confines each computer-based data acquisition system on the overall input voltages that you can monitor. Most ADC boards cannot measure any signal above ±10 to ±13 volts because the input amplifiers on the board are typically powered by either ±12 or ±15 volt power supplies. Any measured signal within two volts of the power supply voltage will saturate the amplifiers and cause significant errors. This means that a board having max signal input of ±10 volts powered by ±12 power supply in single-ended input configurations, where the signal is referenced to ground, cannot have a signal input more than ±10 volts.