Why should I choose isolated voltage output or non-isolated voltage output?

Data acquisition system requirements or safety requirements will drive selection of an isolated voltage circuit. The overall system common mode voltage (CMV), data acquisition’s input normal-mode voltage (NMV Z — the output of the sensor/input of the DAQ) and maximum input voltage (CMV+NMV) will determine whether isolated voltage output is required.

The most common maximum input voltage on data-acquisition systems is +30V DC. The most common system supply voltages using amplified sensors are 12V DC, 24 VDC and 32V DC. If a sensor’s output is 0-5V DC or 0-10 VDC and isolated voltage output is not used in the sensor, there is a very real chance that the DAQ’s maximum input voltage will be exceeded. This will result in catastrophic damage and possible physical injury to the user.

Amplified voltage transducers offer a greater level of noise immunity over their non-amplified ratio-metric (mV/V) cousins. Further, isolated voltage circuits offer additional immunity to system noise from VFDs, RFI and EMI interference due to a reduced path for those signals to compromise the output signal — two output wires are impacted versus two output wires and two input wires.

The disadvantages of isolated voltage sensors include increased amplifier noise within the sensors themselves (high speed switches within the transformer alternating between open and closed) and a slower frequency response of the sensor due to the additional time required for transformers to perform their function as pressure changes occur. Typically, isolated voltage outputs offer <3ms (>333Hz) frequency response while non-isolated voltage outputs offer <1ms and often <0.5ms (>2000Hz).