Whenever a data acquisition (DAQ) system is moved from the controlled environment of the laboratory to validation testing or the manufacturing floor, measurement inputs can become corrupted. Isolating power sources and sensor signals is the most effective method for eliminating undesirable ground loop currents and associated induced electrical noise to both protect equipment and ensure measurement accuracy. Signal conditioning modules can provide up to four levels of isolation.

Figure 1. Data Acquisition Differential Operation. Isolation means there is no electrical connectionbetween the power supply common associated with the instrument’s output and the power supplycommon associated with the PC’s input. The signal conditioning module’s output ground is isolatedfrom the PC’s input ground.

One of the most common roadblocks to successful measurement is crosstalk — the contents of one data acquisition channel being superimposed on another. Crosstalk is caused by capacitance coupling between input channels, and it is common today in the ongoing PCbased low-cost instrumentation revolution, which has motivated the manufacture of single-chip integrated circuit (IC) multiplexers. Often, these multiplexer chips have eliminated the hallmark of traditional instrumentation — an isolation amplifier for each channel. Since multiplexer inputs have capacitance coupling between inputs, at high-speed system sampling rates, crosstalk can occur even when the multiplexer inputs are connected directly to the outputs of an isolation amplifier. While crosstalk causes its share of measurement problems, common-mode voltage (CMV) leads in its capability to distort data.

Figure 2. A typical application where Isolation Allows Measurement in the presence of high CMV.Additional factors to consider regarding measurement accuracy include DC common-mode rejection,AC common-mode rejection, and the instrument’s measurement range and input protection.
The CMV problems that are encountered are related to two specifications on the manufacturer's data sheet: (a) fullscale differential input voltage, and (b) maximum input voltage common to each input. Full-scale input defines the magnitude of differential voltage connected across the instrument's two inputs (normal-mode voltage or NMV) that can be measured successfully. Maximum input voltage common to each input defines how much CMV is allowed simultaneously and in phase on each of the instrument's inputs with respect to power ground for successful measurements.

Figure 3. Four-Way Isolation in isolated analog signal conditioning modules.
Most data acquisition products for the PC permit measurements when the sum of CMV and NMV is equal to or less than the instrument's specified allowed input with respect to ground. Measurements can be made in this case only if the DAQ product's input is configured for differential operation (Figure 1). Typical DAQ products will tolerate a maximum input voltage range of ±30 VDC without damage, but in production environments with 120 to 440 VAC motors, 24 VDC process instrumentation supplies, and the high probability of ground loops, this limit is often exceeded.

This article was written by Dataforth Corporation of Tucson, AZ. For more information, Click Here  /10974-121.

NASA Tech Briefs Magazine

This article first appeared in the August, 2007 issue of NASA Tech Briefs Magazine.

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