Protecting Data Signals Through Isolation

Many of us believe that if we don’t have ground loops we don’t need to isolate analog I/O signals. Yet there are five very real – and very different – reasons to isolate every one of your analog signals! If you’ve had problems on past applications, chances are you experienced one of the following:

1. Signal crosstalk
2. Common-mode voltage
3. DC common-mode rejection
4. AC common-mode rejection
5. Over-range and input protection issues.

Isolating power sources and sensor signals is the most effective way to address these problems; understanding how measurement inputs become corrupted in the first place will illustrate this.

Moving data acquisition systems from the controlled environment of a laboratory to validation testing or the manufacturing floor can lead to problems that are capable of destroying measurement accuracy and possibly even equipment.

Signal Crosstalk

The most common problem – crosstalk – is a condition wherein the contents of one data acquisition channel are superimposed on another. In its most exaggerated form, a nearly exact duplicate of one channel appears on an adjacent channel to which nothing is connected. Crosstalk can cause subtle to major measurement errors that may go undetected.

The widespread use of multiplexers has definitely exacerbated this problem. While multiplexers promise low cost per channel, the hallmark of traditional instrumentation – an isolation amplifier for each channel – is not included. Instead, the system is connected directly to the multiplexer’s inputs – even though multiplexer inputs have capacitance that stores a charge directly proportional in magnitude to the sample rate and impedance of the signal source. This inherent characteristic causes crosstalk.

When the multiplexer’s input is connected directly to the output of an isolation amplifier, the impedance the multiplexer sees is stable and low, with 10 Ω being a typical value. Crosstalk is greatly minimized or eliminated because the impedance of the source is low enough to bleed off the charge on the multiplexer’s input capacitance before the analogto- digital converter reports a value. As source impedance and sample rate increase, the probability of crosstalk also increases.

The key ways to prevent crosstalk, therefore, are:

• Minimize source impedance of the signal source.
• When source impedance cannot be controlled, use an isolation amplifier between the signal source and data acquisition card multiplexer.