Tutorial: Isolating signals to kill crosstalk
In April 2007, we discussed the causes and cures of ground loops in analog instrumentation wiring. A different problem that can cause similar headaches is crosstalk, where the contents of one channel jump onto another to cause reading errors.
A common problem that instrumentation engineers encounter involves a phenomenon where the contents of one data acquisition channel are superimposed on another. This condition, known as crosstalk, can cause measurement errors, ranging from subtle to major, that may go undetected. In its most exaggerated form, a nearly exact duplicate of one channel appears on an adjacent channel to which nothing is connected.
With the PC-based instrumentation revolution came active use of multiplexers and their
promise of low cost per channel. Getting a per-channel cost of $30 to $40 saved money, but along the way, a hallmark of traditional instrumentation has been dropped: an isolation amplifier for each channel. The system under test is connected directly to the multiplexer’s inputs; however, the multiplexer is not always an ideal signal processing device. Its inputs have capacitance that stores a charge that is directly proportional in magnitude to the sample rate and the impedance of the signal source. This inherent characteristic causes crosstalk.
Consider an application where the multiplexer’s input is connected directly to the output of an isolation amplifier. In this situation, the impedance the multiplexer sees is stable and low, with 10 ? being a typical value. Crosstalk is greatly minimized or eliminated altogether since the impedance of the source is low enough to bleed off the charge on the multiplexer’s input capacitance before the analog-to-digital converter (ADC) reports a value.
Even under this nearly ideal impedance situation, a high sample rate can boost crosstalk by minimizing the capacitive discharge time on the multiplexer’s channels. In effect, the capacitance has less time to bleed off its charge before the analog-to-digital conversion takes place, resulting in crosstalk where none existed before.
As source impedance and sample rate increase, the probability of crosstalk increases as well. To prevent this from happening, keep these points in mind:
Minimize the source impedance of the signal source. Use isolation amplifiers to keep it below 100 ohms—although, at very high sample rates, even this value may be too high.
When source impedance cannot be controlled, an isolation amplifier is needed between the signal source and the data acquisition card multiplexer. An instrument with a built-in isolation amplifier is needed on each channel to provide protection from stray signal paths.
These strategies become even more important as the sample rate increases. The best and most predictable results are obtained when an instrument is used that has a fixed scan interval; this helps control any high sample rate bursts.
—Edited by Peter Welander, process industries editor, PWelander@cfemedia.com ,
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