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:
- Signal crosstalk
- Common-mode voltage
- DC common-mode rejection
- AC common-mode rejection
- 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.
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.
While crosstalk causes the most measurement problems, common-mode voltage (CMV) has the greatest capability to distort data.
With popular measurement methods like battery-powered, handheld digital voltmeters (DVM), input readings are almost impervious to CMV problems. Many engineers assume they can extend the success of DVMs to PC-based measurement approaches, though this rarely works. The problems are tied to two specifications on the manufacturer’s data sheet: full-scale input range and maximum input voltage (without damage). Full-scale input range indicates the magnitude of voltage connected across the instrument’s inputs (normal-mode voltage or NMV), which can successfully be measured. Maximum input voltage indicates how much NMV the instrument will tolerate before incurring damage.
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 full-scale input range. Even then, measurements can be made only if the data acquisition product’s input is configured for differential operation, as shown in Figure 1.
So how can a PC data acquisition instrument be used to collect the same measurements that are made so effortlessly and safely with the handheld DVM? The answer is to choose a product that provides isolation.
Isolation means that there is no electrical connection between the common connection associated with the instrument’s front-end input terminals and the power common connection associated with the back end of the instrument and the computer. This leaves the instrument’s front end free to float at a level defined by the magnitude of the CMV, without damage and with complete measurement accuracy.
Figure 2 illustrates a typical application where isolation allows a measurement in the presence of a high CMV. Isolation can be provided as input-to-output, channel-to-channel, or a combination of the two. For the vast majority of multichannel production applications, both input-to-output and channel-to- channel isolation are needed. Each channel’s input can then float with respect to all other channels.
DC Common-Mode Rejection
Whenever a measurement is made in the presence of a CMV, accuracy will be adversely affected. The question that remains is the magnitude of the inaccuracy. This can be determined by looking up the specification for commonmode rejection (CMR) in the product’s data sheet. Any instrument that provides a differential input, isolated or not, is capable of rejecting a CMV to a degree determined by its CMR, which is most commonly defined as a logarithmic ratio of input-to-output CMV (in decibels). The common-mode rejection ratio (CMRR) for most general-purpose, analog- to-digital products for the PC is around 80 dB.
Knowing how a CMV will affect measurement accuracy is essential. To help evaluate an instrument you may already have or might purchase, the table on this page provides a guide to the error caused by a CMV as a function of your instrument’s CMRR. To use it, determine the CMV of the application and look up the instrument’s CMRR specification on its data sheet. The table provides a range of CMRRs in decibels and their equivalent antilog ratios. Plug the appropriate CMV and antilog ratio into the equation shown. The result is the expected measurement error in volts. To determine the instrument’s suitability for a particular application, compare the resulting figure with the NMV you need to measure.
AC Common-Mode Rejection
AC CMVs are as prevalent as DC CMVs, and even more so when you include unpredictable noise sources such as inductive conducted and radiated electromagnetic fields. It’s worthwhile, therefore, to explore how AC CMVs may adversely affect an amplifier’s CMRR and measurement accuracy.
An isolation amplifier’s capability to reject CMVs is tied directly to how well its two inputs are balanced. Under pure DC CMV conditions, any capacitance in the signal source, signal cable, and connectors, as well as within the amplifier itself, is inconsequential. As AC components are introduced, these capacitances form complex and unpredictable impedance, which can force the amplifier out of balance. This unbalanced condition can and will change as a function of frequency. To account for this, most manufacturers specify CMRR at other than ideal DC conditions. Typically, specifications are given at 50 or 60 Hz with 1,000 Ω imbalance between the amplifier’s inputs. This is done to provide a worst-case estimate for CMRR under the most likely source of AC interference: the frequency of the AC power line. Beyond this, a manufacturer cannot predict what particular frequencies different applications may experience.
Measurement Range and Input Protection
An instrument’s measurement range and input protection are other issues to consider. Most production applications will test an instrument’s capability on both ends of the measurement spectrum: from high voltages in the range of several hundred volts to low shunt voltages in the range of tens of millivolts. The system design chosen for these applications should be able to function easily over a variety of measurement ranges, and it should do so on a channelby- channel basis as it is very common to measure voltage and current simultaneously.
Typical input protection allows any input signal within an instrument’s maximum range (without damage to the instrument) to be connected indefinitely, regardless of the measurement range selected. More practical input protection allows input signals many times that of the maximum input range to be connected without damaging the instrument. If the instrument’s maximum range is exceeded and there is inadequate input protection, the result can be damage to the instrument, downtime, and expensive repairs.
To minimize damage, use products designed to tolerate both highvoltage differential transients (such as those defined by ANSI/IEEE C37.90.1) and high common-mode voltages.
Isolated signal conditioning products are the answer to these problems. Isolated signal conditioners protect and preserve valuable measurements and control signals, as well as equipment, from the dangerous and degrading effects of noise, transient power surges, internal ground loops, and other hazards present in industrial environments.
This article was written by John Lehman, Engineering Manager, at Dataforth Corporation, Tucson, AZ, and Roger Lockhart of DATAQ Instruments, Inc., Akron, OH.
For more information, visit: http://info.hotims.com/22918-121 .