In part one of this two-part article we tackled crosstalk and common-mode voltage, two of the most common measurement errors and explained how appropriate use of isolation results in better and more accurate measurement data. In part two we discuss three more measurement problems that can occur with PC-based DA instruments and explain how the proper use of isolation can also mitigate these error sources.
DC Common-Mode Rejection (CMR)
Accuracy will be adversely affected whenever a measurement is made in the presence of a CMV. The question is how great is the magnitude of the inaccuracy and this can be determined by looking up the specification for CMR in the product's data sheet. Any instrument that provides a differential input, isolated or not, can reject a CMV to a degree determined by its CMR. CMR is most commonly defined as a logarithmic ratio of input CMV (VCMV in) to output CMV (VCMV out) in decibels (dB). The common-mode rejection ratio (CMRR) for most general purpose, analog-to-digital products for the PC is around 80 dB. How does this figure apply to the measurement? Here's a simple example:
Assume that you want to measure a 3 VDC normal-mode signal in the presence of a 6 VDC CMV, and assume that the normal-mode signal gain is 1.
- CMRR = 20 log (VCMV in/VCMV out)
- 80 dB = 20 log (6 VDC/VCMV out)
- 4 = log (6 VDC/VCMV out)
- 10,000 = 6 VDC/VCMV out
- VCMV out = 0.6 mV
Measurement Accuracy = 3 VDC + 0.6 mV, or +0.02%
The measurement accuracy determined in this example would be considered reasonable for most applications. The starting CMV was twice the magnitude of the starting signal. The differential amplifier, with its 80 dB CMRR specification, reduced the CMV's effect on the amplifier's output to fractions of a millivolt, with a negligible impact on accuracy. It may appear from this example that 80 dB rejection is suitable for most applications.
Our next example tests that hypothesis, using the real-world application shown in Figure 1.
- 80 dB = 10,000 = 200 VDC/VCMV out
- VCMV out = 20 mV
Measurement Accuracy = 50 mV + 20 mV or +40%
Figure 1. Measurements with high CMV present
The instrument that works well when the spread between the CMV and NMV potentials is narrow (2:1 in the first example) fails when the spread increases exponentially. This is a common situation in many production measurements, such as the 4,000:1 spread of the typical current shunt measurement in Figure 1.
Figure 2. CMRR versus CMV Reduction
General Equation: Voltage Error = CMV/CMRR
Assume that CMV = 100 V, CMRR = 90 dB
Voltage Error = 100/31,623 = 3.2 mV
Compare the 3.2 mV error with your signal of interest to determine overall accuracy.
Because you cannot lower the CMV in these situations, the only solution is to apply an instrument with better CMR.
Here's how the math stacks up for the same application using 120 dB CMRR:
- CMRR = 20 log (VCMV in/VCMV out)
- 120 dB = 20 log (200 VDC/VCMV out)
- 6 = log (200 VDC/VCMV out)
- 1,000,000 = (200 VDC/VCMV out)
- VCMV out = 0. 2mV
Measurement Accuracy = 50 mV + 0.2 mV or 0.4%
As the examples show, knowing how a CMV will affect measurement accuracy is at least as important as knowing that it exists. To help evaluate an instrument you may already have or might purchase, Figure 2 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 in Figure 2 provides a range of CMRRs in decibels and their equivalent antilog ratios, making it unnecessary to work with logarithmic math. Plug the appropriate CMV and antilog ratio into the equation to get the expected measurement error in volts. To determine the instrument's suitability for the application, compare the resulting figure with the NMV you need to measure.
AC Common-Mode Rejection (CMR)
AC CMVs are as prevalent as DC CMVs, and even more so when you include unpredictable noise sources such as motor brushes and inductive conducted and radiated electromagnetic fields (EMFs). Therefore, the assumption of pure DC CMVs may not be supported in actual practice. It's worthwhile to explore how AC CMVs may adversely affect an amplifier's CMRR and measurement accuracy.
An isolation amplifier's ability to reject CMVs is tied directly to how well its two inputs are balanced. Falling back to an ideal example, if 1 VDC is connected to one of the inputs and 1 VDC is connected to the other, the expected output from the amplifier is 0 V. However, because no real-life situation is ever ideal, small tolerance variations within the amplifier, and even in the system under test, will force the amplifier to be slightly out of balance, which will yield inaccuracies. When an AC CMV is applied, a whole new set of inaccuracies is introduced. We've met the culprit in Part 1—capacitance.
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 a 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. For example, it is possible to find that an instrument specified at 100 dB CMR yields much lower rejection of common-mode signals in the presence of higher-frequency noise. However, the product still operates within the specification defined by the manufacturer.
It is difficult to determine how suitable an instrument will be in the presence of noise (frequency) that goes beyond that of the power line; there is no easy answer to this question. However, keep in mind that a product that successfully addresses such an application does not do so by chance. Wide-spectrum AC rejection must be incorporated into the initial product design.
Measurement Range and Input Protection
Up to this point, we've examined some of the more esoteric, yet highly relevant, issues to consider when an instrument is applied to demanding production applications. Other more obvious issues involve the instrument's measurement range and input protection.
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. It should also do so on a channel-by-channel basis, because it is very common to measure voltage and current simultaneously.
Nevertheless, the very nature of these highly variable and wide-ranging dynamic measurements carries an implied need for input protection. It's common for someone to attempt to measure a high voltage on an instrument set to a millivolt range. 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 selected measurement range. More practical input protection allows connection of input signals many times that of the maximum input range, without damaging the instrument.
Finally, if the instrument's maximum range is exceeded and there is inadequate input protection, some consequences can include damage to the instrument and the cost and inconvenience of downtime and repairs. Although many types of input protection abound, none can absolutely or completely protect an instrument. Fortunately, damage can be minimized by using products designed to tolerate high-voltage differential transients (for example, those defined by ANSI/IEEE C37.90.1) as well as high common-mode voltages.
Isolated signal conditioning products 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.
Figure 3 illustrates the 4-way isolation incorporated in Dataforth's line of high-performance, DIN, isolated analog-input signal-conditioning modules. These modules accept input analog voltage or current signals from all types of field sensors. Signals are isolated, filtered, linearized, and amplified providing high-level analog levels suitable for DA and control systems. Dataforth's output modules buffer, filter, isolate, and amplify analog system signals before providing voltages or currents to field devices.
Figure 3. Dataforth's DSCA38 4-way isolated strain gauge signal conditioning module
Dataforth acknowledges and credits Roger Lockhart, DATAQ Instruments, Inc., for the technical content of this Application Note.