Volume 33, Number 2, February, 1999
Ask The Applications Engineer - 27
Q. What problems am I most likely to run into when instrumenting an industrial system?
A. The five kinds of problems most frequently reported by customers of our I/O Subsystems (IOS) Division are:
1. GROUND LOOPS
Any of these problems can be caused by ground loops—inadvertent flows of current through "ground", "common" and "reference" paths connected to points at nominally the same potential. And all of these problems can be eliminated by isolation, the key signal-conditioning attribute we offer in all our signal conditioning series.
Sometimes separate grounding of two pieces of equipment introduces a potential difference and causes current to flow through signal lines. Why would this happen if they were both grounded? Because the earth and metal structures are actually relatively poor conductors of electricity when compared with the copper wires that carry power and signals. This inherent resistance to current flow varies with the weather and time of year and causes current to flow through any wires that are connecting the two devices. Many factory and plant buildings experience potentials of several tens or hundreds of volts. Appropriate signal conditioning eliminates the possibility of ground loops by electrically isolating the equipment. Signal conditioning will also protect equipment, rejecting potentially damaging voltage levels before entering the sensitive measurement system.
Isolation provides a completely floating input and output port, where there is no electrical path from field input to output and to power. Hence, there is no path for current to flow, and no possibility of ground loops.
Q. How is this possible? How can we provide a path for the signal from input to output, without any path for current to flow?
A. It's done by magnetic isolation. A representation of the signal is passed through a transformer, which creates a magnetic—not a galvanic—connection. We have perfected the use of transformers for accurate, reliable low-level signal isolation. This approach employs a modulator and demodulator to transmit the signal across the transformer barrier, and can achieve isolation levels of 2500 volts ac.
One of the most frequently encountered application problems involves measuring a low-level sensor such as a thermocouple in the presence of as much as hundreds of volts of ground potential. This potential is known as common-mode voltage. The ability of a high-quality signal conditioner to reject errors caused by common-mode voltage, while still accurately amplifying low-level signals is known as common mode rejection (CMR). Our 5B, 6B, and 7B Series signal conditioning subsystems provide sufficient common-mode rejection to reduce the impact of these errors by a factor of 100 million to 1!
2. MISWIRING AND OVERVOLTAGE
The answer lies in using rugged signal conditioning on every analog signal lead. This inexpensive insurance policy provides protection against miswiring and overvoltage on each input and output signal line. For example, the use of a 5B Series signal conditioner will provide 240 VAC of protection, even on input lines used to measure sensitive thermocouple signals, with levels in the millivolt range. You can literally connect a 240-VAC line across the same input lines used to measure the thermocouple, without any damage. The use of signal conditioning to interface with field I/O will protect all measurement and data acquisition equipment on the system side.
3. LOSS OF RESOLUTION
For example, a 15-bit plus sign ADC of the type used in our 6B Series would offer resolution of 0.037° on the 0 to 1200° range example, 8 times smaller! On the other hand, if you knew that most of the time the temperature would be in the vicinity of 100°, you could order from Analog Devices a thermocouple signal conditioner with a custom range, calibrated for the exact thermocouple type and temperature measurement range. For example, a custom-ranged signal conditioner with a span of +50° to +150° would offer resolution of 0.024° with a 12-bit ADC, a big improvement over the 0 to 1200° range.
4. MULTIPLE SIGNALS DON'T ALL HAVE THE SAME PROPERTIES
These subsystems incorporate all connections for input, output and field wiring, as well as simple connections for a DC power supply. They offer a choice of output options: 0 to +5 V, 0 to +10 V, 4-20 mA and RS-232/485, and more! Input and output modules are mix-and-match compatible on a per-channel basis and hot-swappable for the ultimate flexibility.
5. ELECTRICAL INTERFERENCE
Lower-frequency noise can be eliminated by choosing signal-conditioning subsystems with excellent common mode and normal mode rejection. Common mode noise present on both the plus and minus inputs can be seen when measuring either the plus or minus input with respect to a common point like ground. Normal-mode noise is measured in the difference between the plus and minus inputs. A typical common mode rejection specification on our signal conditioning subsystems is 160 dB. This log scale measurement means that the effect of any common mode voltage noise is reduced relative to signal by a factor of 108, or 100 million to 1!
Very high frequency noise in the radio frequency bands can cause dc offsets due to rectification. It requires other approaches, including careful circuit layout and the use of RFI filters such as ferrite beads. The performance measures are indicated by our compliance with the EN certifications for electromagnetic susceptibility popularized by the CE mark requirements of the European community. A typical application where this is important would be where a two-way radio is used within a few feet of the input wiring and signal conditioning subsystem. It is necessary to reject measurement errors whenever the radios are transmitting. Good panel layout practice and the use of signal conditioning will ensure the best accuracy in these noisy environments.