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Signal isolation for DAQ and data logging

Admin - Thursday, October 13, 2016
Signal isolation for DAQ and data logging

The Zen series was designed as a data acquisition system for SCADA systems and PLC’s in the industrial arena.

However it is quite different to the plethora of standard data acquisition products on the market. Much of the market is dominated by low cost, high sampling speed, multiplexed units. These were originally developed for controlled lab conditions and scientific use, and were typically connected to scientific packages like LabWindows™ and MATLAB.

The Zen series architecture is built on industrial grade technology proven in the field for many years.

So what’s the difference?

First of all, the A/D type and sampling speeds are quite different.

A PC card typically uses high speed SAR converters which have multiplexers on the front to increase channel count. Although you can get samples quicker, you may have to do a lot of post processing to get a usable reading. The multiplexer itself is not an ideal signal processing device and poses some challenges for the novice and expert alike. The issues relate to the time required for the inputs to settle on each multiplexer change, and how the input impedance of the signal can degrade performance. (Much has been written about this and is generally available, however it is beyond the scope of this paper to go into any further detail on this.)

The analog industrial design engineer has known for many years that to get reliable results from signals that might travel ½ a mile through a plant normally requires a system that can distinguish low level signals from noise, which is sometimes an order or two larger than the signal itself. To combat this the integrating type of converters were invented. These include dual slope, voltage to frequency, and later the Sigma-delta converter. These A/D’s sacrifice signal bandwidth for noise rejection and rugged reliability.

The Zen series has made use of the modern sigma delta A/D to become the workhorse of this new genre of SCADA accessories. However this is only the beginning of the story. Although the A/D is responsible for rejecting noisy signals like 50/60Hz hum, simply replacing the SAR converter with a sigma-delta converter in a multi-channel application would have little benefit.

To really make a rugged industrial system requires every channel to be galvanically isolated from each other. This is exactly how the Zen series is designed. Every input channel is isolated magnetically and optically from all others. Each channel has its own A/D transformer and optocouplers, as well as important EMI filters.

To really make a rugged industrial system requires every channel to be galvanically isolated from each other.

Why use Isolation?

Isolation solves many problems associated with industrial processes. Isolating power sources and sensor signals is the most effective method for eliminating undesirable ground loop currents and induced electrical noise.

Some of the more common problems which isolation solves are:

Cross Talk

Cross talk is when the contents of one data acquisition channel appear on another. This can cause subtle to large measurement errors that can go undetected. The cause of this can be simply sharing a ground in where ground loop currents can flow depending on the size of the signal. This is converted to an unwanted voltage component by the impedance of the earth track. More subtly this can be due to a fast sampling multiplexer input capacitance and a "high" source impedance. Even a "high" impedance of only 100 ohms could be responsible for significant cross talk.

Having an isolated channel with its own A/D stops these problems dead in their tracks. Cross talk is virtually unmeasurable between channels for isolated products in the Zen series.

Common-Mode Voltage

Each instrument will have a CMV specification relating the maximum voltage which can be tolerated on the inputs of channels relative to ground. A good way to visualize this is measuring a stack of 12V batteries typical in a telecom application. If you want to measure the cell voltage on top of the stack the negative of the A/D input will be 36V relative to the ground of the A/D. This is the common mode voltage. Now if, for example, the common mode voltage was rated at 50V, the system would be fine. However the + of the A/D in this case will be 48V - close to the 50V limit. If the voltage strays higher than the 50V limit due to noise or battery charging equipment, the CMV rating will be exceeded, which could distort and degrade the signal leading to an inaccurate reading or damage to the A/D.

Having an isolated input practically eliminates this issue as the A/D ground will float up to the CMV. In this case the CMV limit will be the isolation break down voltage of the transformer and/or optocoupler. For example, the Zen series isolation breakdown barrier is above 3,000V AC, which is much higher than the expected CMV of most production applications.

"The Zen series has broken the isolated price point to around $50 per channel - now there is no excuse not to isolate."
Common Mode Rejection

Every time a measurement is made in the presence of a CMV a loss of accuracy will be encountered. The question is not if, it is how much. Going back to the stacked battery system we are trying to measure a 12V signal on top of a 36V common mode voltage. If you read the spec of your instrument you should find a DC CMR ratio. For a typical PC card A/D this could be 80dB.

Now:

  • 80dB = 20 log (VCMV in/ CCMV out)
  • 80dB = 20 log (12V DC/ CCMV out)
  • 10,000 = 12V DC/VCMV out
  • VCMV out = 1.2mV error

All in all, not too much of an error. However, if we now change the measurement by measuring the current drawn in the 48V system by using a 20mV shunt, the results now look like this:

  • 80dB = 10,000 = 48V / VCMV out
  • Error = 4.8mV (or 24%!)

For a typical isolated system the DC CMR would be in the vicinity of 160dB or 100,000,000. Or in this case, 480nV - practically nothing.

To add to this, AC common mode voltages are even more prevalent than DC CMV’s when you take into account noise sources such as unsnubbered contactors, motor brushes, inductive conducted and radiated electromagnetic fields (EMFs).

Again, an isolated system has many advantages over a non-isolated system, as the isolated system will float to the common mode voltage. It's not uncommon for an isolated conditioner to be able to measure the temperature with a thermocouple which is directly connected to a live mains feed. That is, the T.C. will be floating at 230V AC and still give a measurement accurate to 0.1°C!

Furthermore, because high speed multiplexers are not used in the Zen series input modules, that the designer has the freedom of not having to worry about increasing the capacitance of the input. This opens the way for using modern feed through capacitors in common and differential mode EMI filters. These are designed to reject wideband noise and improve the CMRR. Not only that, but an inherent virtue of the isolated input is that the loop areas of any sensor wiring are kept to a minimum, reducing pickup.

Conclusions

The isolation of industrial signals preserves and protects valuable measurements, as well as expensive equipment, from the effects of ground loops, transient power surges, noise and other hazards present in industrial environments.

In the past the only reason to purchase a non-isolating system was cost. Of course it is less expensive to have only a single A/D than 16 A/D's, as in our 16 channel Zen units. However the cost of commissioning a system, and trying to work out cross talk issues and potential ground loop problems, will soon dissolve any savings made. That, combined with the added multitude of other protection benefits of an isolated system, really points to only one solution for the wise system integrator: Isolate, Isolate, and if in doubt - Isolate again.

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