Thankfully, I could isolate the instruments' power supplies and thus break the ground loop. But, not all engineers have the same luck. They may have signal sources such as strain gauges, thermocouples or pressure sensors that attach directly to metal frames or structures. The potential differences between the ground at each device and the ground at the data-acquisition systems can alter measurement accuracy or damage sensitive equipment.
Because these signals occur on both a sensor's leads, they become common-mode voltages, which can exist as both DC and AC signals. A DC common-mode signal may arise from an offset voltage. Motors, electromagnetic fields, arcing contacts and similar sources can induce AC common-mode signals on connections between sensors and measuring equipment.
Engineers use embedded systems routinely to measure sensors' signals, so they should understand the effects of common-mode signals on their analog-to-digital converters (ADCs). In addition, engineers should understand how to overcome these effects and protect their system's ADCs. All too often, engineers new to microcontrollers (MCUs) connect sensor signals directly to the MCU's ADC inputs. That technique may work in a small stand-alone device such as a personal insulin-measuring unit. But even in office and light-industrial environments that seem benign, an embedded system's ADC inputs may require signal conditioning. (See the "For Further Reading" references for detailed discussions of common-mode signals.)
Often, differential-input front-end circuits, such as amplifiers or filters, can reduce the effects of in-phase equal-amplitude common-mode signals. These circuits simply subtract the common-mode signal on their negative input from the common-mode signal plus normal-mode signal on the positive input (Figure 1). The normal-mode signal (NMS) represents the sensor signal that corresponds to a physical quantity.
[FIGURE 1 OMITTED]
But a differential-input filter or amplifier usually handles common-mode voltages only within a narrow voltage range. A thermocouple-input circuit, for example, may operate properly with only a few volts of common-mode signal. And it may tolerate only a small voltage difference between signal inputs and ground. So, when you plan to measure signals with an embedded system, you may need more protection than a simple differential amplifier can provide.
Companies such as Dataforth, Intelligent Instrumentation and others provide modules that connect to sensors on their input side and produce an isolated and equivalent sensor signal on their output side for ADCs and data-acquisition equipment. These modules use techniques such as optical, magnetic and capacitive coupling to isolate inputs from outputs.
Isolation can exceed 1,000V for common-mode signals, and modules can protect their inputs against high voltages that could occur on sensor wires in an industrial environment. (Sensor leads might short circuit to power-supply or line-power wires.)
Depending on your requirements, you can choose isolation modules that filter signals and provide an excitation or stimulus signal. In addition, modules can provide channel-to-channel isolation in chassis that hold many modules. This capability ensures each sensor's signals experience no ill effects from common-mode signals present from other sensors.
If you don't have a budget for signal-isolation modules, but you still must separate a sensor or two from an embedded system, you might design your own isolation circuit. You can buy isolation-amplifier ICs from Texas Instruments, Analog Devices and other vendors. Also, consider moving the ADC to the sensor's location. I have used a voltage-to-frequency converter (VFC) to drive an opto-coupler that isolated the sensor-VFC circuit from a computer. The Analog Devices AD7742 VFC, for example, offers two multiplexed differential-input channels and a single frequency output. The frequency of an external clock signal sets the VFC's output-frequency range. Many MCUs provide an input that drives a 16-bit counter that software can read and reset. Or, the MCU can read an added external counter.
If you take this approach, remember the VFC now performs the analog-to-digital conversion. Check specifications such as gain error, channel-to-channel isolation and offset error to ensure the VFC provides the required performance. Remember, you still face a small common-mode input range at the VFC's inputs, so moving the converter closer to the sensors won't always eliminate common-mode signal problems; but it can help overcome them.
For Further Reading
* "Common Mode Voltage," AN103, Dataforth, Tucson, AZ.
* "Why Use Isolated Signal Conditioners?" Application Note AN116, Dataforth, Tucson, AZ.
* Rowe, Martin, "Isolation boosts safety and integrity," Test & Measurement World, August 2002.
* Sherwin, Jim, "Understanding Common-mode Signals," EDN, April 17, 2003. pp. 75-82.
by Jon Titus, Senior Technical Editor
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|Title Annotation:||Signal Conditioning|
|Publication:||ECN-Electronic Component News|
|Date:||Jul 1, 2006|
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