Predictive maintenance & condition monitoring


knowledge of each channel’s latency (more accurate than 1/Fsampling, or around 0.5° at 20 kHz) across frequency and temperature. Overall, using a multichannel simultaneous sampling ADC is a generally sufficient solution, yet it has its limitations.


GROUND LOOPS AND THE NEED FOR ISOLATION


Consider simultaneously monitoring across different parts of a single machine, or even across different machines altogether. In this system, we need to have careful consideration of ground loops.


Figure 3. Phase mismatch error sources.


(Figure 3), typically of third order or higher, while preserving minimal in-band magnitude error. For example, a second-order Butterworth filter designed for –80 dB rejection at 16 MHz (sampling frequency) and f3dB of 160 kHz (input bandwidth) could have a phase mismatch of ±0.15° at 20 kHz even with RC mismatch tolerances as low as one per cent. This is not a problem for continuous-time sigma-delta (CTSD) ADCs, like the AD4134,


since they do not have vulnerabilities outside its pass band, eliminating the need for an analogue antialiasing filter. A key difference, however, is that DTSDs are more power scalable than CTSDs. Additionally, there may be other sources of latency, such as the input amplifier and isolation circuitry.


As a solution, both of these multichannel ICs have phase calibration registers to adjust each channel’s phase (Table 1), given the


Grounding and shielding are used in instrumentation to protect measurement signals from unwanted noise and stray electromagnetic fields. The cable used to connect the sensor and the DAQ solution is usually a shielded twisted pair, where the shield is grounded from the sensor side, or from the DAQ side. If, for example, (1) the sensor has a path to ground, (2) the DAQ has a separate one as well, and (3) the cable shield is grounded from both sides, it then forms a ground loop (Figure 4). A ground loop allows current to flow along the shield. Induced currents on the shield from power lines and nearby machinery can then couple the interference on the signal line. For proper grounding, there should ideally be only a single low impedance path to ground from any point in the system. Designing the grounding system requires consideration of the application, environment, and the sensor’s type of isolation. Accelerometers can be (a) case grounded, (b) case isolated, or (c) ground isolated.


a. A case grounded accelerometer has a path to ground when attached to a conductive surface. For a single-channel system, it requires only the DAQ to be isolated. For a multichannel system however, grounding multiple sensors will create a ground loop.


Figure 4. Improper grounding in an accelerometer installation.


TABLE 1. PHASE MATCHING PERFORMANCE AND PHASE CALIBRATION RESOLUTION ACROSS VARIOUS ADCS


AD7768/AD7768-4 AD4134


i. Adhesive mounting provides a varying level of isolation, depending on the thickness of the adhesive.


Channel-to-channel phase matching at 20 kHz (max)


Phase calibration resolution at 20 kHz


Not measured 0.024° 0.88° 0.3°


ii. Integral housing isolation and isolation mounts typically come at a higher/additional cost, but they may be necessary in hazardous environments, such as wind turbines exposed to lightning strikes.


Instrumentation Monthly October 2024


Continued on page 54... 53


b. To avoid this problem, it is then best to have isolated sensors and a grounded multichannel DAQ solution (Figure 5). Many accelerometers have basic case isolation, where the sensing element isolated from the sensor housing, typically through a coated pad.


c. Others achieve ground isolation from the mounting surface using various techniques.


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