Page 29 - EngineerIT November 2021

AUTOMATION AND CONTROL

As described in Example 1, the AD7768-1’s digital filter EXAMPLE 3
also has no rejection at the frequency band around the ADC’s An alternative approach to an IEPE sensor interfacing DAQ signal chain
sampling frequency. In full power mode, the ADC’s effective design is described in CN0540.
sampling frequency is at 16.384 MHz. An active anti-aliasing filter The CN0540 design supports a unipolar input range of 0 V to 24 V.
is constructed with the ADA4945-1 to help the digital filter achieve It is suitable for a non-channel isolated DAQ system interfacing with
adequate overall out-of-band signal rejection across frequency. The IEPE sensors without case isolation, in which case both the IEPE sensor
design is a third-order low-pass filter with multi-feedback architecture and the DAQ signal chain share a common earth ground. This design
and a near Butterworth response. Another low-pass pole is added supports DC coupling to an IEPE sensor. Even though piezo sensors do
by an RC circuit in front of the ADA4610-1 buffer amplifier to help not support responses down to DC, DC coupling gives this signal chain
further increase the aliasing rejection at FS. The overall signal chain the advantage of extracting low frequency vibrations without the start-
frequency response has a –three dB corner at 440 kHz to minimise up delay from a low bandwidth AC-coupling circuit.
the magnitude and phase distortion to the in-band response. The In comparison, the signal chain design shown in Example 2
magnitude droop caused by the AAF at 100 kHz is less than ten mdB. supports a bipolar input signal. It needs to operate in AC-coupled mode
The magnitude response at in order to interface with an IEPE sensor, but this ±12.4 V input range
16.3 MHz is around –108 dB. This, combined with the brick wall and the high input impedance make it more suitable for multipurpose
digital filter of the AD7768-1, produces an alias free signal chain that DAQ systems.
is capable of rejecting all out-of-band signals by at least 105 dB.
Summary
Isolation and power management In conclusion, this article has detailed how the choice of system
The digital and power isolation and power management solution will architecture, sensor type and analysis methods can dramatically
not be discussed in detail here. Solutions such as the ADP1031 can impact the DAQ signal chain design in a condition monitoring system.
provide SPI interface plus ±15 V and 5 V supply voltage across isolation. The design considerations discussed here, along with the example
An ADuM140D high speed digital isolator can be used to provide reference designs, will hopefully help system designers make the best
MCLK and SYNC_IN signals across isolation for across channel sample design choices for their condition monitoring systems. n
synchronisation.
To learn more about Analog Devices’ condition-based monitoring
Table 2. Example 2 DAQ Characteristics system-level solutions, visit analog.com/CbM. In South Africa the
Max input range ±12.4 V pseudo differential contact person is Conrad Coetzee at ccoetzee@arrow.altech.co.za.
Max flat bandwidth (–3 dB) 110 kHz
Sinc5 filter ODR max 1024 kSPS (–3 dB BW = 209 kHz) About the author
Naiqian Ren is an applications engineer with
Dynamic range over 110 kHz BW Better than 105 dB
the Precision Converter Technology Group at
THD at 1 kHz near full-scale input Better than –105 dB Analog Devices in Limerick, Ireland. Naiqian
Gain error drift* 10 ppm/°C joined ADI in 2007 and has a bachelor’s
Offset error drift* 5 μV/°C degree in electrical engineering from the
Power consumption with sinc5 filter 110 mW Dublin City University and a master’s degree
in VLSI systems from the University of
Power consumption with brick wall filter 130 mW
Limerick. He can be reached at naiqian.ren@analog.com.
*Not including error from resistor matching

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