Page 28 - EngineerIT November 2021

AUTOMATION AND CONTROL

constant current source for providing IEPE sensor supply current, For multichannel DAQ systems that do not require channel
an ADA4610-1 precision JFET buffer amplifier, an ADA4945-1 fully isolation, the quad (AD7768-4) or octal (AD7768) version of the same
differential amplifier for ADC driving and the construction of an anti- ADC can be used.
aliasing filter. An ADR444 precision reference source is used to provide
the reference to the ADC with the help of an ADA4528-1 precision op AFE design
amp as the reference buffer. The input signal needs to be buffered to achieve the required impedance.
The buffer amplifier needs to have low input bias current, low noise,
Sensor supply good dynamic linearity, high DC precision and adequate bandwidth. The
The IEPE interface is a two-wire interface, with the sensor output ADA4610-1 JFET op amp is selected based on these requirements. It is
signal (voltage) and the sensor supply (current) sharing the same wire. configured as a unity-gain buffer and requires a ±15 V supply.
An LT3092 is used to construct a low noise 2.5 mA current source The signal then needs to be attenuated and level shifted to fit
across the 30 V supply to power the sensor. The current value can be within the input range of the ADC. It is desirable to convert the
programmed via resistor value in order to support longer cable length/ pseudo differential signal into a fully differential signal. This conversion
higher cable capacitance. improves the measurement dynamic range by six dB and greatly
Some IEPE sensors are not case-isolated, meaning their OUT reduces the second harmonic distortion. The signal then needs to be
terminal may be connected to the local ground. If the sensor filtered to reject aliasing and buffered with a high bandwidth and low
interfacing DAQ is also not isolated, then the DAQ also needs to be output impedance ADC driver amplifier to ensure proper settling of
ground referenced. In this design, the DAQ channel is isolated. This the ADC input. Fortunately, all of these functions can be realised by
helps remove the grounding and supply level constraints, allowing the a circuit design using a single ADA4945-1 fully differential ADC driver
DAQ to be designed with a bipolar supply to support more symmetrical amplifier with minimum distortion and added noise while maintaining
bipolar input signals. excellent DC precision.
In this circuit, the signal is attenuated by 0.33, which allows a
±4.096/0.33 = ±12.41 V full-scale input swing with a 4.096 V ADC
reference. The signal is converted into fully differential with ±4.096 V
amplitude and level shifted to a 2.5 V (middle supply) common-mode
voltage that makes both the FDA output and the ADC input happy.

Figure 11: Only a 2-wire cable is needed to interface with the IEPE sensor.

Input protection
An ADG5421F protection switch is employed to provide input
overvoltage protection to the circuit. The internal switch opens when
the input voltage exceeds the supply range to protect the rest of
the DAQ signal chain. The ADG5421F can withstand up to ±60 V of
input voltage and its low, stable R ON is essential to minimise signal
distortion.
In this design, this switch is also used to provide programmable
options to the signal chain input configuration. Based on the switch Figure 12: Signal conditioning in the analogue front end.
configuration, the signal chain input can be configured as AC or DC
coupled, and the current source can be independently switched in
and out.
An additional TVS is added with a small (10 Ω) series resistor to
help improve the ESD protection of the input node.

ADC selection
The channel isolation requirement drives the need for a single-channel
DAQ solution.
These two examples showcase the versatility of the AD7768-1.
When operating in full power mode, this ADC is capable of achieving
110 kHz of flat bandwidth with the brick wall digital filter (ODR = 256
kSPS) while still achieving 108 dB of dynamic range (with a reference
voltage of 4.096 V). It also supports a sinc5 filter for time domain
waveform capturing, with a maximum output data rate of 1.024 MSPS.
The AD7768-1 also has industry-leading dynamic linearity and DC
performance. This includes having typical THD of –120 dB with a 1 kHz
near full-scale sinusoidal input signal, 300 nV/°C offset error drift and
0.25 ppm of gain error drift. Figure 13: The overall filter responses of the Example 2 signal chain.

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