CN0561

The circuit shown in Figure 1 is a sensor to bits (data acquisition) alias free signal chain for an IEPE sensor, consisting of a current source, instrumentation amplifier, protection circuit, and a fully differential alias free continuous time ADC. The IEPE interface is present on two channels, while the remaining 2 channels can be used for general 24-bit alias free data acquisition. The circuit consists of current source, quad channel protector, instrumentation amplifier, and fully differential sigma delta ADC.

The programmable current source, LT3092, drives the piezoelectric accelerometer with constant current. The output current can be programmed by external resistors and is usually set between 2mA and 20mA, depending on the type of sensor and the cable.

The signal output from the IEPE sensor is AC-coupled to the instrumentation amplifier, whose output is differential signal centered at 2V common-mode. This output is a fully differential voltage optimized for driving the ADC.

The AD4134 is an alias free continuous time sigma delta ADC with resistive input, hence there is no requirement of anti-aliasing filter or a fully differential amplifier. This helps reduce the signal chain size, noise, and phase delays, allowing for precision compact measurements. While piezoelectric accelerometers have bandwidths as high as 30kHz, a wider bandwidth signal chain is chosen with respect to phase delay, thereby achieving better phase matching performance in 2-axis measurements. Further bandwidth limiting occurs in the digital filter of the ADCs; however, the phase delay is known and deterministic.

ICP/IEPE ACCELEROMETER

Any IEPE vibration sensor can be interfaced with the AD4134 reference design because all the IEPE vibration sensors work using the same principle with different offset voltages, noise levels, bandwidths, and sensitivities. An IEPE output signal carries both ac and dc voltages, where the vibration dependent ac voltage is dc shifted to a voltage level between 7V and 13V. This dc level varies from sensor to sensor, and for any given sensor, it has a drift component with respect to time, temperature, and excitation current.

The IEPE sensor must be powered by a current source with a sufficiently high voltage range to fully cover the amplitude of the sensor. A typical excitation voltage of the IEPE sensors is 24V.

The input of the signal chain can receive a signal amplitude of up to 10V p-p with an offset voltage of up to 13V. The dc offset is removed by Series capacitor.

Figure 2 shows an ICP accelerometer block diagram of a sensor powered by a constant current source and connected to a dc-coupled signal chain. The maximum bandwidth of the sensor is proportional to the excitation current and inversely proportional to cable capacitance. The maximum desired output voltage of the sensor and cable type must be taken into account when choosing the level of the constant current, which can be determined by using the following equation:

where:
f_MAX is the maximum frequency of the sensor in hertz.
I_C is the constant current in mA.
1 mA is the power requirement of the sensor.
C is cable capacitance in pF.
V is the maximum peak voltage output from the sensor in volts.

Note that in Equation 1, 1mA is subtracted from the total current supplied to the sensor (IC), where the approximate 1mA powers sensor itself, and the rest of the current drives the cable. This number varies from sensor to sensor.

For example, this reference design is tested using an ICP accelerometer produced by PCB piezotronics, model 333B52 with a maximum peak output of 10V, a 10-foot cable length with a capacitance of 29pF/ft, and an excitation current of 2.5mA. Applying Equation 1, the maximum theoretical bandwidth of the sensor is 82.3kHz.

Neither the cable nor the chosen current level limit the performance of the sensor.

CONSTANT CURRENT SOURCE

Take care when designing the constant current source (CCS) and when considering noise performance. Low current noise is essential because current noise is converted to voltage noise when driving the input impedance of the signal chain.

Figure 3 depicts a 2-terminal current source with resistors R_SET and R_OUT setting the output current to 2.5mA, and capacitor C_SET limiting the bandwidth of the current noise. The internal 10μA reference current source of the LT3092 holds a stable V_SET across R_SET. The V_SET is mirrored across R_OUT, which sets the output current according to Equation 2.

Note that the actual IOUT current is 10μA larger than the output current given by Equation 3 because of the internal reference current flowing from the SET terminal.

The data sheet recommends R_SET = 20kΩ to set the voltage drop across R_SET to 200mV, minimizing the effect of the offset voltage. (The offset voltage is more significant across a smaller V_SET.) The white current noise produced by a resistor is given by Equation 3.

where:
T is absolute temperature in Kelvin.
k is Boltzmann’s constant in (J/K).
R is resistance.

Because resistor current noise is proportional to the square root of the inverse of resistance, increasing the value of R_SET from the recommended 20kΩ to 120kΩ requires a proportional increase in ROUT as well (to keep the output current at the same level), causing a drop in the overall noise current. A capacitor, C_SET, across R_SET is recommended to lower the current noise of R_SET and of the internal current reference of the LT3092. The CSET capacitor bypasses the current noise emitting from the LT3092.

An LT spice simulation of the constant current source, as shown in Figure 3, was performed to optimize component values and placement dependency. A non ideal voltage source was modeled, providing 0.7mV rms voltage noise and 224nA rms of current noise in a 20MHz bandwidth, simulating a keysight E3631 bench power supply with two outputs connected in series, set to 26V total.

Table 1 lists rms noise for various combinations of component values. The rms current noise is simulated for a bandwidth from 1MHz to 100kHz. CCOMP acts like a high-pass filter, passing the noise from the voltage source to the output. Further increasing R_SET and R_OUT helps in the current noise reduction, but it also causes a higher voltage drop over the resistors, reducing the allowable signal swing.

¹ No component required.

When using a long cable with a high level of inductance, stability may become an issue. For more information on compensating inductive loads, refer to the LT3092 data sheet.

To calculate the usable sensor excitation voltage provided by the current source, use the following equation:

where:
V_DD is the supply voltage of the constant current source.
LT3092_DROP is the dropout voltage over the IC itself (typically 1.2V at load current up to 10mA).
R_SET × 10μA gives the dropout voltage over the resistor, which sets the output current level and a current from the internal 10μA flows through the resistor.

In this case, the usable excitation voltage is 23.6V.

INPUT PROTECTION CIRCUIT

The IEPE sensors are typically connected to the DAQ system via long cables. In an environment where many cables and other signal types are present, it is important to protect the DAQ system from any overvoltages that may occur due to operator miswiring or cable damage. The ADG5462F is a user defined input protection switch that automatically detects overvoltage conditions and open circuits the sensor from the data acquisition signal path. For the switches to be active, input signal levels from the sensor must be within the voltages on POSFV and NEGFV Pins. Note that VSS is the negative power supply pin and VDD is the positive power supply pin. The ADG5462F also provides a fault flag (FF) that can be used as an interrupt into a controller to alert the operator that an input fault has occurred. In normal operation, the FF pin is pulled high. However, if any of the Sx pin goes beyond positive fault voltage (POSFV) or negative fault voltage (NEGFV), the FF pin is pulled low to indicate a fault has occurred. If a fault is detected in the AD4134, the sensor input and current source disconnects from the data acquisition system.

ANALOG-TO-DIGITAL CONVERSION

The AD4134, a precision, quad channel, 24-bit, Continuous time Σ-Δ ADC has been chosen for its excellent dc to 390kHz bandwidth precision and the inherent alias free behavior.

The output data rate (ODR) of the ADC can either be programmed (SRC mode) in the AD4134 or set using a continuous external signal (ASRC mode) at the desired sampling speed.

In SRC mode for an ODR to be 375kSPS, calculate the decimation rate as follows:

Decimation Rate = 24MHz/375kHz = 64 = 0x00004000000000
Program ODR_VAL_INT, Bits[23:0] with 0x000040. Program
ODR_VAL_FLT, Bits[31:0] with 0x00000000.

Every time the ODR_VAL_INT, Bits[23:0] and ODR_VAL_FLT, Bits[31:0] are changed, the Bit 0 in the TRANSFER_REGISTER must be set to update the ODR to the new value.

The default settings for this reference design are optimized for an ADC measurement bandwidth of 32kHz, as follows:

• Conversion length: 24 bits
• ASRC mode
• High performance mode
• Status header enabled

MEASURED PERFORMANCE OF THE SIGNAL CHAIN

This signal chain design targets medium to wider bandwidth vibration sensing, where higher harmonics and frequency content above 1kHz are important. In the design, there is a necessary trade-off between the system bandwidth, linearity, and the achievable noise performance. A higher input impedance is chosen to maintain signal accuracy (linearity), which in this design sets a maximum noise performance limit. The signal bandwidth is also set wider that the response of the system over higher frequencies is maintained. A lower noise solution can be achieved with a design that has a narrower bandwidth, thus removing more of the wideband noise.

Noise

Noise performance of the entire signal chain is measured with inputs shorted. Table 3 details the typical noise performance of the signal chain, without any sensor connected.

Figure 4 shows the dynamic range of the dc-coupled solution with shorted input over temperature.

Sensor Noise Contributions

The usual aim of the designer of a data acquisition system is to capture the sensor output signal as accurately as possible. In practice, this means that the performance of the system must be set by the sensor characteristics. The noise performance of the sensor is often one of the key limiting factors of the overall measurement system; understanding this helps to set the performance requirement of the design. For this design, the target is to support sensors that provide vibration data at bandwidths greater than 1kHz, which are used in data acquisition systems targeted at condition-based monitoring for predictive maintenance on rotating or reciprocating plant equipment. Table 4 details the performance level and bandwidth of a small selection of vibration sensors. The main considerations in sensor selection are typically bandwidth, range, noise spectral density (NSD), and power. The ADXL1002 and ADXL1004 sensors are low power devices suitable for a range of vibration applications, where power and bandwidth are important. These accelerometers are suitable for continuous monitoring applications, such as internet of things (IoT) machine monitoring.

For the highest sensitivities and bandwidths, where low noise and sensitivity at higher frequencies are key, piezoelectric sensors are still the most appropriate sensors to use. Due to the wide bandwidth and low noise capabilities of the AD4134, this signal chain is adapted to match the performance level of typical sensors in that wider bandwidth range beyond 10kHz.

In the case of AD4134, the system bandwidth is set to 54kHz, and the signal chain noise performance is aimed at sensors that can achieve >100 dB dynamic range over that bandwidth. For example, piezotronics PCB Model 621B40 accelerometer achieves almost 105dB at 30kHz. This circuit can be adapted for sensors with higher dynamic range and narrower bandwidths by scaling the resistor values and gains of the stages and by utilizing lower output data rates of the AD4134. A complete analysis is beyond the scope of this document, but more information on the trade-off between dynamic range and bandwidth using Output data rates can be found in the AD4134 data sheet.

SYSTEM POWER

The board power design is an optimal solution that allows the complete signal chain to be powered from a single 9V to12V rail.

Power Solution

Figure 5 shows a simplified block schematic of the power section. For compatibility with microcontroller and other development boards with arduino style connections or FPGA mezzanine card (FMCZ) type zed boards, the power solution for the board is designed to run from the supply which is typically provided by both these types of digital platforms.

The supply to the board is directly taken from the power input to the arduino board. To ensure stability of the system, the adapter must be able to support a current of 1.5A.

Although the board does not need 250mA in steady state operation, during the initial power-up phase, there may be inrush currents up to 200mA or higher for up to 30ms.

The power solution comprises four voltage domains: the 9V domain, the 5V domain, 1.8V domain and the +/-15 V domain. It also includes the 2.5mA current source for the IEPE sensor.

The purpose of the provided power solution circuitry is to allow the board to run from a single supply of 9V to 12V, which is used to power the power arduino board, and to generate the other required voltage rails from that one supply. On the AD4134, the raw 3.3V input supply rail is used directly to provide the level translator.

Mainly there are three switch mode dc-to-dc regulators generating voltages required for the operation.

1. LT3471 – Generating +/-15V from 9V to 12V input
2. LT8607 – Generating 5V from 9V to 12V input
3. LT8606 – Generating 1.8V from 5V supply

Additionally, there is a 4.096V reference IC generating the reference for the AD4134 ADC.

5V and 1.8V are needed by the AD4134 ADC, while the +/-15V is needed for in-amp, IEPE interface, and current source circuit. The +/-15V rail is required to supply the LT3092 current source that provides the IEPE sensor with 2.5mA.

Power Measurements

Power measurements are taken directly from the 9V to 12V input rail. The power measurements, therefore, include the contributions from the power solution components themselves.

The bias current to the IEPE sensor from the +15V/-15V rail is constant due to the constant current source and does not vary with ADC settings.

The power consumption of the rest of the system is measured in the different operation modes of the ADC.

Power Consumption

The register settings on the ADC that affect the power consumption are the following:

• Power mode
• Output Data rate
• Filter type
• Sleep mode

  System Default Configuration

  A default configuration of the system for narrow bandwidth measurement is chosen as follows for the ADC settings:

  • ASRC mode with ODR and DCLK as inputs
  • Power mode: High performance mode
  • FIR filter with ODR = 75kSPS
  • VCM pin output: VREF/2
  • MCLK frequency of 48MHz

    The default system configuration is for 28kHz signal bandwidth IEPE sensor.

    Table 5. Power Consumption at Various Data Rates

    ADC Power Mode	ADC Data Rate (kSPS)	9V Rail (mA)
    High Performance	3745	74
    High Performance	375	58
    Low Poswer	75	50