CN0532

This circuit can accommodate variations for different applications. The EVAL-CN0532-EBZ is based on the ADXL1002, with a sensor resonance frequency of 21 kHz. However, the same circuit can be used as an IEPE compatible interface for other sensors in the MEMS accelerometer family. Note that based on the sensor resonance frequency, select the low-pass filter cutoff frequency accordingly.

The AD8541 is configured as a zero gain buffer in this design. Therefore, the output swing of the sensor and the IEPE interface are the same. In another variation, however, to make the sensitivity scale of V_IEPE according to the piezoelectric sensor replacement device, or any other design requirements, adjust the gain of the AD8541 accordingly. Employ careful consideration when selecting the gain resistance so that the output current requirement of the sensor, regulators, and buffer is also met.

A brief description of how to set up the circuit and mechanical mounting, the methods to read output, and what to expect is given in the following subsections. For a complete description of the hardware, see the CN0532 User Guide.

Functional Block Diagram

Figure 4 shows the functional block diagram of the circuit. The solid line, black arrows depict the signal path and the dotted line, black arrows show the current supply path to power the circuit.

Equipment Needed

The following equipment is needed:

• IEPE-compatible vibration measurement equipment, such as the LMS SCADAS from Siemens and the CompactDAQ from National Instruments
• Shaker table or vibration source
• Aluminum block (EVAL-XLMOUNT1)
• Coax cable or properly shielded wires
• The EVAL-CN0532-EBZ

Getting Started

The basic steps for understanding and recreating the test setup follow:

1. Solder a coaxial or shielded cable to the printed circuit board (PCB).
2. Connect the other end to an IEPE-compatible vibration measurement equipment such as the LMS SCADAS from Siemens or the CompactDAQ from National Instruments.
3. Set the acceleration sensitivity (40 mV/G) on the vibration measurement equipment from Step 2 to taking data with the accelerometer sensors.
4. The sensitivity scale of the ADXL1002 may slightly vary from part to part, and the ADXL1002 can be simply calibrated with gravity field or other reference sensors.

For complete details and setup information, see the CN0532 User Guide.

Power Supply Configuration

As previously discussed, the circuit must be supplied with a 4 mA current source, which can be provided by an IEPEcompatible vibration measurement setup or a simple current source. When using a simple current source, the supply line voltage is the output of the sensor.

Test Results

To verify performance of the circuit for vibration measurement applications, the circuit was tested in the Analog Devices vibration lab. The circuit is characterized for its frequency response, noise spectral density, and shock and group delay. Details and results of each test follows.

Frequency Response Measurements

The EVAL-CN0532-EBZ is attached to a shaker table through a mounting interface as shown in Figure 5, and the shaker table generates a controlled mechanical vibration from 100 Hz to 30 kHz, with a fixed 2 g acceleration. The output of the IEPE circuit and the reference vibration measurement (in this case, a laser doppler vibrometer) are recorded. The sensor output is calibrated for the accurate sensitivity scale factor using the low frequency peak amplitudes vs. the calibrated reference. Then, the output of the sine frequency sweep of the sensor is plotted as shown in Figure 6, which is consistent with the transfer function of the ADXL1002. 

In this and any other high frequency vibration tests, mechanical signal path integrity is very important. In other words, from the source to the sensor, there must be no attenuation (due to damping) nor amplification (due to resonance) of the vibration signal. In this example, for instance, an aluminum block (EVAL-XLMOUNT1), four screw mounts, and a thick PCB guarantees a flat mechanical response for the frequency range of interest.

Noise Spectral Density

The equivalent input noise is dominated by the sensor noise of the ADXL1002 because the buffer and shunt resistor are ultralow noise. Noise density characterization at different temperature levels ranging from −40°C to +125°C for the sensor with an IEPE interface are shown in Figure 7 in comparison with five other ADXL1002 outputs without an IEPE interface. The results show that the noise level of the ADXL1002 with an IEPE circuit is similar to those without an IEPE interface. In addition, the noise density variations over temperature range were measured and were within the range of the ADXL1002.

Figure 8 illustrates a sample set of data taken with the EVALCN0532-EBZ stimulated by a 10 kHz sine vibration with 2 g amplitude. The reference sensor shown in this test is an acceleration measurement by a laser doppler vibrometer.

Shock Test

The circuit is also tested with a shock profile as shown in Figure 9. The shock peak acceleration is 10 g, the width is 100 μs, and the shape of the half sine. Note that ringing of the output is expected because the ADXL1002 MEMS sensor can be modeled with an underdamped second-order system.

The reference sensor in this case is a piezoelectric sensor (Model 353C23) with a resonance frequency and characterized group delay of 4 μs. Note that there is a 15 μs phase difference between the reference sensor output and the output of the ADXL1002. Therefore, the overall group delay of the circuit is approximately 19 μs.