Circuit Note CN-0540 Devices Connected/Referenced AD7768-1 Precision 24-Bit, 1024 kSPS, ADR4540 Ultra-Low-Noise, High- Sigma-Delta (-) ADC with Accuracy 4.096 V Power Scaling Voltage Reference ADA4807-1 3.1 nV/Hz, 1 mA, Rail-to- ADA4807-2 3.1 nV/Hz, 1 mA, Dual Circuits from the Lab Rail Input/Output Amplifier Rail-to-Rail I/O Amplifier reference designs are LT3092 200 mA 2-Terminal LTC3459 10 V Micropower engineered and tested for Programmable Current Synchronous Boost quick and easy system Source Converter in ThinSOT integration to help solve LTC2606 16-Bit Rail-to-Rail DAC LT3494 Micropower Low Noise todays analog, mixed- Boost Converters with signal, and RF design Output Disconnect challenges. For more information and/or ADA4945-1 High Speed Fully Differential LT3008 3 A I , 20 mA, 45 V Low Q ADC Driver Dropout Linear support, visit Regulators CN-0540 Circuit Note +5V ADR4540 +26V REF +4.096V BUF BUF +5V LT3092 +5V +3.3V ADA4945-1 +V S FB+ 4.5mA REF+ AVDD1 AVDD2 IOVDD ADG5421F DRDY IN +OUT AIN+ S1 D1 CS VCOM AD7768-1 AAF SERIAL S2 D2 IN+ DOUT AIN INTERFACE SDI OUT +5V REF AVSS DGND SCLK IEPE FAULT FB SENSOR DETECTION FF AND D MODE DISABLE GND SWITCH ANTI-ALIASING DRIVER FILTER V GND SS AD8605 +3.3V +5V +3.3V DIGITAL CONTROL V REGULATE IN STAGE DC TO DC V +3.3V OUT BOOST +26V LDO SCL 2 I C INTERFACE SDA LTC2606 V OUT REF REF Figure 1. Condition-Based Monitoring Signal Chain for an IEPE Piezoelectric Vibration Sensor CIRCUIT DESCRIPTION ICP/IEPE VIBRATION SENSOR The circuit shown in Figure 1 is a data acquisition (DAQ) signal Any IEPE vibration sensor can be interfaced with this CN-0540 chain for an IEPE sensor consisting of a current source, level reference design because all the IEPE vibration sensors work shifting and attenuation stage with a digital-to-analog converter using the same principle with different offset voltages, noise (DAC), a third-order antialiasing filter, an analog-to-digital levels, bandwidths, and sensitivities. An IEPE output signal converter (ADC) driver, and a fully differential - ADC. carries both ac and dc voltages, where the vibration dependent ac voltage is dc shifted to some voltage level between 7 V and The programmable current source drives the piezoelectric 13 V. This dc level varies from sensor to sensor, and for any accelerometer with constant current. The output current can given sensor, it has a drift component with respect to time, be programmed by external resistors and is usually set between temperature, and excitation current. 2 mA and 20 mA, depending on the type of sensor and the cable. The IEPE sensor must be powered by a current source with a The buffered and amplified output of the DAC, together with sufficiently high voltage range to fully cover the amplitude of the level shifting op-amp, shifts the input signal close to a 2.5 V the sensor. A typical excitation voltage of the IEPE sensors is 24 V. common-mode voltage (V ) to balance the input of the anti- OCM aliasing filter and that of the fully differential amplifier (FDA). The input of the signal chain can receive a signal amplitude of The second input of the FDA rail is set to 2.5 V, ensuring input up to 10 V p-p with an offset voltage up to 13 V. The dc offset is headroom requirements are met, and the output is a fully removed by applying a dc offset correction signal, allowing differential voltage optimized for driving the ADC. operation to an arbitrarily low frequency. VDD The antialiasing filter sets the bandwidth of the signal chain to 54 kHz. Piezoelectric accelerometers have bandwidths as high EXCITATION as 20 kHz, but a wider bandwidth signal chain has been chosen CURRENT with respect to phase delay, achieving better phase matching SIGNAL CONDITIONING performance in 3-axis measurements. (Further bandwidth limiting occurs in the digital filter of the ADCs, however, the ICP SENSOR phase delay is known and deterministic.) Figure 2. ICP Accelerometer Block Connection 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 Rev. A Page 2 of 15 22838-002 22838-001