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Sensor interface AFE simplifies sensor conditioning

Posted: 07 Sep 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Sensors? analogue-to-digital converter? ADC? DAC? Sensor conditioning?

The sensor interface offers the ability to interface with 16 differential output sensors. Each sensor connected to the XR10910 has its own inherent offset that if not calibrated out can decrease sensitivity and overall performance of the sensor system. The onboard DAC introduces an offset into the instrumentation amplifier to calibrate the offset voltage generated by the sensors. An independent offset can be set for each of the 16 channels.

The programmable gain instrumentation amplifier offers 8 selectable gains from 2V/V to 760V/V to amplify the signal such that it falls within the input range of the downstream ADC. The XR10910 even includes an integrated LDO that system designers can use to provide a regulated voltage to power the sensors.

The sensor interface operates from 2.7V to 5V supplies and offers a wide digital supply range of 1.8V to 5V. It typically consumes 457uA of supply current and offers a sleep mode which reduces the supply current to only 45uA.

Noise floor performance
Exar's AFE provides 14bit signal path linearity and offers low peak-to-peak noise (2?Vpp at G = 760) and low input voltage noise (35nV/Hz at G = 760), refer to Figures 2 and 3. The low noise performance of the XR10910 combined with the low bias current capability (100pA maximum) allows this AFE to interface with a wide range of sensors and pairs nicely with 3V to 5V 16bit ADCs.

Figure 2: Input Voltage Noise vs Frequency.

Figure 3: 0.1Hz to 10Hz RTI Voltage Noise.

Low power sensor interface for 16 bridge sensors
Many force and pressure sensors utilise a resistive element constructed as a Wheatstone bridge circuit often referred to as a strain gauge. The resistive elements in the bridge change resistance in response to mechanical strain. Strain gauge sensors are commonly used for both force and pressure measurement. The XR10910 is an easy-to-use sensor conditioning interface between multiple bridge sensors and an ADC/MCU or FPGA, as shown in figure 4.

Figure 4: 16:1 Bridge Sensor Interface using the XR10910.

Bridge sensors have a differential output signal (VO+ and VO-). Ideally, the unloaded bridge output is zero (VO+ and VO- are identical). However, in-exact resistive values result in a difference between VO+ and VO-. This bridge offset voltage can be substantial and vary between sensors causing decreased system accuracy. The XR10910 provides the ability to calibrate the bridge offset on each of the 16 bridge sensors using the onboard DAC.

The XR10910 offers an easy to use solution for interfacing with up to 16 bridge sensors. Consuming only 457?A supply current and only 36mm2 of real estate, the XR10910 offers the industry's smallest, lowest power interface for 16 bridge sensors.

Sensors and sensor technology will continue to be integrated into more and more of the electronic products we use every day. Sensor conditioning products will continue to play a key role in how these sensors interface with our ever growing digital and wireless world.

About the author
Debbie Brandenburg is with Exar Corp.

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