Highly integrated sensor interface analog front end simplifies sensor conditioning

Sensors are used in a variety of consumer and industrial markets, from cell phones and tablets to precision inspection, distance measurement, identification/image processing and process instrumentation applications. Many of these systems integrate multiple analog sensors to make the device smarter, more efficient, and more productive. These sensors can be used to detect pressure, temperature, stress, position, temperature, light, flow, sound, speed, heat, and more.

The number of sensors in your home appliance may surprise you. For example, today's washing machines have the ability to detect motor current, drum speed, water level, water flow, water temperature, imbalance, door opening/closing, and even cycle control using touch sensor buttons. The high efficiency straight type washing machine determines the water level of each load based on the weight of the laundry in each load. Sensor and sensor interface technologies enable a variety of efficiency and safety features in today's appliances.

Analog sensors generate electrical signals that are usually very small and are interspersed with noise. In many cases, no two sensors are identical, each carrying its own unique noise signature and introducing its own offset into the signal path. Calibrating the system, distinguishing between signals and noise, and amplifying them are critical to the performance of the end system. An important electronic interface between analog sensors and digital signal processing is to condition the electrical signal to ensure it is within the range of the downstream ADC (analog-to-digital converter). Today's industrial systems utilize multiple sensor and sensor types, creating a greater need for sensor conditioning components. Systems with multiple sensors require multiple levels of calibration and amplification to adequately process the sensor signals and pass them to the downstream ADC. The ever-expanding use of sensors in our electronic instruments and devices has increased the need for more flexible, lower cost sensor interface solutions.

Sensor conditioning capabilities are available through discrete solutions that use multiple components. In many cases, discrete sensor interface solutions abandon the variable gain or adjustable offset of each sensor input. Discrete solutions tend to generate more power than integrated solutions, requiring more space and longer design time. Many vendors offer an integrated sensor interface analog front end (AFE). Most include processors that can be replicated in the customer's end system and may include high levels of digital processing and storage protection that are not required in most applications, adding to the cost. Exar's XR10910 sensor interface AFE provides a lower cost integrated solution that integrates high channel count and functionality in a small footprint and consumes less power than competing devices. The unique feature set of the XR10910 offers greater design flexibility than discrete or other AFE solutions.

XR10910–16:1 sensor interface AFE

The XR10910 sensor interface includes a 16:1 differential multiplexer, a programmable gain instrumentation amplifier, a 10-bit offset correction DAC (digital to analog converter), and an LDO (low dropout regulator). The XR10910's features and features are controlled by the I2C interface and have a performance and feature set that complements today's microcontrollers (MCUs) or field-programmable gate arrays (FPGAs) with embedded ADCs. Figure 1 shows the block diagram of the XR10910.

Highly integrated sensor interface analog front end simplifies sensor conditioning

Figure 1 XR10910 block diagram

The XR10910 is one of the only sensor interface AFEs on the market, offering the ability to interface with 16 differential output sensors. Each sensor connected to the XR10910 has its own inherent offset that can degrade the sensitivity and overall performance of the sensor system without performing a calibration check. The onboard DAC introduces an offset into the instrumentation amplifier to calibrate the offset voltage generated by the sensor. Each of the 16 channels can be set to a separate offset.

The programmable gain instrumentation amplifier provides eight selectable gains from 2V/V to 760V/V to amplify the signal so 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 stable voltage to power the sensor.

Operating over a 2.7V to 5V supply range, the XR10910 offers a wide range of digital power supplies (1.8V to 5V). It typically consumes 457uA of supply current and provides a sleep mode that reduces the supply current to 45uA.

16-bit noise floor performance

Exar's AFE provides 14-bit signal path linearity with low peak-to-peak noise (2μVpp at G = 760) and low input voltage noise (35nV/√Hz for G = 760), see Figures 2 and 3. The low noise performance of the XR10910 combined with low bias current capability (up to 100pA) allows the AFE to interface with a wide range of sensors and pair with a 3V to 5V 16-bit ADC.

Highly integrated sensor interface analog front end simplifies sensor conditioning

Figure 2 Input voltage noise and frequency

Highly integrated sensor interface analog front end simplifies sensor conditioning

Figure 3 0.1Hz to 10Hz RTI voltage noise

XR10910 advantage

● Integrated MUX, DAC, PGA and LDO simplify sensor conditioning applications

● Greater flexibility compared to most integrated AFE solutions

● Lower power consumption than most integrated solutions, with the same or lower power consumption than discrete solutions

● Easy to use simple I2C interface

●6mm x 6mm QFN package

●High channel count simplifies board layout and saves real estate usage

Low-power sensor interface solution for 16 bridge sensors

Many stress and pressure sensors utilize a resistive element constructed as a Wheatstone bridge circuit, commonly referred to as a strain gauge. The resistive element in the bridge changes the resistance according to the mechanical stress. Strain gauge sensors are commonly used for both stress and pressure measurements. 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.

Highly integrated sensor interface analog front end simplifies sensor conditioning

Figure 4 16:1 Using the XR109101 Bridge Sensor Interface

The bridge sensor has differential output signals (VO+ and VO-). Ideally, the no-load bridge output is zero (VO+ and VO- are the same). However, inaccurate resistance values ​​form the difference between VO+ and VO-. Bridge offset voltages can be very large and vary from sensor to sensor, reducing system accuracy. The XR10910 provides the ability to calibrate the bridge offset on each of the 16 bridge sensors using an onboard DAC.

The XR10910 provides an easy-to-use solution for connecting up to 16 bridge sensors. The XR10910 consumes only 457μA of supply current and only 36mm2 of area, providing the industry's smallest, lowest power interface for 16 bridge sensors.

in conclusion

The XR10910 will be the leader in Exar's growing portfolio of easy-to-use sensor interface products. Its feature set is truly unique in the market, complementing the gap between discrete features that are less versatile and a single-chip sensor interface AFE with processing power. Compared to both, the XR10910 offers higher channel count, lower power consumption and smaller footprint.

Sensor and sensor technologies will continue to be integrated into the growing number of electronic products we use every day. Sensor conditioning products such as the XR10910 will continue to play a key role in how these sensor interfaces connect with our growing digital and wireless world.

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