Electronic Design
Chips Implement Sensor Signal Chains From Nanoamps To Bits

Chips Implement Sensor Signal Chains From Nanoamps To Bits

The art of analog design appears to be migrating from the circuit board to the IC. A case in point is a new line of analog front-end (AFE) chips and associated online design software that make the AFEs easy to design into industrial control applications with only a modest amount of practical engineering experience.

These sensor AFEs from National Semiconductor can be customized for specific types of sensors. Actually, calling them “AFEs” understates their functionality. They constitute a full signal chain from sensor input through analog-to-digital conversion, with high-precision digital output and lower noise than might have been possible with a board-level design.

Far from simple, they incorporate a variety of features, including programmable current sources, voltage references, and adjustable sample rates with true continuous background calibration that eliminates offset and gain error over time and temperature without disturbing the measured signal. However, a new version of National’s WEBENCH platform removes the “black art” aspect of designing applications with them. 

At this time, there are two ICs in the family with more coming. Their numerical nomenclature could be confusing, as they’re designated the LMP90100 and the LMP91000. The LMP90100 is a multi-channel, low-power, 24-bit sensor AFE for generic high-performance transmitter and transducer applications, and the LMP91000 is a potentiostat for toxic-gas sensors.

Looking first at the more general-purpose part, the LMP91000 incorporates a 24-bit sigma-delta analog-to-digital converter (ADC) with an eight-channel input multiplexer that can be adapted to handle various configurations of differential and single-ended inputs. Signal gains, sample rates, and diagnostics are programmable for each sensor, and two matched current drives are available to drive whatever sensors are used. The chip draws less than 0.7 mA on average and is guaranteed from –40°C to +125°C.

The LMP91000 is expressly designed for micropower chemical and gas sensing situations such as three-electrode single-gas and two-terminal oxygen sensors. It works by measuring current in a potentiostat that is proportional to the gas concentration and then generates an output voltage proportional to the cell current using a transimpedance amplifier. That transimpedance gain is user-programmable through an I2C-compatible interface, allowing the device to support toxic-gas sensitivities over a range of 0.5 to 9.5 nA/ppm.

The cell voltage and cell output gain are user-selectable. Together, they allow the part to support a wide range of gases and gas concentrations. The same I2C interface that is used for programming enables the user to verify sensor performance. An embedded temperature sensor provides an additional output to monitor temperature. The LMP91000 operates on supply voltages from 2.7 to 5.5 V.

Obviously, executing that kind of a design requires a high level of experience with the entire subject of gas measurement, and that is where the issue of design talent moving from the board to the chip arises. By providing a high degree of programmability along with a library of data about sensors, plus the chip’s operating parameters, the WEBENCH tool has the basis for creating a complete design for a toxic gas measurement system.

Offered in a 28-pin thin-shrink small-outline package (TSSOP), the LMP90100 is available now for $4.95 in 1000-unit quantities. Also available now, the LMP91000 is offered in a 14-pin leadless leadframe package (LLP) and priced at $3.95 in 1000-unit quantities. Use of WEBENCH is free with registration. Additional AFE products targeting faster and higher-performance applications and expanding into other markets such as medical will be introduced later.

National Semiconductor

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