A shortcut to sensor system design

Until now, the design of sensor-based applications required an optimized analog solution tailored for each system. This design work can take anywhere from a few days to a few weeks and often involves many steps, including selecting the relevant components and building a prototype to subsequently create a layout and then test the first printed circuit boards (PCBs) that will be put into production. In order to avoid starting each new task from scratch again and again, solutions consisting of hardware and software components have been developed that not only simplify the work of design engineers, but also save time during the design process. With the new series of high-precision sensor analog front ends (Sensor AFEs), design engineers can create the perfect solution for each new sensor in just a few hours.

Sensor Analog Front End

A single sensor analog front end (AFE) is different from an "analog FPGA" that integrates all functions. The "analog FPGA" chip has too many disadvantages. Because of the need for large-scale packaging, the chip will be very large, which leads to high prices and large power consumption. Therefore, it does not meet the requirements of designers.

National Semiconductor has taken a new path by developing unique integrated circuits tailored for specific measurement tasks, such as measuring/detecting temperature, gas, pressure, pH, several medical counters, weight, etc. Each unique integrated circuit contains the exact right functionality for the specific measurement task without any unnecessary electronic components (ballast). It is very easy to match different sensors with a specific device in the category it measures (such as temperature) (this will be explained in detail later in this article).

The first two sensor AFE devices

Just a few months ago, two devices in the sensor AFE family were introduced: the LMP91000 for temperature sensors and low-speed bridge configuration measurements, and the LMP90100 for gas sensors.

LMP90100

The LMP90100 provides a highly integrated combination of an 8-channel input multiplexer, a high-precision amplifier with adjustable gain factor and a 24-bit Σ-Δ ADC. The device includes current sources, voltage references and other functions. Figure 1 shows the internal structure of the integrated circuit: the user can match all the colored blocks in the figure according to the sensor and measurement task.

Figure 1 LMP90100 internal structure

Other functions that can be turned on or off include sensor monitoring to check for shorts or disconnections (open circuit faults), or offset calibration and amplification. These functions are performed completely in the background and have no impact on the output data stream. In addition, in the event of an external clock failure, the clock management circuit can automatically switch to using the internal clock for power supply.

Since the multiplexer offers 7 single-ended inputs or 4 differential inputs, the device allows the connection of more sensors, which may be based on different technologies. A good example of this is the combination of a thermoelectric component with an analog temperature sensor connection located below the thermoelectric component, where the temperature sensor is used for cold junction compensation. Two thermoelectric components plus two analog sensors, or two three-wire measurement resistors or three thermistors can also be connected directly to this multifunctional component. This allows the sensor management function to constantly check the sensors as needed. At the same time, the management circuit uses a measurement method that is not used so far, that is, always monitoring a single sensor to avoid any interference in the measurement data flow.

Two matched current sources can be adjusted with a maximum current of 1mA step, allowing the use of resistive sensors.

Designers can adjust the gain of the subsequent amplifier stage from 1 to 128 in binary format. When the gain is higher than 16, a buffer immediately after the first amplifier stage can improve the overall measurement. However, this buffer consumes additional power. Designers need to weigh whether the additional power consumption is necessary to improve the measurement results based on the specific application.

The sampling rate of the 24-bit delta-sigma A/D converter is optimized for temperature measurement and is between 1.68 and 214.65 samples/s. Whenever the sampling rate is lower than 13.42, the chip guarantees no distortion at either 50Hz or 60Hz. The designer can adjust the sampling rate of each channel individually. The specific values ​​provided are valid for single-ended operation. If differential channels are used, the designer should note that the sampling rate is divided by the differential channels. With two differential channels, the maximum sampling rate would thus be 214.65/2=107.33. With four differential channels, the sampling rate would thus be 53.6625 conversions/s.

LMP91000

The LMP91000 is a pure analog solution with very low current consumption, which makes it particularly suitable for portable applications. The LMP91000 consumes less than 10μA on average, but it is capable of driving up to 10mA when connected to a new sensor. The LMP91000 can connect sensors with two electrodes operating as a galvanic cell, or with three electrodes operating according to the Ampere principle. When connecting a three-electrode sensor, the LMP91000 can be used as a potentiostat, and when connecting a galvanic cell (to ground or to a reference voltage), it can also act as a buffer. Typical examples of gases suitable for these sensors are listed in Table 1.

These applications can be found in many areas like mining, industrial environments, fire departments, food and medical industries, oil and gas exploration/extraction, and water and wastewater treatment.

Figure 2 shows the LMP91000 acting as a potentiostat.

Figure 2 LMP91000 used as a constant potentiostat

The sensor is equipped with three electrodes: working electrode, reference electrode and counter electrode. If the gas comes into contact with the working electrode, it will oxidize or reduce the electrode. This oxidation process establishes a positive/negative current, while the absolute value of the current varies linearly with the gas concentration. Over time, the electrode will be destroyed as the gas concentration increases. Therefore, regular replacement of the sensor is a mandatory requirement. Each replacement of the sensor will change the current value, which will lead to measurement errors. In order to determine the current status of the sensor life cycle, the possibility of "sensor testing" can be used. For this purpose, the sensor receives a pulse and generates a characteristic output signal. The design engineer is able to analyze the shape of the signal curve to determine the actual condition of the sensor.

The reference electrode is a constant fixed reference potential located in the electrolyte without contact with any gas. By using the reference electrode, the sensor AFE LMP91000 is able to compensate for the measurement errors of the working electrode.

The current on the counter electrode is the same value as the current on the working electrode, but has the opposite polarity; amplifier A1 within the LMP91000 drives this current. In doing so, the device keeps the measurement cell in equilibrium, which is the role of the constant potentiostat derived from the "potential" being compensated. In this way, the LMP91000 helps designers achieve sensitivities in the range of 9.5μA/10×10-6 to 0.5nA/10×10-6 when the maximum bias current driven by RE is 670pA.

When the sensor is first operated, the first step is the accumulation of potential. In order to achieve the required effect, the LMP91000 will drive up to 10mA of current. This requires it to complete this process in just a few hours. In some cases, ordinary discrete circuits will take several days to build up this potential.

WEBENCH Sensor Design Tools

Design engineers can easily evaluate designs with the help of the free online design tool "Sensor AFE Designer" provided by National Semiconductor. Users can access the software directly on the National Semiconductor website. Just click the "Sensor" tab in the WEBENCH box, select the appropriate type of sensor, and then click "Start Design" (see Figure 3).

Figure 3 WEBENCH interface

Here, the user can select the corresponding sensor - in this case a K-type thermal assembly manufactured by Tempco. After selecting the sensor, the software immediately provides a link diagram for the LMP90100 (see Figure 4). All necessary adjustments have been preset by the system: including the selection of the individual sensors and the assignment of the inputs, the system defines all parameters such as current and reference sources and gains. The user can then select the sampling rate, background calibration or sensor test function.

Figure 4 LMP90100 application circuit

In addition to the two sensor AFEs, National Semiconductor also offers an evaluation board. Once the evaluation board is connected to a PC, designers can download the necessary offline software from National Semiconductor's website. The configuration process is the same as for WEBENCH, except that the board is to be used for direct measurement with a real sensor. Sensor data can be displayed as voltage (in V), data (in bits), or temperature (in °C) or pressure (in psi) on a time axis. In this way, users can directly test their processes, measurement tasks, and master measurement tasks using the Sensor AFE. On the left side of the display, the system shows the accuracy that can be expected and the accuracy that is actually achieved. In this case, designers must note that the ENOB formula is valid for statistical values, but not for dynamic values ​​used in normal situations. Since the standard deviation is part of the formula, the ENOB value displayed by the system will drop significantly when the temperature (or pressure, if a bridge circuit is used) does not have a constant value.