As of now, sensor-based application design requires an optimized simulation solution tailored to each system. This type of design work lasts from days to weeks, and often involves many steps, including selecting related components and building prototypes to create layouts later, and then testing the first batch of printed circuit boards (PCBs) to be put into production. In order to avoid starting every new task from scratch again and again, solutions that include hardware and software components have been developed that not only simplify the work of the design engineer, but also save time in the design process. With the new series of high-precision sensor analog front ends (Sensor AFE), design engineers can create the perfect solution for each new sensor in just a few hours.
Sensor Analog Front-End Single Sensor The analog front-end (AFE) is not the same as an "analog FPGA" with all functions and all-in-one. The "analog FPGA" chip has too many drawbacks because of the need for large-scale packaging, the chip will be very large, which leads to expensive prices and a lot of power consumption. Therefore, it does not meet the designer's requirements.
National Semiconductor has opened up new ways to develop tailored, unique integrated circuits for specific measurement tasks such as measuring/detecting temperature, gas, pressure, pH, several medical counts, and weight. Each unique integrated circuit contains exactly the right function for a specific measurement task without any unnecessary ballast. In its measurement category (eg temperature), it is very easy to match different sensors with a specific device (this will be explained in detail later in this article).
The first two sensor AFE devices were launched a few months ago with two sensors in the AFE series: the LMP91000 for temperature sensors and low-speed bridge configurations, and the LMP90100 for gas sensors.
LMP90100
The LMP90100 provides a highly integrated combination of 8-channel input multiplexers. It is a precision amplifier with an adjustable gain factor and a 24-bit Σ-Δ ADC. The device includes a current source, a voltage reference, and other functions. Figure 1 shows the internal structure of the IC: The user can match all colored blocks in the graph based on the sensor and measurement tasks.
Other features that can be turned on or off include: Sensors can monitor for short-circuits or disconnections (open-circuit faults) of the inspection sensor, or offset calibration and amplification. These functions are performed entirely in the background and do not have any effect on the output data stream. In addition, in the event of an external clock failure, the clock management circuitry can be used to automatically switch to using the internal clock supply.
Since the multiplexer provides 7 single-ended inputs or 4 differential inputs, the device allows more sensors to be connected, which may be based on different technologies. A good example of this is the combination of a thermoelectric module with an analog temperature sensor connection located below the thermoelectric module, where the temperature sensor is used for cold junction compensation. Two thermoelectric modules plus two analog sensors, or two three-wire measuring resistors or three thermistors, can also be connected directly to this multifunction module. This sensor management function can continuously check the sensor as needed. At the same time, the management circuit employs a measurement method that is not currently used, ie a single sensor is always monitored in order to avoid any interference with the measurement data stream.
Two matched current sources can be adjusted with a maximum current of 1mA step, allowing the use of resistive sensors.
Design engineers can adjust subsequent amplifier stage gains from 1 to 128 in binary format. When the gain is higher than 16, the buffer immediately after the first amplifier stage can improve the overall measurement effect. However, this buffer consumes extra power. Designers need to measure whether they need to consume additional power to improve measurement results based on the specific application.
The sampling rate of the 24-bit Δ-Σ A/D converter is optimized for temperature measurements between 1.68 and 214.65 samples/s. Whenever the sampling rate is lower than 13.42, the chip ensures that no distortion occurs at either 50Hz or 60Hz. Design engineers can adjust the sampling rate for each channel individually. The specific values ​​provided are valid for single-ended operation. If you use differential channels, designers should be aware that the sample rate is divided by differential channels. With two differential channels, the maximum sampling rate will thus reach 214.65/2=107.33. The sampling rate with 4 differential channels will therefore reach 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 average power, but it can drive up to 10mA when connected to a new sensor. The LMP91000 can connect the sensor with two electrodes that work as a primary battery, or operate according to the amperage principle to connect the sensor with three electrodes. When connected to a three-electrode sensor, the LMP91000 can be used as a potentiostat. It can also act as a buffer when connecting a primary battery (to ground or to a reference voltage).
These applications can be found in many areas such as mining, industrial environments, fire departments, food and medical industries, oil and gas exploration/extraction, and water and wastewater treatment.
The sensor is equipped with three electrodes: working electrode, reference electrode and counter electrode. If the gas contacts the working electrode, it will oxidize or reduce the electrode. This oxidation creates a positive/negative current, and 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 sensors is a mandatory requirement. Changing the sensor each time will change the current value, which will lead to measurement errors. In order to determine the current state of the sensor life cycle, the possibility of "sensor test" can be used. For this purpose, the sensor receives a pulse and produces a characteristic output signal. The design engineer can 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 and not in contact with any gas. By using the reference electrode, the sensor AFE LMP91000 can compensate for the measurement error of the working electrode.
Like the current on the working electrode, the current value on the counter electrode is the same but has the opposite polarity; the amplifier A1 in the LMP91000 drives this current. In doing so, the device can keep the measurement unit in equilibrium, which is the effect of the “potential†derived potentiometer that is being compensated. In this way, when the maximum bias current driven by the RE is 670pA, the LMP91000 helps the designer to achieve sensitivity in the 9.5μA/10×10-6 to 0.5nA/10×10-6 range.
When the sensor is first operated, the first step is the accumulation of potential. In order to achieve the desired effect, the LMP91000 will drive up to 10mA. This requires it to complete the process in just a few hours. In some cases, ordinary discrete circuits take several days to establish this potential.
WEBENCH SENSOR DESIGN TOOLS Design engineers can easily evaluate designs with the help of Sensor AFE Designer, an online design tool provided free by National Semiconductor. Users can access this software directly from National Semiconductor's web site by clicking on the "Sensors" tab in the WEBENCH box, then selecting the appropriate type of sensor and clicking "Start Design".
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 provided a link map for the LMP90100. All necessary adjustments have been preset by the system: including selection of individual sensors and assignment of inputs, the system defines all parameters such as current and reference source and gain. The user can then select the sample rate, background calibration, or sensor test function.
In addition to the two sensor analog front-end integrated circuits, National Semiconductor also offers a companion evaluation board. By connecting such an evaluation board to a PC, design engineers can download the necessary offline software from National Semiconductor's website. Unless the board is to be measured directly with a true sensor, the entire configuration process is the same as WEBENCH. The sensor data can show voltage (in V), data (in bits) or temperature (in °C) or pressure (in psi) on the time axis. In this way, users can directly test their process and measurement tasks and use the Sensor AFE to control measurement tasks. On the left side of the displayed image, the system shows the expected accuracy and the actual accuracy achieved. In this case, the design engineer must note that the ENOB formula is valid for statistical values ​​but does not apply to dynamic values ​​used under normal conditions. Since the standard deviation is a part of the equation, when the temperature (or pressure, if using a bridge circuit) does not have a constant value, the ENOB value displayed by the system will drop significantly.
Sensor Analog Front-End Single Sensor The analog front-end (AFE) is not the same as an "analog FPGA" with all functions and all-in-one. The "analog FPGA" chip has too many drawbacks because of the need for large-scale packaging, the chip will be very large, which leads to expensive prices and a lot of power consumption. Therefore, it does not meet the designer's requirements.
National Semiconductor has opened up new ways to develop tailored, unique integrated circuits for specific measurement tasks such as measuring/detecting temperature, gas, pressure, pH, several medical counts, and weight. Each unique integrated circuit contains exactly the right function for a specific measurement task without any unnecessary ballast. In its measurement category (eg temperature), it is very easy to match different sensors with a specific device (this will be explained in detail later in this article).
The first two sensor AFE devices were launched a few months ago with two sensors in the AFE series: the LMP91000 for temperature sensors and low-speed bridge configurations, and the LMP90100 for gas sensors.
LMP90100
The LMP90100 provides a highly integrated combination of 8-channel input multiplexers. It is a precision amplifier with an adjustable gain factor and a 24-bit Σ-Δ ADC. The device includes a current source, a voltage reference, and other functions. Figure 1 shows the internal structure of the IC: The user can match all colored blocks in the graph based on the sensor and measurement tasks.
Other features that can be turned on or off include: Sensors can monitor for short-circuits or disconnections (open-circuit faults) of the inspection sensor, or offset calibration and amplification. These functions are performed entirely in the background and do not have any effect on the output data stream. In addition, in the event of an external clock failure, the clock management circuitry can be used to automatically switch to using the internal clock supply.
Since the multiplexer provides 7 single-ended inputs or 4 differential inputs, the device allows more sensors to be connected, which may be based on different technologies. A good example of this is the combination of a thermoelectric module with an analog temperature sensor connection located below the thermoelectric module, where the temperature sensor is used for cold junction compensation. Two thermoelectric modules plus two analog sensors, or two three-wire measuring resistors or three thermistors, can also be connected directly to this multifunction module. This sensor management function can continuously check the sensor as needed. At the same time, the management circuit employs a measurement method that is not currently used, ie a single sensor is always monitored in order to avoid any interference with the measurement data stream.
Two matched current sources can be adjusted with a maximum current of 1mA step, allowing the use of resistive sensors.
Design engineers can adjust subsequent amplifier stage gains from 1 to 128 in binary format. When the gain is higher than 16, the buffer immediately after the first amplifier stage can improve the overall measurement effect. However, this buffer consumes extra power. Designers need to measure whether they need to consume additional power to improve measurement results based on the specific application.
The sampling rate of the 24-bit Δ-Σ A/D converter is optimized for temperature measurements between 1.68 and 214.65 samples/s. Whenever the sampling rate is lower than 13.42, the chip ensures that no distortion occurs at either 50Hz or 60Hz. Design engineers can adjust the sampling rate for each channel individually. The specific values ​​provided are valid for single-ended operation. If you use differential channels, designers should be aware that the sample rate is divided by differential channels. With two differential channels, the maximum sampling rate will thus reach 214.65/2=107.33. The sampling rate with 4 differential channels will therefore reach 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 average power, but it can drive up to 10mA when connected to a new sensor. The LMP91000 can connect the sensor with two electrodes that work as a primary battery, or operate according to the amperage principle to connect the sensor with three electrodes. When connected to a three-electrode sensor, the LMP91000 can be used as a potentiostat. It can also act as a buffer when connecting a primary battery (to ground or to a reference voltage).
These applications can be found in many areas such as mining, industrial environments, fire departments, food and medical industries, oil and gas exploration/extraction, and water and wastewater treatment.
The sensor is equipped with three electrodes: working electrode, reference electrode and counter electrode. If the gas contacts the working electrode, it will oxidize or reduce the electrode. This oxidation creates a positive/negative current, and 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 sensors is a mandatory requirement. Changing the sensor each time will change the current value, which will lead to measurement errors. In order to determine the current state of the sensor life cycle, the possibility of "sensor test" can be used. For this purpose, the sensor receives a pulse and produces a characteristic output signal. The design engineer can 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 and not in contact with any gas. By using the reference electrode, the sensor AFE LMP91000 can compensate for the measurement error of the working electrode.
Like the current on the working electrode, the current value on the counter electrode is the same but has the opposite polarity; the amplifier A1 in the LMP91000 drives this current. In doing so, the device can keep the measurement unit in equilibrium, which is the effect of the “potential†derived potentiometer that is being compensated. In this way, when the maximum bias current driven by the RE is 670pA, the LMP91000 helps the designer to achieve sensitivity in the 9.5μA/10×10-6 to 0.5nA/10×10-6 range.
When the sensor is first operated, the first step is the accumulation of potential. In order to achieve the desired effect, the LMP91000 will drive up to 10mA. This requires it to complete the process in just a few hours. In some cases, ordinary discrete circuits take several days to establish this potential.
WEBENCH SENSOR DESIGN TOOLS Design engineers can easily evaluate designs with the help of Sensor AFE Designer, an online design tool provided free by National Semiconductor. Users can access this software directly from National Semiconductor's web site by clicking on the "Sensors" tab in the WEBENCH box, then selecting the appropriate type of sensor and clicking "Start Design".
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 provided a link map for the LMP90100. All necessary adjustments have been preset by the system: including selection of individual sensors and assignment of inputs, the system defines all parameters such as current and reference source and gain. The user can then select the sample rate, background calibration, or sensor test function.
In addition to the two sensor analog front-end integrated circuits, National Semiconductor also offers a companion evaluation board. By connecting such an evaluation board to a PC, design engineers can download the necessary offline software from National Semiconductor's website. Unless the board is to be measured directly with a true sensor, the entire configuration process is the same as WEBENCH. The sensor data can show voltage (in V), data (in bits) or temperature (in °C) or pressure (in psi) on the time axis. In this way, users can directly test their process and measurement tasks and use the Sensor AFE to control measurement tasks. On the left side of the displayed image, the system shows the expected accuracy and the actual accuracy achieved. In this case, the design engineer must note that the ENOB formula is valid for statistical values ​​but does not apply to dynamic values ​​used under normal conditions. Since the standard deviation is a part of the equation, when the temperature (or pressure, if using a bridge circuit) does not have a constant value, the ENOB value displayed by the system will drop significantly.
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