Signal conditioning is usually included in the simulation from the physical sensor signals into digital signals, and the resulting digital signal can be used for data collection, process control, perform calculations, produce display readings and other operations. Analog sensors can measure temperature, pressure, force, flow, mechanical movement, position, pH (acidity) and light intensity and other physical quantities. Since the sensor output voltage, current, or resistance to change is relatively small, usually converted into digital data that must pass before conditioning, so the sensor signal can not be directly converted into digital signals.
conditioning circuit by zooming, cached, or adjusting the magnitude of the analog signal to fit the ADC (ADC) input, and then the signal is digitized by the ADC and sent to a microprocessor or other digital devices ,
cheap nba jerseys, and then completed in the system for further data processing (see Figure 1). Work to make this signal chain is the key to select the measurement can be used with various types of sensors connected to operational amplifiers. ADC chosen designer must be able to handle the signal from the input network, and data acquisition systems have to meet the resolution, accuracy and sampling rate and other requirements of the digital output.
A. Design of the beginning: choose according to the measured physical quantity sensor
sensor into the temperature sensor (such as thermocouples, resistance temperature detector (RTD) and thermistors), pressure, or force sensor (such as strain gauge), solution pH sensor (such as pH electrode), and light detector (such as the PIN used to measure the optical density of the photodiode). Sensors can be further divided into active sensors and passive sensors. Active sensors require an external excitation source (voltage source or current source), passive sensor itself can generate output voltages without external incentives. Active sensors are commonly used in RTD, thermistors and strain gauges, commonly used passive sensors are thermocouples and PIN diodes. To determine which indicators must be met in order to connect with the sensor, the designer must consider the following basic sensor characteristics.
Figure 1
signal source impedance:
high source impedance: higher than 100kΩ
low signal source impedance: less than 100Ω
output signal level:
High signal level: full-scale voltage levels higher than 500mV
low signal level: the level of full scale dynamic range of less than 100mV
:
refers to the sensor output signal generated by the measurable range. Dynamic range and the type of sensor used.
B. The role of the amplifier signal gain
addition to the DC, the amplifier is also the signals from the sensors and adjust the cache and then send it to the ADC analog to digital conversion. There are two key role in the amplifier,
minesota twins jerseys, one of the characteristics of the sensor in accordance with appropriate interface to the sensors,
mlb jerseys, the other is shown in accordance with the ADC is connected to the load characteristics of ADC, these features include the wiring between the amplifier and ADC distance capacity of the load effect and the ADC's input impedance.
in selecting the appropriate sensor connected to the amplifier, the designer must make the sensor to match the characteristics of the amplifier. For the sensor - the amplifier combinations work, some characteristics of the amplifier is more critical than other features. Therefore, the choice of amplifier type, priority should be given some characteristics (see Table 1). For example, pH electrode is a high-impedance sensors, should the amplifier's input bias current as a priority consideration (table bid for the high priority). For this pH sensor, must not be allowed to produce detectable current output signal, it should choose not to require high input bias current amplifier. For this requirement, the high impedance input amplifier for MOS low input bias current to be the best choice. In this application, the gain bandwidth product (GBP) is a secondary factor (Table won the bid for the low priority), since the sensor operates in the low frequency band, this amplifier's frequency response does not prevent an exact reproduction of the sensor signal waveform.
Table 1
C. and sensors to match
1. high-impedance pH sensor with only two 1.5V battery-powered low-power amplifier use (see Figure 2). Although the sensor's output impedance is high (up to 1MΩ or more), but because of this amplifier with multiple input MOS transistors, can still exhibit high impedance of the sensor. The amplifier's input bias current less than 0.1pA (thus drawing the work of the current is very small), the bias voltage less than 1mV. The amplifier has rail-to-rail operation ability and high drive capability, you can send the signal through long-term to far away from ADC. In order to make accurate pH, temperature compensation circuit can be added in the high-precision temperature sensor to measure the pH sensor temperature.
Figure 2
2. strain gauge sensors and other commonly used measurements obtained through the bridge network,
lebron james jerseys, act as a bridge strain gauge is usually the two arms (or four arms.) Strain gauge is a low impedance signal source device, the output signal amplitude only a few hundred microvolts to several millivolts. Figure 3 shows a circuit for measuring not only provides a stable bridge excitation voltage,
cheap minesota twins jerseys, and common-mode voltage rejection (CMR) capability to accurately measure the signals from the sensors, and eliminate all common-mode voltage. Amplifier A1 is very low by the high precision and high precision reference voltage drift driver, the output is very accurate and stable, in the circuit to act as the bridge excitation source
because common-mode voltage is approximately half the excitation voltage, measurement equal to the bridge leg voltage difference between the small. Thus, to make an accurate reflection of the output voltage difference, the amplifier A2, A3 and A4 must have a high common mode rejection ratio (CMRR). In addition, to ensure accurate readings from the sensor, the input offset voltage of these devices (VOS), offset voltage drift (also known as the offset voltage temperature coefficient, TCVOS) and input bias current must be very small . To achieve these goals, where the amplifier A1 to A4 connected as instrumentation amplifier. In this configuration, the voltage gain (AV) is equal to (1 + 2R2/bR2) (aR1/R1),
nba jerseys, where, a and b for the scale factor. .
Figure 3
3. by radiation spectrum techniques can be measured particles emitted radiation, X ray or gamma ray radiation energy distribution. The impact of radiation cross-ray crystallography (scintillation crystal) stimulate ray energy density is proportional to the short-time pulses through the PIN photodiode to the light pulse can be converted into current. Figure 4 is both a PIN photodiode amplifier preamplifier output is current - voltage converter. Pulse amplitude signal contains information of interest to us, therefore, requires that the amplifier has a low input offset voltage and offset voltage drift. Larger bandwidth can speed up the pulse (perhaps only a few nanoseconds) the response speed. The preamplifier output (VOUT) trying to connect to the pulse height analyzer (such as high speed ADC), measured the size of each peak and record the number of occurrences, the resulting distribution is the specific radiation energy spectrum. Feedback resistor value of R1 depends on the PIN photodiode and the issue of the maximum current output to the ADC's maximum voltage, that is, R1 = (Max VOUT) / (Max ISIGNAL). Capacitor C1 PIN photodiode to compensate for the parasitic capacitance. R2 and C1 and C2 are equal to R1, used to compensate the amplifier inverting input of the input bias current.
Figure 4
4. thermocouples generate a voltage signal reflecting the two different temperature difference between the metal lines. Thermocouple temperature sensor includes a sensor node (metal A and metal B direct connection) and a reference junction (the metal A and metal B with copper conductors). Thermocouple signals in access, while the need to control or measure the cold reference junction temperature. Thermocouple signal is small, from about 10μV/oC to about 80μV/oC, the signal source impedance small. Figure 5, the circuit structure of a single amplifier to amplify the differential input to the ADC input and the match level. The differential amplifier gain AV = xR / R, x for the resistance ratio to determine the amplifier's inverting and inverting input maintained between the gain. This configuration helps to inhibit the differential thermocouple wire seizure common mode signal. The circuit requires amplifiers with low offset voltage and low offset drift.
Figure 5
D. Final stage: ADC signal conditioning system
ultimate goal is to make the analog sensor data as quickly as possible, complete and turned into digital form at a lower cost, the task to be completed by the ADC. Many parameters influence the choice of ADC types, including resolution (bits), speed (data throughput), in the form of the input signal (AC or DC), precision (DC and AC), reaction time (sampling period began with the first time between the effective digital output) and power level. Output (ie, with a microcontroller or digital signal processor interface) of the important parameters including the transmission mode (serial or parallel), the processor receiving the input signal level range, the availability of power supply voltage and power consumption. Table 2
how to use the number of signal conditioning applications successive approximation register (SAR) or the integral ADC. Two types of DC signal can be a good deal, but the structure of the SAR converter can better cope with rapid alternating signal (see Table 2). In a variety of ADC, SAR converters the most adaptable, not only can provide high-resolution (up to 16-bit), can also provide high throughput. Integrating ADC conversion method used for the conversion takes a long time, but with a signal averaging effect, reducing the noise in this ADC has a distinct advantage. Δ-Σ converters of alternating medium-frequency signal is the best, very high resolution and conversion accuracy. This converter has the advantage of high resolution (up to 24 bits), the disadvantage is the low conversion rate (waiting time is very long). There are also channels of ADC and sub-district type (subranging) ADC to choose from, these two high-speed ADC devices are very suitable for converting high frequency AC signal.