Part 5 of this series will focus on "real" applications, and the skills and experience we've learned so far will be applied to help us easily stabilize a complex circuit. We will design a general-purpose single-supply buffer amplifier (which buffers 2.1V to 4.1V reference), and a single 5V supply allows it to operate linearly, providing a large output current ("13mA") at -40°C The drift of the +125°C operating temperature range is 0.4V. Although this circuit can be used in many applications, we will briefly explain the reasons for this design and explain why there are no ready-made circuits to do this. We use integrated technology here to develop a device network to provide a stable circuit that proves beneficial for many op amp applications.
technical background:
In practice, a common application of Wheatstone bridges is pressure measurement. As shown in Figure 5.1, many of these pressure sensors have significant second-order nonlinearities as the applied pressure changes.
Figure 5.1 Typical actual sensor output ratio plus pressure
Figure word (upper and lower): the relationship between bridge output and pressure at room temperature, ideal sensor, actual sensor;
Axis word: X axis: pressure, Y axis: bridge output when Vexc = 1V (V/V or Vbridge)
In addition to the non-linearity associated with the applied pressure changes, many pressure sensors have nonlinear characteristics in terms of offset and range as a function of temperature. Used for school
A modern solution to these errors is to have an electronic circuit built into the pressure sensor and then use the electronic circuit and the pressure sensor as a module.
Digital calibration is performed as the temperature changes. One IC suitable for this type of application is the Burr-Brown product PGA309 supplied by Texas Instruments (eg
Figure 5.2 shows). The output voltage has been digitally calibrated, and the signal conditioning IC includes an analog sensor linearization circuit.
A portion of the output voltage is fed back to the voltage excitation pin of the sensor to linearize the second order nonlinearity at a 20:1 modified ratio. therefore,
The VEXC pin will adjust its voltage as the applied pressure of the sensor changes. One limitation of this circuit is its sensor excitation pin VEXC, working
The temperature range is limited to a maximum output current of 5 mA. Here we have encountered a dilemma, how to use an impedance to stimulate the required current
More than 5mA sensor.
Figure 5.2: Modern Digital Calibration Sensor Signal Conditioner
Graphics words (left and right, up and down): nonlinear teletype sensor, linearization circuit, reference, analog sensor linearization circuit, linearized DAC, fault monitor, automatic zero PGIA, over/under scale limiter, analog signal Adjust circuit, external temperature, digital temperature compensation, internal temperature, temperature ADC, control register interface circuit, linear Vout, digital calculation.
Design requirements:
Figure 5.3 details the main design indicators. We want to supply power with a 5V power supply with a 10% tolerance. We need a uniform gain buffer because we don't want to introduce any errors into the PGA309 linearization loop. Since the PGA309 has a wide programmable range on the VEXC pin, we need to accommodate a voltage range from 2.1V to 4.1V. Our smallest sensor has a resistance of 300Ω. Therefore, for a maximum output voltage of 4.1V, we need to provide at least 13.6mA. The PGA309 linearization circuit has a bandwidth of approximately 35 kHz. Due to the way the loop is closed, our buffer bandwidth is at least equal to or greater than the bandwidth of the linearized loop. We set the target to a small signal closed-loop bandwidth of 100 kHz. For sensor applications we are interested in, a large signal response with a 1V/μs swing rate is sufficient. The design should be stable over a temperature range of -40 ° C to +125 ° C. Because we don't want to introduce any additional errors into the final application circuit due to the buffer, we need a circuit that does not have any cross-over distortion in the op amp's common-mode input range. We will briefly discuss this issue as it is a problem for almost all CMOS single-supply rail-to-rail input (RRI) op amps.
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