Single power source of the design of the buffer circuit

Part 5 of this series will focus on 'real' application, so far we have to learn the skills and experience will be used, help us easier to stabilize a complex circuit. We will design a general single power buffer amplifier ( 2. Buffer 1 v to 4. 1 v reference) 5 v single power supply linear to enable it to work, can provide larger output current ( 》13mA) And in - 40 ° C to + 125 ° C temperature range of drift is 0. 4V。 Although the circuit can be used in many applications, but we will briefly introduce to give the reason of this design, and explain why no ready-made circuit can be used to complete the work. We adopt integrated technology to develop the network device here, in order to provide a proof is good for many operational application the stability of the circuit.

technical background:

in practical application, a common application of wheatstone bridge is pressure measurement. As shown in figure 5. 1, with the added pressure change, a lot of the pressure sensor has obvious of second order nonlinear characteristics.

in figure 5. 1 typical sensor output than the actual stress figure word (

Up, down) : bridge output at room temperature and pressure relationship, ideal sensor, the actual sensor;

axes words: X: pressure, Y axis: Vexc bridge output (= 1 v Or Vbridge V/V)

in addition to changing with the added pressure caused by nonlinear, many pressure sensor with temperature changes in the offset and scope has nonlinear characteristics. Used to school

is the error of a modern solution is built-in electronic circuit in the pressure sensor, and the electronic circuit and pressure sensor as a module,

as the change of temperature for digital calibration. A kind of suitable for such purpose IC is Burr - provided by Texas instruments Brown PGA309 ( Such as

in figure 5. 2) 。 The output voltage is through digital calibration of the sensor and the signal adjustment consists of IC an analog sensor linear circuit, the circuit will be part of the output voltage feedback

to sensor voltage pin, which part to improve the proportion of second order nonlinear linearization. Therefore,

VEXC pins will be along with the change of sensor and a pressure to adjust the voltage. This is a limitation of the sensor circuit incentive pin VEXC, within the scope of work

as temperature limit in 5 ma on the maximum output current. Here we ran into a dilemma, namely how to incentive for current in an impedance sensor

more than 5 ma.

in figure 5. 2: modern digital calibration sensor signal conditioner

figure word ( The left and right sides, top and bottom) Telex: nonlinear type sensor, linear circuit, reference, analog sensor linear circuit, linear DAC, fault monitor, automatic zero PGIA, over/owed scale limiter, analog signal adjusting circuit, external temperature, digital temperature compensation, temperature, internal temperature ADC, control register interface circuit, and linear Vout, digital computing.

design requirements:

in figure 5. Three main design index is given in detail. We want to use a tolerance of 10% 5 v power supply to the power supply. We need a unity gain buffer, because we don't want to introduce any error in the PGA309 linearization loop. Because PGA309 on VEXC pin has a very wide range of programmable, so we need to accommodate from 2. 1 v to 4. 1 v voltage range. Our youngest sensor resistance of 300 Ω. Therefore, for maximum 4. 1 v output voltage, we need to provide at least 13. 6 ma of current. PGA309 linearizer is about 35 kHz bandwidth. As a result of the closed loop way, our buffer bandwidth must be at least equal to or greater than the linear loop bandwidth. We will aim at 100 KHZ small signal loop bandwidth. For the sensor application which we are interested in, the large signal response when we have 1 v/u s swing speed is enough. In from the design 40 ° C to + 125 ° C temperature range should be stable work. Because we don't want to in the final application due to the reason of the buffer circuit in introducing any extra error, so we need a in the op-amp input common-mode range will not have any crossover distortion of the circuit. We will briefly discuss the question, because it is almost all CMOS single input power rail to rail ( RRI) Op-amp is a problem.