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# Signal Conditioning and Digital Output to a Computer

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A calibrated RTD with Ã?± =0.0041/Ã??C, R =306.5ââ??¦ at 20Ã??C, and P_D(dissapation constant) = 30mW/Ã??C will be used to measure a critical reaction temperature. Temp must be measured between 50Ã??C and 100Ã??C with a resolution of at least 0.1Ã??C. Devise a signal conditioning system that will provide an appropriate digital output to a computer. Specify the requirements on the ADC and appropriate analog conditioning to interface your ADC.

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## A photocell with a 35-ms time constant is used to measure light flashes.

1. A photocell with a 35-ms time constant is used to measure light flashes. How long after a sudden dark-to-light flash before the cell output is 80% of the final level?

2. A manufacturer specification sheet lists the transfer function of a pressure sensor as 45±5% mV/kPa with a time constant of 4±10%. A highly accurate test system applies a step change of pressure from 20 kPa to 100 kPa.

a. What is the range of the sensor voltage outputs initially and finally?
b. What range of voltages would be expected to be measured 2 sec after the step change is applied?

3. A pressure-measurement system uses a sensor that converts pressure into voltage according to the transfer functions, Vp = .5&#8730;p . This voltage is then converted into a current. As pressure varies from 0 to 100 psi, the current varies from 4 to 20 mA.

a. Find the transfer function equation for the conversion of voltage to current.
b. What pressure change, &#916;p, will cause the current to change by 1 mA from 19 mA to 20 mA.
c. What pressure change, &#916;p, will cause the current to change by 1 mA from 4 mA to 5 mA? Why is the pressure change not the same as in the previous case, even though the current changed by 1 mA in both cases?
d. Is the relationship of current versus pressure linear or nonlinear?

4. Develop a voltage-to-current converter that satisfies the requirement I = 0.0021Vm. If the op amp saturation voltage is ±12 V and the maximum current delivery is 5 mA, find the maximum load resistance.

5. A sensor varies from 1 to 5 k&#937;. Use this in an op amp circuit to provide a voltage varying from 0 to 5 V as the resistance changes.

6. A system is needed to measure flow, which continuously cycles between 20 and 30 gal/min with a period of 30 sec. The required output is a voltage varying from -2.5 to +2.5 V for the cycling flow range. The sensor to be used has a transfer function of &#8730; Q volts, where Q is in gal/min, and an output impedance of 2.0 k&#937;. Tests show that the output of the sensor has 60 Hz noise of 0.8 V rms. Design a signal-conditioning system, including noise filtering, and evaluate your design as follows.

a. Plot output voltage versus flow, and comment on the linearity.
b. Determine the noise on the output as percent FS.

7. A process involves moving speed, load weight, and rate of loading in a conveyor system. The variables are provided as high (1) and low (0) levels for digital control. An alarm should be initiated whenever any of the following occur: