1. Suppose a temperature sensor has a linear transfer function as v A+ B\'t, whe

Question

1. Suppose a temperature sensor has a linear transfer function as v A+ B't, where t is the temperature at °C and v is the output voltage. Its output voltage is 250 mv at 100 °C and 450 mV at O °C. (a) Find its sensitivity and the transfer function. (b) Suppose the application requires the sensor to measure temperatures ranging from 10 C to +50 °C. Find the output range of the sensor (c) Design an analog interface between the temperature sensor and the ADC (Analog-to- Digital Convertor). The analog input range for this 12-bit ADC is 0 to 5 V. The sensor output signal should completely fill the ADC input span. Find the transfer function of the interface circuit.

Explanation / Answer

1)

v = A + B * t

Given

250mV at 100oC & 450 mV at 0oC

250 = A + B * 100 ........(1)

450 = A + B * 0 .........(2)

From (2) we have

A = 450mV

Putting A =450 in (1)

250 = 450 + B * 100

B = (250-450)/100 = -2mV/oC

Thus, the sensitivity of the linear transducer is -2mV/oC.

Transfer function is

( v - 450 mV)/-2mV = T

b) ouput range of sensor.

First

for 10oC, v = 450 + (-2 x 10)

= 430 mV

for 50oC,

v = 450 + (-2 x 50)

= 350mV

The output range of sensor is 350mV to 430 mV.

c)

The sensor voltage ranges from 250 mV to 450 mV.

The ADV voltage ranges from 0 to 5V.

So, transfer function at interface = ( Range of ADC)/(Range of sensor)

= ( 5 -0) /( 450 mV -250 mV)

= 5/ 200 mV

= 0.025 V/mV

V = A + B*t

0 = A + 0.025 x 250

A = -6.25V

Hence the transfer function at interface is

V = - 6.25 + (0.025 * B)

where B is mV (sensor voltage)

V is ADC voltage

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