Statement: Consider the amplifier between stations B and C of the temperature me

Question

Statement: Consider the amplifier between stations B and C of the temperature measurement system shown in Figure 5.1. (a) Determine the minimum input impedance of the amplifier (in ) required to keep the amplifier's voltage mea- surement loading error, ev, less than 1 mV for the case when the bridge's output impedance equals 30 and its output voltage equals 0.2 V. (b) Based upon the answer in part (a), if an operational amplifier were used, would it satisfy the requirement of ev 107 . Here we need Z > 5970 , thus any typical op amp will have sufficient input impedance (c) G- 48.6

Explanation / Answer

a) Bridge output is Vo = 0.2 V and output impedance is Ro = 30 Ohm

Let Zm be the input mpedance of amplifier .Zm be should be such that loading error should be ev should be less than 1 mV.That is voltage across Ro is less than 1 mV and rest should be across Zm.

Let Voltage across Ro be Vro = 0.99 mV and Vzm ne voltage across amplifier

0.2 V = Vro + Vzm

200 mV = 0.99 + Vzm

Vzm = 199.01 mV

Now current through Ro & Vzm is given by

I = 0.99 mV / 30 Ohm = 0.033 mA

Zm = Vzm / I = 199.01 mV / 0.033 mA = 6030 Ohm                 ............1

This is the required minimum input impedance of amplifier.

b)OpAmps have input impedance in the range of 10 Mega Ohm range.

From result in a) we require inut impedance greater than 6030 Ohm.

Thus any operational amplifier will satisfy our input impedance requirement.

c)Now PT 100 RTD sensor has resistance of 108.657 Ohm at 72 deg.F

Now Bridge output voltage Vb is

Vb = [ {108.657 / (108.657 +25 )} - {25 / (25 +25 )} ]*5 V

Vb = [0.8129 - 0.5 ]*5 V

Vb = 1.5645

Hence gain of amplifier required G = 9 V /1.5645 = 5.75

G = 5.75

This is the required gain of amplifier

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