Lecture 5 Analogue Electronics Third Science

1
Lecture 5Analogue ElectronicsThird Science

• A Case Study
• Measuring and Displaying temperature

2
Measuring Temperature

• There are dozens of ways of measuring temperature
  electronically.
• For this example we will make use of a transducer
  called a Platinum resistance thermometer.
• This consists of a spiral of fine platinum wire
  whose resistance changes with temperature.

3
Platinum Resistance PT100

• This device exhibits a resistance of 100? at 0C
  and the resistance changes linearly at a rate of
  0.1? per C

• This resistance can be measured by a constant
  current technique or by a Wheatstone Bridge
  arrangement.

4
The Constant Current Arrangement.

• An op-amp is used to supply a constant currant
  through the PT100. If this current is known then
  the resistance can be calculated from the voltage
  measured across the device.

5
How it Works

• The Current passing through the sense resistor
  generates a voltage which is fed back to the op
  amp.
• The action of the op amp is to always try to
  minimise the voltage difference between its input
  terminals.
• The op amp will act to drive enough current
  through the circuit to keep the sense voltage the
  same as the voltage set by the Set Current
  resistor.

6
The Next Stage

• Given Iconst 10mA.
• Rsense 100?
• ?Vsense 1V.
• Vout 1V Rplat 10mA
• 2.02V at 20C
• i.e. We have a 0.02V signal on a 2V background.

7
The Final Stage

• The final stage involves a simple amplification
  to magnify the 0.02V to 2V for instance. Ie a
  gain of 100.
• This signal could then be displayed on a digital
  voltmeter and read directly in degrees C.
• The by 100 amplification could be easily included
  in the diff amp stage.

8
Review

• This exercise has demonstrated the use of an op
  amp to control a current.
• It has introduced the idea of offset and scaling
  of a signal.
• We have also seen a diff amp being used as a
  subtractor!
• These are all standard operations, particularly
  when preparing signals for use with computers.

9
Virtual Earth
10
The Summer
11
Servo Systems

• If we arrange for the output of an op-amp
  (through a suitable amplifier) to control a motor
  and include a means for the op amp to know the
  motor position we will have a servo system.
• Applying a voltage to the input of the
  amplifier will produce a out which will drive
  the motor until an equal voltage returns from the
  pot, at which point the motor stops.

12
Servo System Outline