Transcript 2/11 Lecture Notes

Chem. 133 – 2/11 Lecture
Announcements
• Lab today
– Will cover 4 (of 8) set 2 labs (remainder covered on Tuesday)
– Period 1 will extend one day
• Website/Homework
– Have posted text data for problem 1.2.1
– Also posted solutions for homework problems not collected
– Problem 17.3 doesn’t give enough information to solve, but if
you look at Example 17.2 (and assume a range of 0 to 1 for
transmittance), it is not too hard
• Today’s Lecture
– Transducers (pretty brief)
– Amplifiers (covering qualitatively)
– Noise (if time)
Electronics - Overview
• Generic Instrument Block Diagram
Covered
last time
Analog Electronics
Exciter
Digital
Electronics
sample
Digital to
Analog
(control)
Transducer
Analog
Signal
Processing
Analog to Digital
Conversion Board
Digital Signal
Processing
Memory
Long-term
Storage
(Disk)
Signal
Display
Covering
today
Electronics
Transducers
Definition:
Classifications:
•
•
A transducer is a
device that converts a
physical (or chemical)
property into an
electrical signal
•
•
By output measure (V,
I, R, frequency)
By phenomenon
measured (charged
particle flux,
temperature, light
intensity, surface
modification)
Internally vs. Externally
Amplified
Transducers
Charge Particle Detectors
• Measurement of
electrons, molecular
ions and charged
aerosol particles
• Most common type
for GC and MS
edetectors
Charge Collector or
Faraday Cup
Can detect currents > 10-15 A
I
Transducers
Charged Particle Detectors
Detection Process
Electron Multiplier (MS detector)
1.
2.
M-
e- e
3.
I
Cathode
Dynodes
4.
Charged particle hits
cathode
Electrons emitted from
collision
Amplificaion occurs with
each stage
Current (electron flux)
increases before anode
Example: if each stage produces 6
useful electrons out per ion in,
amplification in current would be x63
or x216. With greater amplification,
single particle detection is possible
Transducers
Measurement of Temperature
• Applications:
– Temperature control (e.g. GC ovens)
– Infrared light (IR spectrometers)
• Resistance based
– Thermistors and platinum resistance
thermometers (both have R = f(T))
• Voltage based
– Thermocouples (voltage generated by metal
junction which depends on T)
Transducers
Detection of Light
1. Vacuum tube types
-
-
-
Based on photoelectron effect
Current based detectors
Photocells (see diagram)
All have minimum energy
(maximum wavelength) where
electron ejection just occurs
Photomultiplier tube
(combination of photocell and
electron multiplier)
Photomultiplier tube allows
detection of single photons
e-
I
Transducers
Detection of Light
2. Solid state types
- Typically less expensive than vacuum tube
types
- Tend to operate better at longer wavelengths
- Based on promotion of electrons to conducting
bands
- Photodiodes (I proportional to intensity)
- Photoconductivity cells (R dependent on
intensity)
- Photovoltaic cells (V dependent on intensity)
- Advanced devices (discussed in spectroscopy
section)
- Arrays (1D or 2D sets of detectors)
1D Photodiode
Array
+
e- e
n
e-
p
-
Reversed-bias
photodiode:
High impedance
until photons
arrive
Some Questions on Transducers
1.
2.
3.
4.
5.
List a transducer with a (primary) current signal.
List a transducer with a (primary) resistance signal.
List a transducer that can be used to measure charged
particles.
What is the main reason that a photomultiplier tube is
more sensitive than a photocell?
Give an example of a transducer that is readily
available in an array form.
Operational Amplifiers
• General Use: Analog Signal Processing
• Common Uses
– voltage amplification
– current amplification (removal of effect of
internal resistance)
– current to voltage conversion
– differential amplifier to remove common noise
• This time – only covering qualitatively (no
calculations problems)
Operational Amplifiers
• Function
– Requires power (+15 V/
-15 V)
inverting
– Has inverting and
input
noninverting inputs
– Output voltage is equal to
(gain)x(V+ – V-) (“real” op
amp)
– Main thing to know about
real op amp is you can not
connect the two input wires
+15 V
output
+
-15 V
Operational Amplifiers
• “Ideal” Op Amp
– V+ = V- (infinite gain)
– I+ = I- = 0 (infinite input
resistance)
feedback circuit
output
+
• Useful Circuits
– All use feedback circuits
– Example: voltage follower
(current amplifier)
– V(output) = V(electrode)
+
electrode
with
Velectrode
Operational Amplifiers
• “Ideal” Op Amp
– V+ = V- (infinite gain)
– I+ = I- = 0 (infinite input
resistance)
feedback circuit
output
+
• Useful Circuits
– All use feedback circuits
– Example: voltage follower
(current amplifier)
– V(output) = -V(electrode)
+
electrode
with
Velectrode
Operational Amplifiers
• Other Useful Circuits
– Inverting amplifier
• in text
• Vout = -RfVin/Rin
• useful for amplifying voltage
signals
– Differential amplifier
• in text
• Vout = (Rf/Rin)(V1 - V2)
• allows removal of noise
common to V1/V2
– Current to voltage convertor
• Calculate Vout
Rf
transducer with
current I
+
Noise
Introduction
• Why worry about noise?
– Both noise and signal affect sensitivity (the ability to
detect low concentrations
– While it is easy to increase the signal, noise often will
also increase (e.g. inverting op amp amplifier circuit)
– It is possible to reduce noise without also reducing
the signal (e.g. differential op amp amplifier circuit or
transducers with internal amplification)
– If we know the source of the noise we can make
improvements more easily
Noise
Definitions
Noise
1)
“variability in a measurement due to (random) errors” (textual)
2) the standard deviation in the values (σ) (mathematical) or the root
mean square value (more common in electronics – based on assumption
of sine wave form of noise)
3) peak to peak noise (graphical and roughly 6σ)
Peak to Peak Noise
Voltage (mV)
45.00
44.00
43.00
42.00
Peak to peak
41.00
40.00
0
0.2
0.4
0.6
Time (min.)
0.8
1
1.2