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
Analog Electronics
Transducer Analog Signal Processing
Digital Electronics
Analog to Digital Conversion Board
Memory Signal Display
Long-term Storage (Disk)Digital Signal
Processing
Exciter sample
Digital to Analog (control)
Covering today
Covered last time
ElectronicsTransducers
Definition:• A transducer is
a device that converts a physical (or chemical) property into an electrical signal
Classifications:• By output measure
(V, I, R, frequency)• By phenomenon
measured (charged particle flux, temperature, light intensity, surface modification)
• Internally vs. Externally Amplified
TransducersCharge Particle Detectors
• Measurement of electrons, molecular ions and charged aerosol particles
• Most common type for GC and MS detectors
Charge Collector or Faraday Cup
Ie-
Can detect currents > 10-15 A
TransducersCharged Particle Detectors
Detection Process1. Charged particle hits
cathode2. Electrons emitted from
collision3. Amplificaion occurs with
each stage4. Current (electron flux)
increases before anode
CathodeDynodes
M-
e-e-
I
Electron Multiplier (MS detector)
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
TransducersMeasurement 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)
TransducersDetection 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
I
hνe-
TransducersDetection 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)
+
-
n
p
Reversed-bias photodiode:High impedance until photons arrive
e-
e-
e-
1D Photodiode Array
Some Questions on Transducers
1. List a transducer with a (primary) current signal.2. List a transducer with a (primary) resistance
signal.3. List a transducer that can be used to measure
charged particles.4. What is the main reason that a photomultiplier
tube is more sensitive than a photocell?5. 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)
– Has inverting and 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
-15 V
+
-
inverting input output
Operational Amplifiers• “Ideal” Op Amp
– V+ = V- (infinite gain)– I+ = I- = 0 (infinite
input resistance)• Useful Circuits
– All use feedback circuits
– Example: voltage follower (current amplifier)
– V(output) = V(electrode)
+
-
output
feedback circuit
+
-
electrode with Velectrode
Operational Amplifiers• “Ideal” Op Amp
– V+ = V- (infinite gain)– I+ = I- = 0 (infinite
input resistance)• Useful Circuits
– All use feedback circuits
– Example: voltage follower (current amplifier)
– V(output) = -V(electrode)
+
-
output
feedback circuit
+
-
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
+
-
transducer with current I
Rf
NoiseIntroduction
• 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
NoiseDefinitions
Noise1) “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
40.00
41.00
42.00
43.00
44.00
45.00
0 0.2 0.4 0.6 0.8 1 1.2
Time (min.)
Vol
tage
(mV
)
Peak to peak
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