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