Dual Beam Spectrophotometer Solution PDF

Dual Beam Spectrophotometer:
ADI Solutions Application Brief
Introduction
A spectrophotometer is an instrument that can measure intensity as a function of the light source wavelength. A spectrophotometer is commonly used
for the measurement of transmittance or reflectance of solutions and
transparent or opaque solids or gases. The use of spectrophotometers
spans various scientific fields, such as physics, materials science, chemistry,
biochemistry, and molecular biology. Spectrophotometers are widely used
in many industries including semiconductors, laser and optical manufacturing,
printing, and forensic examination, as well as in laboratories for the study of
chemical substances.
The most common spectrophotometers are used in the UV and visible
(UV/VIS) region of the spectrum, with some of these instruments operating
in the near-infrared region as well.
Here we introduce a general spectrophotometer with a simplified optical
system as an example.
System Design Considerations and Major Challenges
Stability, drift with time, and drift with temperature are very important factors
during spectrophotometer design. To achieve this objective, low drift and an
accurate signal chain are required.
In addition, high dynamic range is important in order to analyze a wide range
of compounds. A programmable gain transimpedance amplifier can provide
high dynamic range with lower noise than two stages of amplification. For
best performance, the system must strongly reject any external electrical and optical noise.
Theory of Operation
This simplified optical system generates modulated light which passes through both reference and sample
cells. The photodiodes generate a current proportional to the light energy hitting their active area, and the
transimpedance amplifiers convert this current to a voltage. The signal is then converted and demodulated
into a dc voltage for the precision Σ-∆ analog-to-digital converter (ADC), which can be independent or
integrated in the microcontroller. The demodulation step will strongly reject any noise or other signals that are
not in phase with the modulated signal from the LED. The ADC measures the amplitude of the reference and
sample channels. The ratio of the two amplitudes is related to the concentration of the sample solution.
The main advantage of a dual beam system is the ability to generate stable results even with changing
external conditions such as amplitude fluctuations in the light source and to compensate for different
sample holder materials. The modulated light method helps to eliminate the effect of ambient light and
other noise sources that are not in phase with the modulating clock.
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System Block Diagram
1. AMP
5. REF
7. SWITCH
2. DIFF
AMP
1. AMP
8. INTERFACE
4. MICROCONTROLLER
...
6. MUX
Below is the system block diagram of a general spectrophotometer including a simplified optical system, sample and reference cells, dual channel signal
conditioning circuit, microcontroller (ADC integrated), communication interface, and power management.
REFERENCE
6. MUX
...
3. ADC
1. AMP
7. SWITCH
RS-232
USB
PHY
GPIB
2. DIFF
AMP
1. AMP
SAMPLE
9. POWER MANAGEMENT
DC-TO-DC
LDO
Note: The signal chains above are representative of a system block diagram. The technical requirements of the blocks vary, but the products listed in the following table are representative of ADI’s solutions that meet some of those requirements.
1. Operational
Amplifier
2. Difference
Amplifier
3. Analog-to4. Analog
Digital Converter Microcontroller
AD8615,
AD7798, AD7799,
AD8271, AD8278
AD8605, AD8626
AD7190, AD7192
5. Reference
6. Multiplexer
7. Switch
8. Interface
ADR4525,
ADR3425,
ADR291
ADG704,
ADG708,
ADG1609
ADG733,
ADG1636
ADM3251E
ADUCM361,
ADuC7061
9. Power
Management
ADP2441,
ADP2370,
ADP160,
ADP7102,
ADP7182
Main Products
Part Number
Description
Benefit
AD8615
Precision 20 MHz CMOS single RRIO operational amplifier
Low bias current at room temperature, high speed, low noise,
low offset op amp
AD8605
Precision, low noise, CMOS, RRIO op amp (single)
Low bias current at room temperature, high speed, low noise,
low offset op amp
AD8626
Precision, low power, single supply, JFET amplifier in MSOP
Wider power supply range, low bias current @ 0°C to 50°C,
low offset drift
AD8271
Programmable gain precision difference amplifier
Low gain drift and high speed, suitable for the drive ADC
AD8278
Low power, wide supply range, low cost difference amplifiers,
G = ½, 2
Low power consumption, enough bandwidth
AD7798
3-channel, low noise, low power, 16-bit, Σ-∆ ADC
with on-chip in-amp
Low power consumption and high integrated Σ-∆ ADC,
high resolution and high accuracy
AD7799
3-channel, low noise, low power, 24-bit, Σ-∆ ADC
with on-chip in-amp
Low power consumption and high integrated Σ-∆ ADC,
high resolution and high accuracy
AD7191/AD7192
4.8 kHz ultralow noise, 24-bit Σ-∆ ADC with PGA
Low power consumption, low noise and high integrated Σ-∆ ADC,
high resolution and high accuracy
ADuCM361
Low power precision analog microcontroller, ARM Cortex-M3
with single Σ-∆ ADC
Low power consumption, high precision 24-bit Σ-∆ ADC,
4 mA to 20 mA loop applications, small package
ADuC7061
Low power precision analog microcontroller, dual Σ-∆ ADCs,
flash/EE, ARM7TDMI
Low power consumption, low cost, 24-bit Σ-∆ ADC,
4m A to 20 mA loop applications, small package
Operational Amplifier
Difference Amplifier
Analog-to-Digital Converter
Analog Microcontroller
| Dual Beam Spectrophotometer: ADI Solutions Application Brief
2
Main Products (Continued)
Part Number
Description
Benefit
ADR4525
Ultralow noise, high accuracy 2.5 V voltage reference
Low drift, very good stability and low noise reference, low hysteresis,
and many other choices for output voltage in the ADR45xx family
ADR3425
Micropower, high accuracy 2.5 V voltage reference
Low drift, good stability, and many other choices for output voltage
in the ADR34xx family
ADR291
Low noise micropower precision voltage reference (2.5 V)
Low power consumption, pretty good drift and stability
ADG704
CMOS, low voltage, 2.5 Ω, 4-channel multiplexer
Low leakage and low on resistance help to build a highly
accurate system
ADG708
CMOS, low voltage, single 8 to 1 multiplexer
Low leakage and low on resistance help to build a highly
accurate system
ADG1609
4.5 Ω RON, 4-channel, ±5 V,+12 V, +5 V, and +3.3 V multiplexer
Wider power supply range, low leakage, and low on resistance
help to build a highly accurate system
ADG733
CMOS, 2.5 Ω, low voltage, triple SPDT switch
Low leakage and low on resistance help to build a highly
accurate system
ADG1636
1 Ω typical on resistance, ±5 V, +12 V, +5 V, and +3.3 V
dual SPDT switches
Wider power supply range, low leakage, and low on resistance
help to build a highly accurate system
Isolated single-channel RS-232 line driver/receiver
High integrated isolated RS-232 transceiver
Reference
Multiplexer
Switch
Interface
ADM3251E
Power Management
ADP2441
36 V, 1 A, synchronous, step-down dc-to-dc regulator
Small 3 mm × 3 mm LFCSP package, high efficiency
ADP2370
High voltage, 1.2 MHz/600 kHz, 800 mA, low quiescent current
buck regulator
Small 3 mm × 3 mm LFCSP package, few peripheral components,
and small solution size
ADP160
Ultra low quiescent current, 150 mA, CMOS linear regulator
Low power consumption, integrated output discharge resistor,
small package with only two 1 μF external capacitor
ADP7102
20 V, 300 mA, low noise CMOS LDO
High input voltage, low noise LDO
ADP7182
–28 V, 200 mA, low noise linear regulator
High input voltage, low noise negative LDO
System Benefits
More Functionality/
Integrated Solutions
Reduce equipment size with parts that add more functionality per square inch such as the AD7190/AD7192 with
integrated PGA or the ADuCM361 analog microcontroller: ARM Cortex®-M3 with on-chip 24-bit Σ-∆ ADC.
Faster Time to Market
Speed time to market by reducing and simplifying development efforts with ADI’s tools such as Photodiode
Wizard, signal chains, reference designs, and more.
More Accurate and
Faster Solutions
Use multichannel, 24-bit Σ-∆ ADCs such as the AD7799 or the 4.8 kHz AD7192/AD7193/AD7194 family and
higher speed amplifiers like the AD8615.
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Superior Services and User Experience
Broad Product Portfolio
ADI provides the broadest product portfolio for analytical test and measurement instrumentation applications including:
• Sensor interfaces
• Pressure and vacuum measurement
• FET input amps—mostly precision—for photodetectors
• Low noise, low drift amplifiers and references
• Capacitive sensor measurement
• Strain gauge measurement
• Analog switches and multiplexers
• Temperature measurement
• Signal conditioning and processing
• Temperature sensors
• ADC drivers—typically lower bandwidth
• Digital temperature sensors
(less than 5 MHz)
• Multichannel temperature monitors
• 24-bit Σ-∆ ADCs, low bandwidth
• Current/voltage measurements and references
• Signal generation
• Current sense amplifiers—high and low side
• Precision DACs—voltage and current sources
• Low drift amps and voltage references
• Direct digital synthesis (DDS)—waveform generators
• Power management
Design Resources
Circuits from the Lab ® Reference Designs
Design Tools/Forums
• Dual-Channel Colorimeter with Programmable Gain
Transimpedance Amplifiers and Synchronous Detectors
(CN0312) —www.analog.com/CN0312
• Chemical Analysis Signal Chains—
instrumentation.analog.com/en/chemical-analysis/
segment/im.html
Application Notes/Technical Articles
• Analog Photodiode Wizard —
www.analog.com/en/content/photodiode_wizard/fca.html
• Programmable Gain Transimpedance Amplifiers
Maximize Dynamic Range in Spectroscopy Systems —
www.analog.com/library/analogdialogue/archives/
47-05/pgtia.pdf
• ADIsimPower™: ADI Voltage Regulator Design Tool —
designtools.analog.com/dtPowerWeb/dtPowerMain.aspx
• ADIsimOpAmp™: ADI Op Amp Design Tool —
www.analog.com/en/amplifier-linear-tools/topic.html
• ADuCM361 Design Tools—
ftp.analog.com/pub/MicroConverter
• EngineerZone®: Online Technical Support Community —
ez.analog.com
To view additional resources, tools, and product information, please visit instrumentation.analog.com.
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