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. instrumentation.analog.com Simplifying Design for Your Competitive Edge 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. instrumentation.analog.com | 3 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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