Circuit Note CN-0312 Devices Connected/Referenced 3-Channel, Low Noise, Low Power, 16 Bit, AD7798 - ADC with On-Chip In-Amp AD8615/ Precision Single/Quad Rail-to-Rail AD8618 Input/Output Operational Amplifier Circuits from the Lab reference circuits are engineered and Ultralow Noise, High Accuracy 2.5 V tested for quick and easy system integration to help solve todays ADR4525 Voltage Reference analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0312. Programmable Gain Precision Difference AD8271 Amplifier ADG633 CMOS, 5 V/+5 V/+3 V, Triple SPDT Switch CMOS, 2.5 Low Voltage, Triple SPDT ADG733 Switches Dual-Channel Colorimeter with Programmable Gain Transimpedance Amplifiers and Synchronous Detectors The circuit provides an efficient solution for many chemical EVALUATION AND DESIGN SUPPORT analysis and environmental monitoring instruments used to Circuit Evaluation Boards measure concentrations and characterize materials through CN-0312 Circuit Evaluation Board (EVAL-CN0312-SDPZ) absorption spectroscopy. System Demonstration Platform (EVAL-SDP-CB1Z) Design and Integration Files The photodiode receiver conditioning path includes a Schematics, Layout Files, Bill of Materials programmable gain transimpedance amplifier for converting the diode current into a voltage and for allowing analysis of CIRCUIT FUNCTION AND BENEFITS different liquids having wide variations in light absorption. The circuit shown in Figure 1 is a dual-channel colorimeter that The 16-bit sigma delta (-) analog-to-digital converter (ADC) features a modulated light source transmitter and a synchronous provides additional dynamic range and ensures sufficient detector receiver. The circuit measures the ratio of light absorbed resolution for a wide range of photodiode output currents. by the sample and reference containers at three different Using the modulated source and synchronous detector rather than wavelengths. a constant (dc) source, eliminates measurement errors due to ambient light and low frequency noise and provides higher accuracy. Rev. 0 Circuits from the Lab circuits from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices Tel: 781.329.4700 www.analog.com be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) Fax: 781.461.3113 2013 Analog Devices, Inc. All rights reserved. CN-0312 Circuit Note 15pF GUARD RING 33k LED CLK 4.7pF VREF +5V +5V 1M +5V ADG733 ADG633 ADG633 AD8271 +5V 0.22F AD8615 ADG733 +V S +3.3V AD7798/ AD8615 10k VREF VREF VREF AD7799 VREF 0.1F 0.1F 10k LED CLK AIN1+ REFERENCE V AIN1 S 1F VREF DOUT/RDY AIN2+ CHAN0 G0/G1 DIN AIN2 SCLK VREF AIN3+ CS BEAM SPLITTER AIN3 SAME AS CHANNEL 1 10k 10k SAMPLE VREF REFIN(+) REFIN() 10k +5V ADG633 BLUE 0.1F +5V AD8618 0.83V 33 ADR4525 VREF VOUT VIN 10k 10k 2.2F 2.2F LED CLK BLUE GND 42. 2 GREEN CHANNEL RED CHANNEL Figure 1. Dual-Channel Colorimeter with Programmable Gain Transimpedance Amplifiers and Lock-In Amplifiers (Simplified Schematic: All Connections and Decoupling Not Shown) with a gain dependent on the feedback resistor and the photodiode CIRCUIT DESCRIPTION shunt resistance. In addition, any op amp input voltage offset The AD8618 quad op amp forms three simple current sources to appears across the photodiode, causing the photodiode dark drive the LEDs with a constant current. The EVAL-SDP-CB1Z current to increase. generates a 5 kHz clock that modulates one LED by using the Figure 2 shows a typical transimpedance amplifier with a single ADG633 single pole, double throw (SPDT) switch to turn its feedback resistor and its ideal transfer function. current sources reference voltage on and off. Setting the current I sources for the other two LEDs to 0 V keeps them off while not D R F in use. I D I B The beam splitter sends half the light through the sample container and half the light through the reference container. The containers V = I R O D F V = 0V D absorb different amounts of light depending on the type and concentration of material in each container. The photodiode Figure 2. Transimpedance Amplifier Transfer Function on the opposite side of each container generates a small current proportional to the amount of light received. Because some solutions under test may have very strong absorption characteristics, it is sometimes necessary to use large feedback The first stage of each channel consists of an AD8615 op amp resistors to measure the very small currents generated by the configured as a transimpedance amplifier to convert the photo- photodiode, while at the same time being able to measure the large diode output current to a voltage. The AD8615 makes a good currents corresponding to the highly diluted solutions. To address choice for a photodiode amplifier because of its very low input this challenge, the photodiode amplifiers in Figure 1 include two bias current (1 pA), input offset voltage (100 V), and noise different selectable gains. One gain is set at 33 k , while the other is (8 nV/Hz). Although the signal is later ac-coupled, it is still set at 1 M. When a single SPDT switch is connected to the output important to minimize dc errors in this stage to avoid losing of the op amp to switch the feedback resistors, it can result in a dynamic range. The op amp input bias current multiplied by transimpedance gain error due to the on resistance of the ADG633. the feedback resistor value appears at the output as an offset voltage. The op amp input offset voltage appears at the output Rev. 0 Page 2 of 7 11406-002 11406-001