Dual-Channel Colorimeter with Programmable Gain Transimpedance Amplifiers and Synchronous Detectors (CN0312)
The circuit shown in Figure 1 is a dual-channel colorimeter that features a modulated light source transmitter and a synchronous detector receiver. The circuit measures the ratio of light absorbed by the sample and reference containers at three different wavelengths.
The circuit provides an efficient solution for many chemical analysis and environmental monitoring instruments used to measure concentrations and characterize materials through absorption spectroscopy.
The photodiode receiver conditioning path includes a programmable gain transimpedance amplifier for converting the diode current into a voltage and for allowing analysis of different liquids having wide variations in light absorption. The 16-bit sigma delta (Σ-Δ) analog-to-digital converter (ADC) provides additional dynamic range and ensures sufficient resolution for a wide range of photodiode output currents.
Using the modulated source and synchronous detector rather than a constant (dc) source, eliminates measurement errors due to ambient light and low frequency noise and provides higher accuracy.
Figure 1. Dual-Channel Colorimeter with Programmable Gain Transimpedance Amplifiers and Lock-In Amplifiers (Simplified Schematic: All Connections and Decoupling Not Shown)
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