CN0363 is a dual-channel colorimeter featuring a modulated light source transmitter, programmable gain transimpedance amplifiers on each channel, and a very low noise, 24-bit Σ-Δ analog-to-digital converter (ADC). The output of the ADC connects to a standard FPGA mezzanine card. The FPGA takes the sampled data from the ADC and implements a synchronous detection algorithm. By using modulated light and digital synchronous detection rather than a constant (dc) source, the system strongly rejects any noise sources at frequencies other than the modulation frequency, providing excellent accuracy. The dual-channel circuit measures the ratio of light absorbed by the liquids in the sample and reference containers at three different wavelengths. This measurement forms the basis of many chemical analysis and environmental monitoring instruments used to measure concentrations and characterize materials through absorption spectroscopy.
A clock set to a user-programmable frequency modulates one of the three LED colors (R,G,B) with a constant current driver built around the AD8615 op amp, the ADG819 switch, and the AD5201 digital potentiometer. The beam splitter sends half the light through the sample container and half through the reference container. The ADA4528-1, configured as a transimpedance amplifier, then converts the photodiode current into an output voltage square wave, whose amplitude is proportional to the light transmitted through the sample or reference containers. The transimpedance amplifiers use the ADG633 single-pole, double-throw (SPDT) switches to select one of two transimpedance gains. The AD7175-2 Σ-Δ ADC samples the voltage and sends the digital data to an FPGA for digital demodulation.
For a thorough circuit description, please visit CN0363 Circuit Note.