The design features a modulated light source and digital synchronous detection for precision in chemical analysis and environmental monitoring.
A dual-channel colorimeter is an essential tool in both scientific research and industrial applications, prized for its enhanced measurement accuracy and reliability. This instrument operates by utilizing two separate channels—one to analyze the sample and the other to measure a reference—allowing simultaneous comparisons that help correct for variability in light intensity and sensor responses. Such capabilities are crucial for precise quantification in chemical concentrations and biomedical assays. The CN0363 is a dual-channel colorimeter reference design from Analog Devices (ADI) equipped with a modulated light source transmitter, programmable gain transimpedance amplifiers on each channel, and an ultra-low noise, 24-bit analog-to-digital converter (ADC).
The ADC’s output links to a standard FPGA mezzanine card, which processes the data using a synchronous detection algorithm. This design, utilizing modulated light and digital synchronous detection instead of a steady source, effectively eliminates noise at frequencies different from the modulation frequency, enhancing accuracy. The device measures the ratio of light absorption in sample and reference containers at three distinct wavelengths, a crucial function in various chemical analysis and environmental monitoring tools that utilize absorption spectroscopy to determine material concentrations and characteristics.
A clock, set to a user-defined frequency, modulates one of the three LED colors using a constant current driver that incorporates the AD8615 op-amp, the ADG819 switch, and the AD5201 digital potentiometer. A beam splitter divides the emitted light, directing half through the sample container and the other half through the reference container. The ADA4528-1, functioning as a transimpedance amplifier, converts the current from the photodiode into a voltage square wave, the amplitude of which corresponds to the amount of light transmitted through the sample or reference. This setup includes ADG633 single-pole, double-throw (SPDT) switches in the transimpedance amplifiers, which allow for the selection between two different gains. The output voltage is then sampled by the ADC, which relays the digital data to an FPGA that performs digital demodulation.
The dual-channel setup calculates the ratio of light absorbed by the liquids in both the sample and reference containers across three different wavelengths. This key measurement underpins a wide range of chemical analysis and environmental monitoring tools that assess concentrations and characterize materials via absorption spectroscopy.
Instead of using synchronous detection hardware as outlined in CN-0312, this circuit captures time-sampled data and employs an FPGA for digital synchronous detection. The FPGA also generates the AC excitation signal that powers the LEDs, and a numerically produced sine wave synchronizes with this signal via a digital phase-locked loop. The input signal is then multiplied by this digital sine wave and a version shifted by 90 degrees, yielding two low-frequency signals that demodulate in-phase and quadrature components of the input signal at the modulation frequency. The magnitude calculation involves taking the square root of the sum of the squares of these two components.
ADI has tested this reference design. It comes with a bill of materials (BOM), schematics, assembly drawing, printed circuit board (PCB) layout, and more. The company’s website has additional data about the reference design. To read more about this reference design, click here.
