Part Details for AD7798 by Analog Devices Inc

The AD8618 quad op amp forms three simple current sources to drive the LEDs with a constant current. The EVAL-SDP-CB1Z generates a 5 kHz clock that modulates one LED by using the ADG633 single pole, double throw (SPDT) switch to turn its current source’s reference voltage on and off. Setting the current sources for the other two LEDs to 0 V keeps them off while not in use.

This circuit uses three photodiodes that are sensitive to different wavelengths (red, green, and blue), to measure light intensity levels over the light spectrum where plants are photosynthetically active. The measured results can be used to optimize the light source to match the requirements of the specific plants, enhance the growth rate, and minimize energy losses.<p>This circuit uses three precision current to voltage conversion stages that drive a single-supply, low power, low noise, 16-bit, Σ-Δ analog-to-digital converter (ADC) with three differential inputs.<p>The circuit deviates from the traditional approach by eliminating all mechanical and optical components, and uses only electrical components to achieve the same goal.<p>The circuit consumes less than 10 mW typical, making it ideal for battery operated portable field applications.<p>The printed circuit board (PCB) is designed in an Arduino- compatible shield form factor and interfaces to the EVAL- ADICUP360 Arduino-compatible platform board for rapid prototyping.

Figure 2 shows a simplified schematic of an electrochemical sensor measurement circuit. Electrochemical sensors work by allowing gas to diffuse into the sensor through a membrane and interacting with the working electrode (WE). The sensor reference electrode (RE) provides feedback to maintain a constant potential with the WE terminal by varying the voltage at the counter electrode (CE). The direction of the current at the WE terminal depends on whether the reaction occurring is oxidation or reduction. In the case of carbon monoxide, oxidation takes place; therefore, the current flows into the working electrode, which requires the counter electrode to be at a negative voltage (typically 300 mV to 400 mV) with respect to the working electrode. The op amp driving the CE terminal should have an output voltage range of approximately ±1 V with respect to VREF to provide sufficient headroom for operation with different types of sensors (Alphasense Application Note AAN-105-03, Designing a Potentiostatic Circuit, Alphasense, Ltd.).

This circuit is a portable gas detector, using a 4-electrode electrochemical sensor, for simultaneous detection of two distinct gases. The potentiostatic circuit uses an optimum combination of components designed to provide single-supply, low power, and low noise performance, while offering a high degree of programmability to accommodate a variety of sensors for different types of gases.