Photovoltaic as Sensor Circuits and Digitally Controlled Potentiometers

Sensors are energy conversion devices. Sensors or transducers convert the physical world of light, pressure, temperature, flow, level, acceleration, force, etc. to electrical signals but generally they can’t do so without help. Excitation and/or signal conditioning electronics are almost always needed to interface the sensor and provide adjustable calibration, amplification, linearization, and level transformation functions. Digitally controlled potentiometers add variability and programmability to the sensor circuit and provide an automated alternative to manually adjusted mechanical trimmers. The results are accuracy, speed, reliability, packaging flexibility, and labor and cost savings.

Photovoltaic detectors are used to sense and measure light energy in industrial, medical, consumer, and scientific instrumentation applications. Solid state photovoltaics respond to wavelengths ranging from the far infra-red through the visible spectrum and into the ultra-violet and therefore tend to excel in precision photometric applications. The basic signal conditioning circuit for photovoltaics is the current-to-voltage or trans impedance amplifier of Figure 1. The chief shortcoming of this classic circuit is the inability of one value of feedback resistor to adequately accommodate the four, five, or more decades of dynamic range of current produced by many of the photo detectors. Even when RF is made adjustable, the finite resolution of the feedback resistance fails to fully resolve the dynamic range problem.

The Figure;

The circuit of Figure 2 combines two Intersil X9258T digitally controlled potentiometers with an AD822 low noise dual op amp to create a flexible, digitally calibrated, wide dynamic range trans impedance amplifier topology that can be used with virtually any photovoltaic detector technology. A second feature of the circuit in Figure 2 is the independence of gain of both DCP1 element and wiper resistances. Using the pot wiper as an input terminal effectively moves element tempco and wiper contact resistance errors inside the feedback loop of A1, thus removing them as gain-error terms and thus improving the time and temperature stability of the gain setting. DCP2 is used to null the amplifier zero point. It varies the voltage at the non-inverting input of A2 by ±2mV with a resolution of 16μV.