N c is the noise generated due to the detector capacitance, the cable capacitance, the stray capacitance, and the bias resistor to the detector. N i can be minimized by selecting a detector with low leakage current and by lowering the operating temperature of the detector. The topic of cooling the detector is discussed below. N c can be minimized by using a short cable between detector and preamplifier and by minimizing any stray capacitance due to PC board layout and coaxial connectors. The high voltage bias resistor should be chosen to have low noise, typically less than 0.5db above thermal noise with a value greater than 1 GOhm for detectors with low leakage current.
V b is the power supply voltage.
R is the bias resistor. If the leakage current in the detector is high, care should be taken to calculate the voltage drop across the bias resistor. Typically the leakage current in the detector doubles for every 8 o C increase in temperature. The power supply voltage minus the voltage drop across the bias resistor should be equal to the manufacturer's recommended operating voltage for the detector at a given temperature. The A250 circuit design has been optimized so that its noise contribution is negligible compared to the total noise of the system.