PDF《CREATING A BIPOLAR INPUT RANGE FOR》

8 shows the software'

s normalized reading versus DDC112 output code and input signal. Using a normalized range makes it easy to read +Full Scale = 157pC FFFFFh = 1,048,575 INPUT SIGNAL DDC112 OUTPUT CODE 1.0 EVALUATION FIXTURE SOFTWARE READING Zero 69F5Ch = 434,012 CFull Scale = C104pC 00000h 0.412 C0.0039216 TINT = 500?s Range = 250pC R = 20M? V = 4.1V

4 noise, the spectral density of the voltage noise is propor- tional to the value of resistance. The one on the right is the Thevinen equivalent: a current source in parallel with a noiseless resistor. Notice that the spectral density of the current noise is inversely proportional to the value of resis- tance. That is, the bigger the resistance, the smaller the current noise. The DDC112 measures current and therefore, the noise contribution of the resistor is best modeled using the current source equivalent circuit. As seen in Figure 10, the current noise is proportional to the inverse of the resistor'

s value. Notice that the thermal noise power of the resistor is independent of its value. This comes from the physical nature of thermal noise in a resistor and is explained in detail in Reference 1. As just described qualitatively, the resistor'

s noise contribu- tion seen at the DDC112'

s output decreases as its value increases. Now, to get some quantitative results to calculate the actual amount of additional noise produced by the resistor, equations are needed. In general, for a linear sys- tem, the mean-squared output noise as a function of the system'

s transfer function and input noise is given by where S(?) is the spectral no........