Capacitive-Transimpedance Amplifier

FIGURE 7.4 Visual image of individual detector elements. Pixels are 5, 2.45, and 1.60 mm long and 145 pm wide, with 10-pm spacing. There is also a 10-pm guard electrode present between each detector element. (From Babis et al., Performance evaluation of a miniature ion mobility spectrometer drift cell for application in hand-held explosives detection ion mobility spectrometers, AmZ. Bioanal. Chem. 2009, 395, 411-419. With permission.) 

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One recent approach to increased sensitivity in IMS instruments operating at atmospheric pressures has been the use of a microfaraday array coupled with capacitive-transimpedance amplifier (CTIA). Figure 7.4 provides a picture of a... 

In this subsection, we will discuss noise sources that arise from the nanopore itself, Erec, and the head stage during nanopore sensing. For the noise analysis, we use the simplified electrical models shown in Figure 29.3. Here, and model the resistance and capacitance of the nanopore, respectively. These vary with nanopore diameter, thickness, and material. Erec is modeled by the series resistance and a double-layer capacitance Cj, where R is much smaller than Rj. Note that in this chapter, we pay attention to the resistive-feedback transimpedance amplifier (TIA) due to its simple hardware structure and robust reliability, rather than a capacitive-feedback TIA, which requires a disruptive periodic reset [12,15]. Readers who are interested in the capacitive-feedback TIA can refer to the paper of Kim et al. [17]. 

Noise analysis and critical role of the input capacitance. In terms of noise the adoption of the transimpedance configuration is not necessarily an improvement with respect to the first solution, as the feedback affects the signal and noise in the same way. Let us analyse in detail the noise sources that characterise the transimpedance amplifier considering that the noise performance of the circuit mainly depends on the equivalent impedance of the device connected to its input. For this purpose the device will be composed of a resistor Rm in parallel to a capacitor C, , Fig. 6. All the noise... 

Figure 7.20 The capacitor feedback transimpedance amplifier (CTIA) gain is determined by the drawn capacitance in the feedback loop.

The noise is expressed as noise density in units of V/(Hz), or integrated over a frequency range and given as volts rms. Typically, photoconductors are characterized by a g-r noise plateau from 10 to 10 Hz. Photovoltaic detectors exhibit similar behavior, but the 1/f knee may be less than 100 Hz and the high frequency noise roU off is deterrnined by the p—n junction impedance—capacitance product or the amplifier (AMP) circuit when operated in a transimpedance mode. Bolometers exhibit an additional noise, associated with thermal conductance. 

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