Readout circuits capacitance

Fig. 10. The proposed structure of the SET-hased readout circuit for THz detector. The quantum detector cell (QDC), which is the triple Q-dot system, is capacitively coupled through the sense electrode to a carbon based nanotubes RF-SET. A bias-electrode is used to control the bias potential on the sense electrode.

Fig. 11. An example showing all the capacitances of the top quantum dot (QDl) interconnecting with the other structures of the QCD device and SET readout circuit elements.

Most capacitive readout circuits are extremely sensitive. For example, a laF change of a capacitor biased at 1 V corresponds to a charge difference of only six electrons, but can readily be detected by many readout circuits. The high sensitivity translates into an equally high susceptibility to errors and noise. 

Electronic noise manifests itself as random fluctuations of voltage or current. Two kinds of electronic noise are relevant to capacitive readout circuits flicker or 1/f noise, and thermal noise. The former dominates at low frequencies and can be... 

For illustration, the circuit from Fig. 6.1.10 is redrawn in Fig. 6.1.13 with the noise sources vn and i from the amplifier and bias resistor shown explicitly. The variances are determined from Equation (12) for the resistor, and the datasheet of the amplifier. The noise specification of amplifiers includes also an input referred current noise, which (for amplifiers with CMOS inputs) is usually negligible, and for amplifiers with bipolar input is so large that BJT input stages simply cannot be used in most capacitive readout circuits. 

Fig. 6.1.13 Electronic noise sources in capacitive readout circuit...

Capacitive readout circuits, such as that shown in Fig. 6.1.13 can be readily implemented with off-the-shelf components using operational amplifiers for single ended [25, 26] or differential designs [27, 28]. Special purpose chips for capacitive sensing [29, 30] are also available. 

A monolithic three-axis accelerometer with three independent capacitive readout circuits on a single chip is described elsewhere [7] (Fig. 6.1.14). The circuit is similar to Fig. 6.1.10 and achieves 0.085 aF/v/Hz resolution with 100 fF sense capacitors. The noise is actually dominated by Brownian noise in the sensor itself, as tests in vacuum demonstrate. The actual capacitance resolution is therefore somewhat better than stated. This circuit uses correlated double sampling (CDS) for biasing and to reject flicker noise. 

Many other integrated solutions are documented in the literature. These include single-ended and differential solutions employing chopper stabilization [1, 20, 24, 31], circuits using correlated double sampling [2, 3, 32] and applications for capacitive readout circuits and corresponding implementations [15]. 

Fig. 7.18.3 shows the output of a prototype capacitive sensor fabricated by Bosch, showing a linear dc output voltage depending on the RME content in the fuel mixture. The sensor utilizes a coaxial capacitive probe and a dedicated readout circuit operating on a 5 V supply. 

The JFET switch Is closed by the step application of a -5 volt signal. Whenever this voltage step Is applied to the open switch, the Inherent junction capacitance of the JFET causes charge to be Injected Into the readout circuit. The fluctuation In this charge generates an electronic readout or scanning noise current (n ), which Is Independent of the photodiode signal current 1. ... 

Figure I. Schematic diagram of photodiode array readout circuit. Cj diode Capacitance. Cj readout capacitor.

Modulation-based readout circuits have gained popularity for interfacing with capacitive and inductive MEMS sensors to reduce the noise levels of l// noise and direct current (dc) offset in the circuit. Modulation-based circuits are based on sigma-delta (2A) converter [56,62,65], successive approximation register (SAR) analog-to-digital converter (ADC) [56,66], chopper modulation [54,55,58], pulse-width modulation (PWM) [57,61,63], and frequency modulation (FM) configurations [42,48,51,64,68],... 

M.S. Arefln, M.B. Coskun, T. Alan, J.-M. Redoute, A. Neild, M.R. Yuce, A microfabricated fringing field capacitive pH sensor with an integrated readout circuit. Applied Physics Letters, 104 (2014) 223503. 

N. Yazdi, H. Kulah, K. Najafi, Precision readout circuits for capacitive microaccelerometers, in Proceedings of the IEEE Sensors Conference, 2004, pp. 28-31. 

Several of the readout circuits can be nonlinear over part of their dynamic range due to low detector resistance (reducing injection efficiency in DI circuits) or providing increased saturation levels with the addition of the detector capacitance (direct integrator and SFD). The dynamic range for these circuits cannot be evaluated without the detector attached. 

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