Readout circuits

We shall assume that the spectrometer basically consists of an entrance slit of area A focused on an exit slit at 1 1 magnification (Fig. 1). The recorded image is blurred because of diffraction and aberrations in the optics between the two slits and because of electronic blur in the readout circuit. The ultimate aim is to deconvolve or restore the spectral image so as to retrieve the spectral pattern at the entrance slit the latter pattern is called the object. 

An alternative approach is to form an hybrid array [20], The detector array is then formed in a mercury cadmium telluride substrate and the readout circuits are formed in a silicon substrate. The hybrid arrays are presented in part two. Detector arrays having detector elements which are provided with individual read-out leads formed in or on a non-active supporting substrate are presented in chapter 2.1. The detector elements may however be directly connected to a read-out chip which is bonded to the detector chip. A flip-chip bonding technique using indium bumps may be used as shown in figure 3 [21]. 

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. 

Johnson thermal noise. Noise generated by thermal agitation of electrons (electron kinetic energy fluctuations) in a resistor sets a lower limit on noise present In a detector readout circuit. Called Johnson thermal noise, this non-perlodlc noise exists In all conductive elements, and Is a function of temperature and resistance, described by ... 

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.

Figure 5.10 I Schematic diagram of a thermocouple gauge. An electric current heats the filament, and a thermocouple monitors the filament s temperature. Gas molecules coUide with the filament and cool it. So the higher the pressure, the lower the filament temperature will be. A readout circuit usually converts the measured filament temperature into pressure units for display.

Chou, J.C., Kwan, P.K., Chen, Z.J. Sn02 Separative Structure Extended Gate H -Ion Sensitive Field Effect Transistor by the Sol-Gel Technology and the Readout Circuit Developed by Source Eollower. Jpn. J. Appl. Phys. 42, 6790-6794 (2003)... 

Figure 7.1 A wireless point of care system with integrated biosensors and readout circuits.

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. 

Recent progresses in micro- and nanotechnology have led to the design of implantable, swallowable, wearable, or portable wireless biosensors to continuously monitor and detect important physiological parameters. The miniaturization and integration of biosensors, readout circuits, embedded microcontrollers, and wireless transceivers on a single chip have opened the way for new possibilities in medical applications. 

Figure 7.12 The block diagram of a wireless capsule system with biosensors, readout circuits, and transmitter.

The architecture of a wireless capsule system named the lab-in-a-pill (LIAP) is presented in Fig. 7.13 [113]. It consists of pH and temperature sensors and a custom-made application-specific integrated readout circuit. The pH sensor is a micro-fabricated ISFET with Ag/AgCl reference electrode. The temperature sensor is an n-channel silicon diode. The system consumes 15.5 mW. The circuit has a power saving feature to operate it for 42 h. 

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