Capacitive Parasitics

Capacitance, parasitic 176 Carbon black 12, 33, 47, 57, 60 - -, processing 27, 30 Carbon fiber reinforced polymer (CFRP) 103... 

The circuit equivalent taking into account parasitic effects consists of a contact resistance (Rs), a lead inductance Ls in series as well as a stray capacitance C() and the resistance of the substrate material between the leads 1/G0 parallel to the impedance of the sample. These parasitic effects, resulting from the compact arrangement of the electrodes can be eliminated by an offset adjustment. Capacitive parasitic effects are acquired by compensation measurements. The electrode array comprises some IDCs, which have almost negligible parasitic pathway effects, like position 5-5 (Fig. 11.1), where the conductor pathways are perpendicular to each other.22 These positions show the smallest capacities, which result only from the IDC and can be taken to determine the parasitic capacitive values of the other positions.24 Inductive parasitic effects Ls are acquired by data fitting and elimination from the sample impedance. Resistances of the conductor path (Rs generally <10 mQ) and conductivity of the substrate material (1/G0, >20mQ, exceeds measurement limit of the impedance analyser) are unaccounted. 

Figure Bl.28.8. Equivalent circuit for a tliree-electrode electrochemical cell. WE, CE and RE represent the working, counter and reference electrodes is the solution resistance, the uncompensated resistance, R the charge-transfer resistance, R the resistance of the reference electrode, the double-layer capacitance and the parasitic loss to tire ground.

A substrate is a robust element that provides mechanical support for the die. It can be mounted with more than one die such packages are called multichip modules. Because parasitic capacitance effects are directiy proportional to the dielectric constant, substrate material should have a low dielectric constant. 

The frequency response or switching speed of the bipolar transistor is governed by the same processes which control the speed of thep—n junction, the capacitance associated with the movement of charge into and out of the depletion regions. To achieve high frequencies the dimensions of the active areas and parasitic circuit elements must be reduced. The two critical dimensions are the width of the emitter contact and the base thickness, W. The cutoff frequency,, is the frequency at which = 57 / - b /t > where is the emitter-to-coUector delay time and is the sum of the emitter... 

To achieve the lowest possible delay a bipolar switching transistor developed by IBM minimizes parasitic resistances and capacitances. It consists of self-aligned emitter and base contacts, a thin intrinsic base with an optimized collector doping profile, and deep-trench isolation (36). Devices must be isolated from each other to prevent unwanted interactions in integrated circuits. While p—n junctions can be used for isolation, IBM s approach etches deep trenches in the siUcon wafer which are filled with Si02 to provide electrical insulation. 

The main advantages that compound semiconductor electronic devices hold over their siUcon counterparts He in the properties of electron transport, excellent heterojunction capabiUties, and semi-insulating substrates, which can help minimise parasitic capacitances that can negatively impact device performance. The abiUty to integrate materials with different band gaps and electronic properties by epitaxy has made it possible to develop advanced devices in compound semiconductors. The hole transport in compound semiconductors is poorer and more similar to siUcon. Eor this reason the majority of products and research has been in n-ty e or electron-based devices. 

In some cases it is possible to form bridges of metal using air as the dielectric (150). However, if more than two levels of wiring are required then dielectric spacing is necessary. The ideal dielectric film has excellent adhesion and alow dielectric constant to minimize parasitic capacitances. The most common films include siUcon oxide, siUcon nitride, and a number of spin-on dielectrics (216). 

Figure 3-35 The symbol of a power MOSFET with the parasitic capacitances.

The overall ability of a power supply to attenuate disturbances at its input is expressed as its PSRR (power supply rejection ratio). In graphs, PSRR is usually plotted as a function of frequency. We will invariably find that the rejection ratio is very low at higher frequencies. One reason for this is that the Bode plot cannot really help because the open-loop gain is very small at these frequencies. The other reason is, even a tiny stray parasitic capacitance (e.g., across the power switch and inductor) presents such a low impedance to noise frequencies (whatever their origin) that almost all the noise present at the input migrates to the output unimpeded. In other words, the power stage attenuation (which we had earlier declared to be Vo/Rin) is also nonexistent for noise (and maybe even ripple) frequencies. The only noise attenuation comes from the LC filter (hopefully). 

Note It is almost impossible to see the noise spikes. First of all, various parasitics help limit/absorb them somewhat (though they can still retain the capability to cause controller upset). Further, the moment we put in an oscilloscope probe, the 5 to 20pF of probe capacitance can also absorb the spikes sufficiently, and we would probably see nothing significant. In addition, probes pick up so much normal... 

Figure 11-13 Create a Metal StandofF Near the Switch to Close the Loop of the Noise Current Injected Through the Parasitic Mounting Capacitance of the Fleatsink...

Recommendation 4 (see Figure 11-13) Create a metal standoff near the switch to close the loop of the noise current injected through the parasitic mounting capacitance of the heatsink. 

Figure 11-14 The Suggested Metal Standoff Technique to Return the Noise Current Injected Through the Parasitic Capacitance of the Heatsink... 

With recent trends toward microminiaturization and utilization of very thin conductor lines, close spacings, and very thin insulation, greater demands are being placed on the insulating layer. Reductions in such parasitic capacitance can... 

Fig. 11.1. Two basic types of current ampliflers. (a) Feedback picoammeter. It consists of two components, an operational amplifier (op-amp) A, and a feedback resistor 1 fb- a typical value of the feedback resistor used in STM is 10 fl. The stray capacitance Cfb is an inevitable parasitic element in the circuit. In a careful design, Cfb 0.5 pF. The input capacitance Cm is also an inevitable parasitic element in the circuit. Those parasitic capacitors, the thermal noise of the feedback resistor, and the characteristics of the op-amp are the limiting factors to the performance of the picoammeter. (b) An electrometer used as a current amplifier (the shunt current amplifier). The voltage at the input resistance is amplified by the circuit, which consists of an op-amp and a pair of resistors R, and R2. The parasitic input capacitance Cm limits the frequency response, and the Johnson noise on Rm is the major source of noise. Also, the input resistance for this arrangement is large.

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