Different configurations used in electrical capacitance detection. A variation in the capacitance produces a variation in the output voltage or a variation in the total charge . In (a) charge is kept constant (measurement device with infinite input impedance) and and . In (b) voltage is kept constant (measurement device with zero input impedance) and and . The advantage of the second approach is that the output does not depend on the parasitic capacitances represented by .
Circuit diagram of the transimpedance amplifier. The negative feedback keeps voltage in the capacitor constant and equal to . This is an implementation of the capacitance measuring approach described in Fig. 1(b). The output is proportional to , , and but does not depend on the parasitic capacitances represented by .
PMOS and NMOS parameters as a function of temperature. The amplifier was designed considering a simple model for the transistors working in saturation where . Parameters , , and were extracted from the output vs and transfer vs curves.
Schematic for the two stage unbuffered operational amplifier. M1 and M2 form the -channel differential input pair. CC is a compensation capacitor for improving stability of the circuit. I5 is an external current source.
Open loop gain vs frequency at 23 K. The dc gain is . The gain bandwidth is approximately 4 MHz. The two poles corresponding to the two amplifier stages are located at and .
Transimpedance amplifier power consumption as a function of temperature. The sensor is connected to the amplifier but with its input terminal connected to ground (no capacitance variation).
Transimpedance amplifier input referred noise as a function of temperature. The sensor is connected to the amplifier but with its input terminal connected to ground (no capacitance variation).
Micromechanical oscillator resonance curves. A 2 mVp.p. sine wave with a 1 mV dc offset was applied to the sensor input terminal. Frequency sweeps were performed to find the torsional mode resonant frequency of the oscillator at different temperatures: 8, 30, 40, and 50 K.
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