CMOS Current Feedback Operational Amplifier-Based Relaxation Generator for Capacity to Voltage Sensor Interface

Abstract

This paper presents a simple relaxation generator, suitable for a sensor interface, operating as a transducer of capacitance to frequency/period. The proposed circuit employs a current feedback operational amplifier, fabricated in I3T25 0.35 μm ON Semiconductor CMOS process, and four passive elements including a grounded capacitor (the sensed parameter). It offers a low-impedance voltage output of the generated square wave. Additional frequency to DC voltage converter offers output information in the form of voltage. The experimental capacitance variation from 6.8 nF to 100 nF yields voltage change in the range from 21 mV to 106 mV with error below 5% and sensitivity 0.912 mV/nF evaluated over the full range of change. These values are in good agreement with simulation results obtained from the Mathcad model of frequency to DC voltage transducer passive circuit.

Design of Signal Generators Using Active Elements Developed in I3T25 CMOS Technology Single IC Package for Illuminance to Frequency Conversion

“…More details and the CMOS topology of CCCII can be found in [4]. The voltage follower (VF) with transfer function V out = V in has been also included in the same IC (once again, more details are in [4]). The OTA represents the last important active cell of the IC.…”

“…It is not typical in the case of similar solutions (integrator using current conveyor) because I Cmax is frequently given by the saturation voltage (V sq ) and it is, therefore, allowed only in linear dynamics of the current conveyor (see [14,15,17]). The constant value of 10 results from mirroring gains of the internal topology of the CCCII cell [4]. Hence, R 1 is not considered in the equations and it allows generating larger output levels than in the case working within linear dynamics of the AE.…”

“…We suppose the validity of Equations (1)-(3) and expectation of saturating V sq (V Y,X > ±0.5 V). Hence, the equation for f 0 has an identical form as Equation (4). The difference is in the inverted polarity of CCCII output Z and additional voltage follower in the circuitry.…”

“…Many standard and modern active elements (AEs) [2,3] fulfill requirements for their implementations in these generators. However, using them leads to very limited electronic adjustability of the repeating frequency in the generators (e.g., see comparison in [4]). Such phenomenon is caused by the implementation of nonadjustable active elements in key parts of a generator (e.g., see once again comparison in [4], where the AD844 device is used in an integrator or comparator).…”

“…Therefore, we can evaluate only the complexity of real circuitry based on available information. in [4]. The voltage follower (VF) with transfer function Vout = Vin has been also included in the same IC (once again, more details are in [4]).…”