A controlled carrier current measuring sensor/transducer signal amplifier system comprising a carrier oscillator for providing a carrier voltage oscillatory signal of known frequency and fixed voltage magnitude. A differential amplifier has the carrier voltage oscillatory signal supplied to one of its inputs. A reference impedance and a variable impedance sensor transducer are connected in a series circuit across the output of the differential amplifier and a first feedback signal is fed back to a second input of the differential amplifier with the first feedback signal comprising the sum of the voltage across the reference impedance and the variable impedance sensor transducer. A buffer amplifier has its input connected to the juncture of the reference impedance and the variable impedance sensor/transducer for deriving an output voltage representative of the instantaneous value of the voltage across the variable impedance sensor/transducer. The output voltage signal from the output of the buffer amplifier is fed back to a third input to the differential amplifier in opposition to the first feedback signal for regulating the current supplied through the reference impedance and the variable impedance sensor/transducer to a substantially constant value whereby the output voltage signal from the output of the buffer amplifier provides a measure of the impedance of the variable impedance sensor/transduer. Preferably, the system further includes shielded conductors interconnected between the reference impedance and the variable impedance sensor/transducer and between the juncture thereof and the input to the buffer amplifier and the output from the buffer amplifier is fed back to the shielding of the shielded conductors whereby the effects of stray capacitance on any measurement obtained from the variable impedance sensor/transducer are minimized.

A preamplifier for amplifying signal voltages from a signal source with high source impedance has a signal input connected to the signal source, a first supply voltage terminal connected to the first power supply voltage potential, a second supply voltage terminal connected to a second power supply voltage potential and a signal output supplying an output signal. In this preamplifier, the first and second power supply voltage potentials can be shifted in parallel depending on the output signal. Alternatively or cumulatively, a known preamplifier may be developed so that the potential of a shield for the signal source can be controlled by the output signal.

An apparatus for detecting contamination of a capacitive sensor, comprising a guard electrode surrounding a portion of the sensor, and a circuit for supplying an excitation signal to the sensor and an identical excitation signal to the guard electrode. The circuit periodically isolates the guard electrode from the excitation signal, and detects the difference between the sensor output when the guard electrode is supplied with the excitation signal and when the guard electrode is isolated from the excitation, the detected difference being a function of the contamination of the guard electrode.

A probe developing an admittance with a material whose condition is to be continuously measured is capacitively coupled with a variable frequency oscillator, preferably one having a digital output stage, for generating a variable frequency signal having a period varying with the admittance developed at the probe. Both the oscillator input and output are capacitively coupled to the probe. In a preferred embodiment, a set point circuit including a plurality of resistors coupled in parallel with the oscillator output and a plurality of switches coupled in parallel with the probe conductor and each with a different one of the resistor, provides easy circuit adjustment for use of the system with a variety of materials. A preferred digital circuit employing a pair of flip flops, fixed timer and one shot generate a fixed period variable duration duty cycle digital voltage level signal for transmission, if desired, or for conversion into a variable current signal.

The non-linear conductance and capacitance characteristics of electrically non-homogenous materials over a given frequency range are used to identify and analyze such materials.