Very high impedance input circuit

Claim 2

A very high impedance input circuit, comprising: an amplifier of positive gain g composed of first and second dual input operational amplifiers, each of said amplifiers being inverter-connected with a resistive negative feedback and said amplifiers being cascade coupled so that their inputs are substantially at virtual ground; an input resistor of value R1, one terminal of which is connected to one input of the first operational amplifier; a resistor of value R5 positive-feedback connected between the output of the second operational amplifier and the when input signals are coupled to said first operational amplifier, said voltage amplifier is isolated from said first and second operational amplifiers; and a second storage capacitor one terminal of which is connected to a reference potential and the other terminal oF which is coupled to the junction of said means for coupling and said first switching means.

Claim 3

An input circuit according to claim 1 wherein said second switching means operates to continuously couple applied AC signals and intermittently couple applied DC signals to the one input of said first operational amplifier. 3. An input circuit according to claim 2 further including, a third capacitor connected in series circuit relationship with said second switching means and said input resistor R1, and a third switching means operating in phase opposition with said second switching means and connected to a point between said second switching means and said capacitor.

Claim 4

An input circuit according to claim 3 wherein the capacitance of said second capacitor is determined as a function of the period during which said second switching means provides an input signal coupling path to said input circuit such that the potential variation at the terminals of said second capacitor during this period is lower than the resolution of the system.

Claim 5

An input circuit according to claim 1 and further including: a variable gain amplifier having an input connected to the output of the input circuit and having an output other terminal of the input resistor, so as to establish the relationship g 1 + R5/R1 and to make the apparent input impedance of the amplifier practically infinite, the circuit further comprising in association with said operational amplifiers; a first means of drift stability control including a first capacitor inserted in the loop formed by the two operational amplifiers and the positive feedback resistor for opening said loop for drift signals having a frequency lower than that of signals applied to the input circuit; a second means of drift stability control including at least one circuit including a negative-feedback voltage amplifier having an input terminal connected to the output of the second operational amplifier; a first switching means connected to said voltage amplifier for decoupling said voltage amplifier from said first and second operational amplifiers; means connected to said first switching means for coupling the output of said voltage amplifier to said other input of the first operational amplifier; a second switching means for coupling input signals to said one input of said first operational amplifier; means for operating said first and second switching means in phase opposition to one another such that connected to said input of said second means of drift stability control, switching means operative with said first switching means for decoupling said voltage amplifier from said variable gain amplifier, the output of said variable gain amplifier being further connectable to a utilization device.