Method and circuit for reducing effects of distributed capacitance associated with large thin film resistors

An isolating differential amplifier comprising in combination:

• (a) a first inverting input, a second non-inverting input, a first output;
• (b) an amplifier circuit having a second inverting input, a second non-inverting input, and an output coupled to the first output, and first and second supply voltage terminals, and a reference voltage conductor;
• (c) an electrically isolated doped layer;
• (d) an insulative layer on the isolated doped layer;
• (e) a high resistance thin film resistor on the insulative layer having a first terminal coupled to the first output and a second terminal coupled to the second non-inverting input;
• (f) a first input resistor coupled between the first and second non-inverting inputs;
• (g) a first voltage division resistor coupled between the reference voltage conductor and the second non-inverting input;
• (h) means for reducing phase shift of a signal flowing through the thin film resistor, the phase shift reducing means including:
• (i) first means for applying an AC reference voltage to the second terminal of the thin film resistor;
• (ii) second means for applying a reference voltage on the reference voltage conductor to a first part of the isolated doped layer;
• (iii) third means for applying a first signal produced on the first output to both the first terminal of the thin film resistor and a second part of the epitaxial layer to thereby cause a first current to flow through the thin film resistor and produce a first voltage gradient therein and to also cause a second current to flow through the epitaxial layer and produce a second voltage gradient therein, the first and second voltage gradients being sufficiently nearly equal to prevent substantial charging currents from flowing through parasitic capacitance between the thin film resistor and the epitaxial layer, whereby phase shift of the first current as it flows through the thin film resistor is substantially avoided.