US 4,843,339 A · Isolation amplifier including precision voltage-to-duty-cycle converter and low ripple, high bandwidth charge balance demodulator

Abstract

An isolation amplifier includes a voltage-to-duty-cycle modulator, a non-galvanic isolation barrier, and a demodulator converting a duty-cycle-modulated signal transmitted across the isolation barrier to an analog voltage replica of the analog input voltage. The modulator circuit includes a first current switching means which produces a first current that is switched between positive and negative values in response to an output from a comparator that can be referenced to a noise-synchronized signal. The first current is summed with an input current and the difference is integrated and input to the comparator, the output of which produces the duty-cycle-modulated signal. The demodulator includes a second current switching circuit for producing a second current that is switched between positive and negative levels in response to the duty-cycle-modulated signal received across the isolation barrier and includes circuitry for algebraically summing the input current with the second current and integrating the result to produce the analog output voltage. The modulator circuit and demodulator circuits are fabricated on a single semiconductor area to produce the close matching between components of the first and second current switching circuits. The area is cut in half to produce two chips, which are connected to two isolation barrier capacitors. In the demodulator, a sample and hold circuit synchronized with the duty-cycle-modulated signal transmitted across the isolation barrier samples the output of the integrator.

Claims38

Dependent claims

A modulator/demodulator circuit comprising in combination:

(a) a modulator circuit including
i. means for receiving an analog input current;
ii. first switching means for producing a first current that is switched between positive and negative values in response to a signal on a first conductor;
iii. first integrating circuit means for integrating an algebraic sum of the analog input current and the first current to produce an output voltage that increases or decreases in accordance with whether the algebraic sum of the input current and the first current is positive or negative;
means for producing a duty-cycle modulated signal on the first conductor having a first level if the output voltage exceeds a reference voltage sional that is synchronized with a noise signal;
(b) means for coupling the first conductor second conductor;
and
(c) a demodulator including
i. second switching means for producing a second current that is switched between positive and negative levels in response to the duty cycle modulated signal on the second conductor;
ii. second integrating means for integrating the second current to produce an analog output signal that is accurately representative of the analog input current.

7 more elements

An isolation amplifier comprising in combination:

(a) a modulator circuit including
i. means for producing an analog input current in response to an analog input voltage;
ii. first current switching means for producing a first current that is switched between positive and negative values in response to a signal on a first conductor;
iii. first integrating circuit means for integrating an algebraic sum of the analog input current and the first current to produce an output voltage that increases or decreases in accordance with whether the algebraic sum of the analog input current and the first current is positive or negative;
iv. means for producing a duty-cycle-modulated digital signal on the first conductor having a first level if the output voltage of the first integrating circuit means exceeds a threshold voltage and a second level if the output voltage of the first integrating circuit means is less than another threshold voltage;
(b) a non-galvanic isolation barrier having an input coupled to the first conductor and an output coupled to a second conductor;
and
(c) a demodulator including
i. second current switching means for producing a second current that is switched between positive and negative levels in response to the duty cycle modulated digital signal on the second conductor;
ii. second integrating circuit means for integrating the second current to produce an analog output voltage that is accurately representative of the analog input voltage, wherein the second integrating circuit means includes
a first operational amplifier having a first input responsive to the second current,
a second input coupled to a demodulator ground reference voltage,
a first capacitor having a first terminal coupled to the first input of the first operational amplifier and a second terminal connected to an output of the second operational amplifier,
a sample and hold circuit having an analog input coupled to the output of the first operational amplifier, an analog output conductor coupled to a first terminal of an output resistor, a second terminal of the output resistor being connected to the first input of the first operational amplifier, the analog output voltage being produced on the analog output conductor of the sample and hold circuit, the sample and hold circuit including a sample control input coupled to the second conductor.

11 more elements

A method of operating a modulator/demodulator circuit comprising the steps of:

(a) producing an analog input current;
(b) switching a firsat current between positive and negative values in response to a signal on a first conductor;
(c) integrating an algebraic sum of the analog input current and the first current to produce an output signal that increases or decreases in accordance with whether the algebraic sum of the input current and the first current is positive or negative;
(d) producing a duty-cycle-modulated digital signal on the first conductor having a first level if the output signal exceeds a first threshold signal and a second level if the difference is less than a second threshold signal;
(e) switching a second current between positive and negative levels in response to the duty=cycle-modulated digital signal;
(f) integrating the second current to produce an analog output signal that is accurately representative of the analog input current by
i. applying the second current to a first input of a first operational amplifier having a second input coupled to a demodulator ground reference voltage, a first capacitor having one terminal coupled to the first input of the first operational amplifier and a second terminal coupled to an output of the first operational amplifier,
ii. applying a signal on the output of the first operational amplifier to an analog input fo a sample and hold circuit, an analog output conductor of the sample and hold circuit being coupled to a first terminal of an output resistor, a second terminal of the output resistor being coupled to the first input of the first operational amplifier, and
iii. applying the signal on the second conductor to a sample control input of the sample and hold circuit, the analog output voltage being produced on the analog output conductor of the sample and hold circuit.

6 more elements

A demodulator circuit comprising in combination:

(a) first circuit means for receiving a digital input signal that is modulated to represent an analog input signal and producing in response thereto a switch control signal including a sequence of pulses;
(b) a first current source;
(c) switching means for electrically coupling the first current source to a first summing node during the pulses of the switch control signal;
(d) integrating circuit means having a first input coupled to the first summing node and an output for integrating a current flowing from the first summing node to the first input;
(e) a sample and hold circuit having an input coupled to the output of the integrating circuit means and an output, and a sample control input responsive to the digital input signal, wherein the sample and hold circuit includes
i. a first sample and hold circuit having an input coupled to the output of the intergrating circuit means, and
ii. a second sample and hold circuit having an input coupled to an output of the first sample and hold circuit, the second sample and hold circuit having an output producing the analog output signal;
(f) a feedback resistor coupled between the output of the sample and hold circuit and the first summing node, the output of the sample and hold circuit means producing an analog output signal that accurately represents the analog input signal, the sample and hold circuit operating to substantially reduce ripple voltage on the analog output signal.

5 more elements

A sample and hold circuit comprising in combination:

(a) a first sample and hold circuit including
i. a first operational amplifier having a first input receiving an analog input signal, and a first output,
ii. a second operational amplifier having a first input and an output,
(b) a second sample and hold circuit including
i. a third operational amplifier having a first input coupled to the output of the second operational amplifier, and a first output,
ii. a fourth operational amplifier having a first input and an output,
(c) first switching means for coupling the first output of the first operational amplifier to the first input of the second operational amplifier throughout a first interval, and then decoupling the first output of the first operational amplifier from the first input of the second operational amplifier at the end of the first interval;
(d) second switching means for coupling the first output of the third operational amplifier to the first input of the fourth operational amplifier throughout a second interval that begins after the decoupling.