Split-path linear isolation circuit apparatus and method

A split-path isolation apparatus for receiving a wideband input signal voltage referenced to a first ground and accurately amplifying the input signal voltage to provide a wideband output signal voltage referenced to a second ground that is isolated from the first ground by an isolation barrier, comprising:

• a high path amplifier that amplifies the wideband input signal voltage and couples a low-frequency to high-frequency portion thereof through a transformer having a primary winding and first and second secondary windings that forms a first portion of the isolation barrier;
• a low path amplifier that amplifies the wideband input signal voltage and couples a DC to low-frequency portion thereof through a single-input, multi-output optocoupler, a first output of which forms a linearity controlling feedback path around the low path amplifier and a second output of which forms a second portion of the isolation barrier;
• and
• a signal combiner including the transformer that receives the DC to low-frequency portion and the low-frequency to high-frequency portion of the amplified input signal voltage that is coupled across the isolation barrier with the first and second secondary windings co-acting with the primary winding to produce a substantially zero net magnetic flux in the transformer over the DC to high-frequency range of the wideband output signal voltage, the frequency portions being substantially overlapped in a crossover frequency region such that the signal combiner produces the wideband output signal voltage having a substantially flat amplitude response without requiring frequency response adjustments in the crossover frequency region.

A method for accurately amplifying a wideband input signal voltage referenced to a first ground to provide a wideband output signal voltage referenced to a second ground that is isolated from the first ground by an isolation barrier, comprising:

• splitting the wideband input signal voltage into a low path signal having a first 3-db rolloff frequency and a high path signal having a second 3-db rolloff frequency, the first and second rolloff frequencies being substantially overlapped in a crossover frequency region;
• coupling the high path signal across the isolation barrier with a transformer having a primary winding and first and second secondary windings;
• coupling the low path signal across the isolation barrier with a first output of a single-input, multi-output optocoupler;
• closing a feedback loop of an amplifier in the low path with a second output of the optocoupler;
• and
• combining the low path signal and high path signal coupled across the isolation barrier via the transformer to generate the wideband output signal voltage having a substantially flat amplitude response without requiring frequency response adjusting steps in the crossover frequency region with the first and second secondary windings co-acting with the primary winding to produce a substantially zero net magnetic flux in the transformer over the DC to high-frequency range of the wideband output signal voltage.

In an oscilloscope having a chassis ground, an improved apparatus for accurately displaying an amplitude response versus time of a first input signal that is referenced to a first isolated ground, comprising:

• a first isolation barrier that separates the first isolated ground from the chassis ground;
• a first split-path isolation amplifier receiving the first input signal and including a high path amplifier employing a transformer having a primary winding and first and second secondary windings to couple a high-frequency portion of the first input signal across the first isolation barrier, and a low path amplifier that employs a single-input, multi-output optocoupler to couple a DC portion of the first input signal across the first isolation barrier;
• a first signal combiner including the transformer combining the DC and high-frequency portions of the first input signal coupled across the first isolation barrier to produce a first wideband output signal that is referenced to the chassis ground and that accurately represents the first input signal with the first and second secondary windings co-acting with the primary winding to produce a substantially zero net magnetic flux in the transformer over the DC to high-frequency range of the wideband output signal voltage;
• and
• a display subsystem electrically connected to the first wideband output signal for rendering an amplitude response versus time display of the first input signal.