Reading an analog signal with an external ground

(Please excuse my intentional ambiguity)

I have created a "frequency-shifter" that takes input in the form of an un-grounded AC sine wave, and outputs a square wave at a multiplied frequency. The system consists of an Arduino Nano plus some supporting hardware.

Currently I can control the frequency multiplier from a physical potentiometer on the project board. However, I would like to begin changing the multiplier based on analog input from a sensor.

Here is the issue... my entire Arduino system is grounded to one leg of the incoming AC sine wave. The analog signal I want to read references a ground on an external system and I am unable to connect the two ground references.

Is there an Arduino/Software method I can use to read this analog signal? Will I need additional hardware?

There are voltage to frequency chips that can measure and communicate analog voltages using optocouplers.

Otherwise, a ground connection is required. Consider grounding the Arduino to the device whose output to be measured, and transmitting the digital results by some wireless means.

Here is the issue... my entire Arduino system is grounded to one leg of the incoming AC sine wave. The analog signal I want to read references a ground on an external system and I am unable to connect the two ground references.

There is no good way to isolate analog-DC signals... There are 3 common ways to isolate signals -

  1. Electro-magnetic isolation (a transformer). Transformers only work for AC. They do work well to isolate the AC power line or isolate analog audio signals.

  2. Optical isolation. Opto-isolators are generally not linear although you might find one that's linear-enough over a limited range.

  3. Electro-mechanical isolation (a relay). "Digital-only" (on-off).

There is no good way to isolate analog-DC signals.

That would depend very heavily on what you mean by "good way".

Voltage to frequency converters with optoisolators are available that work much better (higher linearity and dynamic range) than the Arduino ADC to measure voltages, and offer 1500V isolation.

If you have a sine wave, and only its frequency is important, that sine can be converted to a rectangle and transmitted through an optocoupler. But that circuit deserves a power supply, so that the converter better should be tied to and powered by the Arduino.

On the output side an optocoupler can be used for isolation.