measurement Measuring large differential voltage

Posted on Feb 5, 2014

A high voltage power supply for an electron beam system. The voltage regulator is an old analog system that works via a simple comparator and reference signal from a POT. My problem is the feedback voltage from the high voltage tank (tank meaning large steel oil filled tank with all HV components inside) is upward of 124 volts DC and is floating in respect to ground. The feedback signal comes from a 50k resistor

measurement Measuring large differential voltage
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which is part of a large resistance network inside the tank. Part of that network also runs to an analog 0-1mA meter that is a kV meter so I don`t want to mess with the resistance. I am trying to replace the analog controller with a microcontroller (ATmega) to replace the closed loop analog control with digital control. I don`t want to mess with the resistance network inside the tank so this has to piggyback existing system and not unbalance the resistance. Below is an overly simplified schematic of the power supply. As you can see R1 is the feedback resistor, two wires are taken from each of its terminals and run to the voltage regulator. R3 represents beam current measurement and overload protection (different circuit). The ground is chassis/earth ground. R2 represents the numerous resistors inside the tank from the negative side of the power supply. R4 and R5 is my planned divider to a difference amp (ignore the TL801 part number). My idea is to use a high impedance divider using 11 megs of resistance and then feeding that scaled down signal into a unity gain difference amplifier or high impedance instrumentation amplifier. The amplifier would be powered from an isolating DC-DC power supply and the signal isolated via an isolation amplifier ( TI ISO124 ). From there I can scale it further using a POT to the uC ADC. Any thoughts

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