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Isolating High Voltage from your Data Acquisition Card by h

2016-10-12 08:57  
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Measuring high voltages with a data-acquisition card and a resistive voltage divider can be done easily, but at some peril to your computer.  The application of a voltage above 10 or 12 volts through the serial, parallel port or USB bus can destroy your laptop in a fraction of a second.
This design, using Vishay IL300 (or Avago HCNR200) Analog Opto-Isolators will reduce the danger to your PC.  The IL300 and HCNR200's have one photo-emitter and two photo-receptors. The photo-receptor on the left hand side is connected to the inverting input of an opamp acting as a servo amplifier.  In this way the non-linearities of the photo devices are nulled out.  As shown below, a positive going voltage to the input will result in a positive going voltage on the output.  You can easily rearrange connections so that a negative-going voltage on the input will result in a positive or negative going output.  Bipolar operation is possible, albeit with an additional opto-isolator.

To keep the integrity of the isolation, each side of the opto-isolator is powered by its own power supply. The power for the output stage is borrowed from the USB bus of the Measurement Computing 1208FS Data Acquisition Card.  As the USB bus is a bit noisy I used a small choke and a 100uF tantalum capacitor on the power bus.  The devices shown on the 3D rendering of the prototype seem to be pretty gigantic -- but they were the parts I had on hand and the cost was about nil! As shown below, I used LMC6492 surface mount, CMOS operational amplifiers. These are relatively low noise devices (25nV Rt Hz) and I happened to have a bunch of them on hand,

Since the high voltage environment may have a bit of noise, a 10 Hz, 2-pole Low Pass Filter is incorporated on the output stage.  With careful layout, this design is capable of 12 bit resolution.

Isolating High Voltage from your  Data Acquisition Card by hcnr200

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