| 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.
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