I
NSTRUTECH
C
ORPORATION
ITC-18/PCI
U
SER
’
S
M
ANUAL
The ITC-18 is connected to a host computer over a shielded cable to a host
interface card. Host interfaces boards are available for ISA bus, PCI bus and
NUBUS. Moving the ITC-18 from one platform to another is a simple matter of
replacing the host interface board.
This configuration is unique to data acquisition units. Most data acquisition
devices are printed circuit-boards installed directly in the computer itself. The
interior of a computer is an electrically noisy environment. At the very high speeds
of modern computers, even a short length of a conductive trace on a PC board acts
as an antenna. Certain kinds of data acquisition devices are not affected by such
noise (e.g. slow-speed integrating A/D converters), but high speed high-resolution
A/D and D/A converters like the ones used in the ITC-18 are affected by such
noise. Mounting the electronics in an external box is the most effective way of
reducing this noise.
Optical Isolation:
The analog electronics of the ITC-18 are optically isolated from the digital
electronics and the computer. This provides complete electrical isolation between
the computer and the measurement.
The optical isolation is provided to eliminate computer electrical noise from the
analog measurements. Computers contain digital electronics that switch at high
speed, producing large electrical transients. The computer ground functions as the
return path for these transients, resulting in substantial high-frequency ground
noise. The ITC-18 has a completely separate analog ground that is isolated from
the computer ground. Computer ground noise is not coupled into the measurement.
Optical isolation is almost essential if low-noise analog outputs are provided. If the
analog outputs are not electrically isolated from the computer ground, the analog
output ground will contain computer-generated noise. This is one of the major
ways by which computer-generated noise is introduced into a measurement system.
The optical isolation of the ITC-18 brings with it another benefit that simplifies
complex measurement systems. Since the analog inputs and outputs of the ITC-18
float with respect to ground, the ITC-18 and computer are not a source of ground
loop problems.
Analog Circuitry:
The ITC-18 provides eight 18 bit A/D converters that feature individually
programmable input range of +1, +2, +5, or +10 volts. Using separate A/D
converters for each analog input channel eliminates crosstalk between channels and
improves the frequency response of the analog inputs.
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