MTI AccuMeasure 9000, User Manual

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3.6 Probe Cables
MTII capacitance probes are equipped with special, low-noise coaxial cables. Standard coaxial cables are
not acceptable for connecting the probes to probe amplifier. Cables are available in either 50-ohm or 95-
ohm sizes. Specifications:
Diameter Operating Temperature
50

Cable 1.95mm (0.076 inch) -90 to +200ºC (-130 to 390ºF)
95

Cable 1.6mm (0.063 inch) -90 to +200ºC (-130 to 390ºF)
Table 1: Standard Probe Cable Characteristics
Most probes are shipped with 95

, low noise cables. If cable size is important, please contact the factory.
Cables may be spliced only if the spiced section is 100% shielded. Routing the cable through relays and
other switching devices will interfere with the measurement system.
Cylindrical feed-through assemblies may be used. Since the cable shield is
not at ground potential, the
feed-through must have provisions that guarantee that the cable shield will be isolated. This is typically
accomplished by selecting a triaxial feed-through, utilizing the center and middle conductors and allowing
the outside conductor to ground with the equipment. When selecting a triaxial feed-through, it is also
important that the center conductor (tip) of remain 100% shielded by the middle (fringe guard conductor).
Contact the factory for information on MTII-manufactured feed-through assemblies.
3.7 Target Grounding
The target being measured by the probes connected to the Accumeasure™ 9000 must be grounded
directly to the board power supply grounds for the probe to target gap to be accurately measured.
In general, it is recommended that grounding practices for sensitive equipment be applied when using the
Accumeasure
TM
amplifier. For additional information on grounding practices refer to H.W. Ott’s Noise
Reduction Techniques in Electronic Systems (John Wiley and Sons, 1976) or to Chapter 24 of the Analog
Devices Applications Reference Manual (1993).
3.8 Static Electric Discharge Precaution
The face of the probe and the center conductor of the BNC for probe input is static
sensitive. Due to the nature of capacitance amplifiers it is possible to damage the
sensitive input circuitry if there is a static electric discharge into the center conductor of
the probe input BNC or the center sensing area of the probe itself. Protection circuitry
has been incorporated to minimize this effect but a discharge of sufficient amplitude
will cause damage. The probes may be handled without ground straps, but high static
charges on the target may arc over to the probe.
!
NOTE
A poor ground return and the connection of the board output connectors to equipment such as
oscilloscopes, chart recorders, or data loggers can create ground loops, which will cause errors due to
excessive of 50-60 Hz noise on the carrier and gap output signals.