732B/734A
Instruction Manual
3-16
preferably at the same location the guards are grounded. If an instrument in the system
has a grounded input or output, select it as the common earth ground point for all guards
in the system.
Figure 3-9 shows a connection to the GUARD binding post. The GUARD is used in that
setup to prevent spurious ground currents from degrading the accuracy of the
measurement taking place.
Note
For best results, connect GUARDS to GROUND in a star configuration.
Connect a separate lead between each instrument’s GUARD to the common
GROUND. Do not connect GUARDS in a daisy-chain configuration.
With line power disconnected, you can confirm the integrity of your guarding and
grounding scheme by checking each instrument with an ohmmeter to find hidden or
internal connections between guard, common, and ground. If you have any question
about proper guarding and grounding, draw a guarding diagram for your system of
instruments, and make sure all instrument guards are tied together and grounded at only
one point.
For more information about grounding and guarding, suggested references are
Grounding and Shielding Techniques in Instrumentation, by Ralph Morrison, 1977,
John Wiley & Sons; and Noise Reduction Techniques in Electronic Systems by Henry
W. Ott, 1976, John Wiley & Sons.
Monitoring Oven Temperature 3-16.
As shown in Figure 3-7, MONITOR/EXT BAT IN pins 4 and 8 connect to the two
terminals of the oven temperature thermistor. To monitor the oven temperature, measure
the resistance between these two pins. The nominal resistance at normal operating
temperature is between 36.5 kΩ and 42.5 kΩ. The resistance should change with time by
no more than 900Ω per year. The TC (temperature coefficient) of the thermistor is
approximately 2 kΩ per °C.
You will be able to use this measurement to gain confidence that oven temperature
regulation is working during changes in ambient temperature. Your observations of the
thermistor resistance will reveal when the ambient temperature is too extreme for the
oven temperature regulation circuitry to work properly.
Resetting the IN CAL Indicator 3-17.
If the IN CAL indicator is off, the 732B may not meet its specifications listed in Section
1. The indicator goes off if power to the battery voltage has dropped too low, or if the
oven temperature has gone too high or too low. If IN CAL trips off, you will need to
recalibrate the 732B, or verify its output before you reset the IN CAL indicator. Refer to
Section 5 for the procedure to calibrate the standard and reset the IN CAL indicator.
Monitoring The IN CAL Indicator State Remotely 3-18.
As shown in Figure 3-7, three of the MONITOR/EXT BAT IN pins are connection
points for monitoring the state of the IN CAL indicator. Figure 3-10 illustrates the IN
CAL status output circuit inside the 732B. Proceed as follows to use this connector to
remotely monitor the IN CAL indicator:
1. Connect an external +5V logic level to pin 3 (common to pin 2).
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