Lake Shore 372, Руководство пользователя, Страница: 208 / 214

196 c
HAPTER
8: Service
Model 372 AC Resistance Bridge and Temperature Controller
8.12.2 Signal Level
Troubleshooting
The Model 372 provides user access to all the error monitoring points in the circuit for
troubleshooting convenience. Access to these points is provided through the
diagnostic monitor output BNC connector on the back of the instrument. To select
the desired monitoring point, refer to section 4.7.4 (Monitor Output Voltages). The
reference output provides an excellent way to trigger the oscilloscope when looking
at the measurement AC signal in the presence of noise. Note that the reference
output signal follows the input signal that is presented on the diagnostic monitor
output BNC connector. For example, if the monitor output is configured for the
control input ADC, the control input reference frequency will be presented on the
reference output. Otherwise, the measurement input reference frequency will be
presented on the reference output.
Be aware that the attachment of an earth-grounded oscilloscope to the monitor and/
or reference output jacks may induce ground loop related noise into the system that
is not normally present. Conversely, it is also possible that the addition of the ground-
ing path through the oscilloscope may actually quiet an improperly grounded system.
If in doubt, a differential input to the oscilloscope should be used. A battery-operated
oscilloscope is another option.
8.12.2.1 Nominal Signal Levels
For the measurement input, measuring a 100 k
)
“balanced” (or floating) resistor at a
voltage excitation of 632 mV and a resistance range of 200 k
)
, with CMR on, the fol-
lowing AC sine-wave signals will be observed at the appropriate signal points:
For the control input, when measuring a 10 k
)
“balanced” resistor with a current
excitation of 10nA (20 k
)
resistance range), 1 VAC Peak can be observed on the moni-
tor output when it is configured for the control input ADC.
The given levels may vary by 1 or 2%. At the above levels, the observed sine wave
should be very clean and noise free. These signal levels are representative of a “bal-
anced” resistance measurement. For the measurement input, if one side of the mea-
sured resistance is common to measurement common, large deviations from these
voltages will be seen. Also, be reminded that CMR must be turned off in that condition
for an accurate measurement.
As excitation is reduced, environmental noise will be seen on the signals. At voltage
excitations below 200
μ
V, the measurement signal levels at CSPOS, CSNEG, VCMPOS
and VCMNEG get small enough that a common oscilloscope my not adequately see
the measurement signal. However, common mode noise may be quite visible on the
CSPOS and CSNEG signals.
CSPOS 4.8 VAC Peak
CSNEG 4.8 VAC Peak
VCMPOS 0.228 VAC Peak
VCMNEG 0.228 VAC Peak
VDIF 4.5 VAC Peak
Measurement Input ADC 1 VAC Peak
OFF No signal
TABLE 8-12 Nominal signal levels