Theory of Operation
Component Test
2
2-23
Test the auto diode circuit by placing a diode across the input terminals and viewing the
input waveform to the Fast A/D Converter (TP19.) A rectified waveform indicates that
the analog circuit is working properly.
Component Test
2-48.
Component Test graphs the current vs. voltage relationship encountered when the test
probes are placed across an unknown component. All power in the circuit under test must
be off to make this test.
Component Test uses the low impedance source path with a
±
3.2 volt frequency-
selectable sine wave oscillator to provide the voltage stimulus. This stimulus signal is
generated digitally on U24 and converted to an analog signal using a 10-bit current
output DAC (U1). Two op amps on U30 convert the current output of the DAC into a 6.4
volt peak-to-peak sine wave voltage at U30-4. U30 also provides a single-pole low-pass
filter at 82 kHz to remove glitches on the DAC output. The Component Test source is
routed to the
Ë
input through R31, RT1, and R103. This gives a nominal output
impedance of 11.1 k
Ω
. This oscillator allows frequencies from 2 Hz to 18.75 kHz.
Component Test uses the 8-bit Fast A/D Converter (U10) to make the current and
voltage readings. The voltage sense path is kelvin connected to the volt/ohm input
through R99 and R20. The voltage signal is then routed through U30 and divided by 10
at U30-87. A 1V dc bias is added to the signal at TP29. With the gyrator, AC, and peak
circuits turned off (U14-10 and U14-11 asserted high, U14-9 asserted low), the signal is
routed through U9 to TP19 and into U10-16.
The current readings are obtained through a virtual ground connection between common
and the measurement ground (GND3). Any current that flows through a device under test
flows into the I-V amp on U30, creating a voltage proportional to the current. For a short
circuit between
Ë
and
â
, the voltage at TP28 is 309 mV rms
±
12%. This
voltage is routed through U30-87. Like the voltage measurement, a 1V dc bias is added
to the signal at TP29; the gyrator, AC, and peak circuits are turned off, and the signal is
routed through U9 to TP19 and U10-16.
The current and voltage readings are taken on separate cycles of the oscillator stimulus.
The switching between voltage and current takes place entirely in U30.
The two component test modes are described as follows:
•
In normal mode, a lissajous is taken every four cycles (for high frequencies, the
lissajous requires more cycles due to settling and processing time). On the first cycle,
U30 is configured for voltage, and the front end is allowed to settle. On the next
cycle, the voltage scan data is acquired and stored in U24. During the third cycle, the
voltage data is read by the microprocessor (U25), U30 is configured for current
readings, and the front end is again allowed to settle. The fourth cycle is used for
collection of the current data by U24.
•
In Touch Hold, an additional two cycles are required to complete an “open leads”
check.
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