Keithley SourceMeter 2400, Руководство по обслуживанию, Страница: 58 / 137

2400-902-01 (G - Feb 2006)
Model 2400 Service Manual 4-4
Troubleshooting
CHAR SET test
The character set test lets you display all characters. Perform the following steps to run the
character set test:
1. Display the MAIN MENU by pressing the MENU key.
2. Select TEST, and press ENTER to display the SELF-TEST MENU.
3. Select DISPLAY-TESTS, and press ENTER to display the following menu:
FRONT PANEL TESTS
KEYS DISPLAY-PATTERNS CHAR-SET
4. Select CHAR-SET, and press ENTER to start the character set test. Press any key
except EXIT to cycle through all displayable characters.
5. When finished, abort the character set test by pressing EXIT. The instrument returns to
the SELF-TEST MENU. Continue pressing EXIT to back out of the menu structure.
Principles of operation
The following information is provided to support the troubleshooting tests and procedures cov-
ered in this section of the manual. Refer to the following drawings:
Figure 4-1 — Analog circuitry overall block diagram
Figure 4-2 — Power supply block diagram
Figure 4-3 — Output stage simplified schematic
Figure 4-4 — Digital circuitry block diagram
Analog circuits
Figure 4-1 shows the overall block diagram for the Model 2400.
D/A converters control the programmed voltage and current, or voltage compliance and current
compliance. Each DAC has two ranges, a 10V output or a 1V output. The DAC outputs are fed
to the summing node, FB. Either the V DAC or the I DAC has the ability to control the main
loop. If the unit is set for SV (source voltage), it will source voltage until the compliance cur-
rent is reached (as determined by the I DAC setting), and the current loop will override the volt-
age loop. If, however, the unit is set for SI (source current), it will source current until the
compliance voltage is reached (as determined by the V DAC setting), and the voltage loop will
override the current loop. A priority bit in the Vclamp/I clamp circuit controls these functions.
The error amplifier adds open-loop gain and slew-rate control to the system to assure accuracy
and provide a controllable signal for the output stage, which provides the necessary voltage and
current gain to drive the output. Sense resistors in the HI output lead provide output current
sensing, and a separate sense resistor is used for each current range. The 1A range uses 0.2V
full-scale for a full-range 1A output, while all other ranges use 2V output for full-scale current.
Voltage feedback is routed either internally or externally.