Keithley SourceMeter 3A 2420, Service Manual

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4-6 Troubleshooting
D/A converters control the programmed voltage and current, or voltage compliance and current
compliance. Each DAC has two ranges, a 10V full-scale output or a 1V full-scale 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
current 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 compli-
ance 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 accura-
cy 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 and 3A ranges use
0.2V full-scale for a full-range output, while all other ranges use 2V output for full-scale current.
Voltage feedback is routed either internally or externally.
There are four voltage ranges: 0.2V, 2V, 20V, and 60V. The feedback gain changes only for
the 20V and 60V ranges, resulting in three unique feedback gain values. A multiplexer directs
the voltage feedback, current feedback, reference, or ground signal to the A/D converter. An
opto-isolated interface provides control signals for both DACs, analog circuit control, and A/D
converter communication to the digital section.
Power supply
Figure 4-3 shows a block diagram of the Model 2420 power delivery system.
Analog Board
+15V +5V F -15V
F F F
Output Stage
-85V -42V +42V +85V O
Constant Frequency
Low Noise Floating
Switching Supply
+24Vdc
D
Line
Neutral
+5Vdc
D
+12Vdc
PFC
DC/DC
Converter
D
Digital
Circuits
+24dc
DC
Regulators
Figure 4-3
Power supply block diagram