Keithley SourceMeter 2425, Руководство по обслуживанию

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Model 2425 Service Manual Troubleshooting 4-7
Analog circuits
Figure 4-2 shows a block diagram of the analog circuits.
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 voltage 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 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.
Figure 4-2
Analog circuitry block diagram
V DAC
I DAC
Control
-42 +42
O
+
-
V Clamp
I Clamp
Error
Amp Output
Stage
Sense
Resistors
FB
VFB
IFB
VFB
IFB
MUX
+7
S
A/D
-150 +150
Output
HI
S+
Output
LO
S-
O
Remote
Protection
Protection
Guard
Out
Guard
Sense
O
There are four voltage ranges: 0.2V, 2V, 20V, and 100V. The feedback gain changes only
for the 20V and 100V ranges, resulting in three unique feedback gain values. A multi-