Keithley TEC SourceMeter 2510, Service Manual, Page: 68 / 120

Models 2510 and 2510-AT Troubleshooting 4-11
A constant current is forced through the DUT (thermistor) as well as a reference resistor (in
this example, 10k
Ω
for the 10k
Ω
range). As various phases of the measurement cycle, the volt-
ages across the DUT and 10k
Ω
reference resistor are measured, and the actual DUT resistance
is computed from the ratio of the two voltages. Since the resistance of the 10k
Ω
reference
resistance is accurately known (as determined during instrument calibration), the DUT resis-
tance and a given temperature can be accurately determined, and internal software converts the
resistance into an equivalent temperature reading.
A/D converter
The Model 2510 unit uses a multi-slope charge balance A/D converter with a single-slope
run-down. The converter is controlled by a gate array. Commands are issued by the MPU to the
gate array, and the gate array IC sends A/D reading data back to the MPU for calibration and
processing.
PID hardware loop control
Figure 4-7 shows an overall diagram of the hardware aspects of the PID control loop.
68332
HDAC
U127
ADC
2
Ω
MUX
U142
U147
ITHP
0.1
Ω
VTHP
REF
ZERO
THP_FBK
PC_TEMP
+15V MEAS
TEMP_REF
“D”
Amplifier
U151
Figure 4-7
PID hardware control loop
2510-902-01 (E - Mar 2010)(Service).book Page 11 Tuesday, March 16, 2010 1:04 PM