Keithley 6517 Service Manual page 82

V
IN
A. Volts Function
I
IN
B. Amps Function
Q
IN
C. Coulombs Function
R
X
V Source
V
SRC
D. Ohms Function
Figure 5-6
Preamp configurations
Offset compensation
Although the input preamplifier has low offset voltage and
input bias current, offset compensation circuitry is included
to further diminish any residual offsets. The offset compen-
sation circuitry consists of a DAC and buffer amplifiers.
During the offset correction process, offset parameters are
read by the MPU, and offset compensation constants are
calculated and stored for reference. During normal
operation, serial offset compensation data from the MPU is
+
V
OUT
-
V V
OUT = IN
R
F
-
V
OUT
+
V I R
OUT = -
IN F
C
F
-
V
OUT
+
-Q
V
IN
OUT =
C
F
R
F
-
V
OUT
+
-V R
V
SRC
OUT =
R
X
converted into parallel data by U403 and U404, and fed to a
12-bit DAC, U407. After the digital-to-analog conversion
process, offset signals are buffered by U401 and U402 and
applied to appropriate nodes of the preamplifier input stage
to null offsets.
Prescaler
A prescaler circuit, made up of U215 and various feedback
elements, selects the overall signal gain depending on
selected range and function. Analog switches located in
U210 and U211 select among the six feedback resistors
R219-R224 to set the U215 gain factor. Prescaler absolute
voltage gains of × 0.05, × 0.5, and × 5 are available depending
on the pair of feedback resistors selected.
2V analog output
The 2V analog output provides a 0-2V output voltage analo-
gous to the input signal. U209 is the 2V analog output buffer
amplifier, and R214 and R215 set the absolute gain of U209
to × 0.2.
Multiplexer
Elements of U211 and U212 form the signal multiplexer that
switches among the various signals to be applied to the A/D
converter input. In addition to the preamp, temperature, and
humidity signals, the multiplexer also switches among refer-
ence and zero signals at various phases of the measurement
cycle.
A/D input buffer
F
The A/D input buffer U214 is the final circuit used to process
the signal before it is applied to the A/D converter input.
Note that the voltage gain of U214 is set to unity.
Temperature and humidity circuits
Both the temperature and humidity inputs require additional
buffering and amplification. U504 and associated compo-
nents provides these functions for the temperature input,
while U502 and corresponding components perform similar
functions for the humidity input.
Since both the temperature and humidity input voltages are
referenced to digital common, special circuits are necessary
to isolate the analog and digital sections. U501 and one-half
of U213 provide the necessary isolation for the humidity sig-
nal, while U503 and the other half of U213 isolate the tem-
perature signal.
Troubleshooting
5-9