Keithley 486 Instruction Manual page 146

SECTION 6
Theo y
of
operafion
i c147 1
ID)
Ft206
I
R214 R211
! 1 LJ146 we'!'
.
Multiplexer
! Anal og Filter
" 1.."
L-------------------_______________I
* See table for values
of CF and RF
Figure 6-5. Anslog Filter Circuitry
filter is enabled when the ANALOG or ANALOG+DIGI-
TAL filter is selected and enabled (front panel FILTERin-
dicator on). Table 3-6 in Section 3 of this manual lists the
effects of the analog filter on rise time and settling time.
After filtering, the conditioned voltage signal is routed to
the multiplexer via a xl buffer W146). The buffer Mates
the input stage
from
the analog output circuitry.
6.2.3
Multiplexer
The multiplexer circuitry sekts among the various sig-
nals that are part of the instrument measurement cycle
and connects them to the A/D converter. The multi-
plexer circuit is shown in Figure 6-6. Device U132 is not
psrt of the multiplexer but serves as a xl buffer to isolate
the A/D converter from the rest of the analog circuitry.
Figure 6-7 shows the general switching phases for the
various signals. During each phase, an integration is per-
formed by the A/D converter, and the resultant data is
used by the microprocessor to calculate the final reading.
6.2.4 +2.6V Reference
Current measuemenis are based on comparing the un-
lawwn signal with an internal +2&V reference. During
each measurement cycle, the unknown signal is sampled
and then compared with signal common and the +2.W
reference values.
The +2.8V reference is derived from a 6.4V reference
source that is shown on sheet 2 of schematic 486-106 or
487-106. VR103 provides a highly stable 6.4V reference,
while U114 and R147 provide a constant current to mini-
mize zener voltage variations. R188 and R192 (on sheet 1
of the schematic) divide down the 6.4V value to the final
+2.8V reference.
6-3