Model 1019 - Keithley 181 Service Manual

I
MODEL 181
THEORY OF OPERATION
FIGURE 4-7. NVPA Showing Filter and Drift Correction
4-22 Drift Correction
The offset resistors A423 and R424 bring the amplifier ~ni" the!
operating rang". The "f&et can only be nulled 1" 2m" referred
to the input. At the output on X1000. 2V can be presents The
AID would be overloaded by i:2,4V which means the 2V
component must be offset t" 0" for OV,",
This offset c"rrec,,"n IS pr"v,ded by C404. 0406 and low
leakage buffer U4OlE. When Q420 IS on and when th" A/D IS
not lookIng at preamp zer" (during the A/D zero cycle1 Cl406
turns on and stores the offset voltage present on the DC
amplifier output "n C404. Then this capacitor holds this value
during the next two conversions 1.5 secj until it 1s updated.
This correction occurs two hmes a second. Leakage off this
capac~toi will yield offsets on the 200 m,lliv"l, range wiih ,nput
shorted. The major swrce of leakage is U4OlB and 0406.
These two devices are selected for leakage to mantain less
than 15 microvolls of drift over a conversion cycle of 50
millisecond. This is the time it takes 1" do a full preamp
signal/zero conversion. C405 prevents preamp slewing ibet~
ween signal and zero FET switching) from affecting the charge
on c404
4-23 Input Protection
CR402 and CR403 prevent input stage foldover which would
severely drop input impedance during overload. This also
prevents long recovery time caused by unbalancing power
dissipation between Q413A and 04138. 0410 and Q411 pre-
vent the amplifier from getting lost during a range change or
severe overload by clamping the summing junction at * 1V.
CR&J, CR405, R427 and R428 provide a ground referenced
f0.7V threshold for the protection circuitry. High impedance
( ) 10Q ohm1 is not affected by the protection circuit for input
voltages less than 1 volt because of low leakage FET pair Q415
and 0416. However, for inputs greater than 1 volt. the input
""Itage is clamped and current limit is provided through R436.