Equipment For Zero Hop Compensation - Keithley 595 Instruction Manual

1. Disconnect alI input~signals from the instrument and cap
the METER INPUT connector. Make sure the GUARD
to chassis ground link is installed.
2.Remove the two screws securing the top cover and
remove the cover from the instrument.
3. Select the current function and place the instrument on
the 2OpA range.
4. Enable ZERO CHECK.
5. Using the DMM set to the 2OOniVDCrange, monitor Test
Point 1 (TPl) on the preamp board (see Figure 8-2)
referenced to GUARD. Locate the offset adjustment
potentiometer (R315) on the preamp boards The pot is
accessible through a small hole in the shield closest to
the rear of the instrument.
6. Adjust R.315 for a reading of OOO.OOOmV ~.OlOmV on the
DMM.
8.5.10 Zero Hop Compensation
Zero hop is adjusted at the factory and need not be per-
formed unless components in the compensation circuitry
were replaced or if pot R303 was accidentally adjusted.
Equipment Needed: Oscilloscope with a Xl probe and an
RC filter. See Figure 8-3 for filter component values and
Setup.
Perform the following procedure to adjust zero hop:-
1. Set the Model 595 to the capacitance function, 200pF
range, 0.07 S delay, 0.05 SQ Step V and enable ZERO
CHECK.
2. Connect the l8OpF capacitor to the input of the Model
595.
3.
Turn pot R303 (see Figure 8-2) fully counter clockwise.
4. Set the scope to O.lV/div., DC coupled and 20mSld.iv.,
and monitor the waveform at Tl'l withes respect to
GUARD.
5. Adjust the scope so that the straight line trace is at the
center of the CRT (OV).
6.
Disable ZERO CHECK and turn R303 clockwise until the
adjustable portion of the trace is at OV.
7. Enable ZERO CHECK, set the scope sensitivity to
5mV/div and again center the straight line trace to the
center of~~~the CRT (OV).
8. Disable ZERO CHECK and adjust R303 until the ad-
justable portion of the trace is within one division
(rt5mV) of the OV line,
El
HI
PROBE
LO
L---J
SCOPE
Figure 8-3. Equipment for Zero Hop Compensation
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