Shielded Fixture Construction; External Feedback Procedure - Keithley 6512 Instruction Manual

Operation
2.9.2 Shielded fixture construction
Since shielding is so critical for proper operation of external
feedback, it is recommended that a shielded fixture similar to
the one shown in Figure 2-16 be used to house the feedback
element. The fixture is constructed of a commercially avail-
able shielded fixture modified with the standard BNC con-
nectors replaced with triaxial female connectors. For
convenience, a banana jack can be mounted on the box to
make the necessary PREAMP OUT connection. Alternately,
a wire could be run through a rubber grommet mounted in a
hole in the side of the box. Note that input low is connected
to chassis ground within the shielded box. This connection
can be made by using a small solder lug secured with a
screw.

2.9.3 External feedback procedure

Use the following procedure to operate the Model 6512 in
the external feedback mode.
5
TO 6512 INPUT JACK
HI
LO
GND
ITEM DESCRIPTION
1 SHIELDED FIXTURE
2 FEMALE TRIAXIAL
3 BANANA JACK
4 TRIAXIAL CABLE
5 TRIAXIAL CABLE
Figure 2-16
Shielded fixture construction
2-22
LO
SOLDER LUG
2
HI
TO PREAMP OUT
FEEDBACK
ELEMENT
SHIELDED
FIXTURE
237-ALG-2
7078-TRX-3
CABLE
PARTS LIST
MFR. PART NUMBER
POMONA #2390
KEITHLEY 7078-TRX-TBC
KEITHLEY BJ-9-2
KEITHLEY 237-ALG-2
KEITHLEY 7078-TRX-3
1. Connect the feedback element between the PREAMP
OUT terminal and the Input High terminal.
2. Place the instrument in the external feedback mode by
pressing the SHIFT then VOLTS buttons in that order.
The AMPS and VOLTS indicators will illuminate simul-
taneously in the external feedback mode.
3. The display will show the voltage measured at the out-
put of the input preamplifier (PREAMP OUT). Howev-
er, the "V" exponent will not appear as in the volts
mode. For example, with a 150mV output the display
will show:
(External feedback may be temporarily digitally cali-
brated as outlined in paragraph 7.4.16.)
4. The external feedback mode may be cancelled by press-
ing one of the four functions keys (VOLTS, OHMS,
COUL, or AMPS).
INPUT LOW (INNER SHIELD)
1
INPUT HIGH (CENTER CONDUCTOR)
FEEDBACK ELEMENT
3
A. CONSTRUCTION
PREAMP OUT
HI
LO
GND
S
CABLE
B. EQUIVALENT CIRCUIT
150.00 m
2
4
FROM SIGNAL
TO RANGING
AMP AND A/D
6512 INPUT AMP