Keithley 7077, Instruction Manual

Page: 48 / 62

Service Information
4-10
4.4.6 Differential and common model isolation
tests
Differential and common mode isolation tests check the
leakage resistance (isolation) between high (H) and guard
(G) (differential), and from high and guard to chassis (com-
mon mode) of every row and column. The tests are per-
formed by applying a voltage (42V) across the terminals and
then measuring the leakage current. The isolation resistance
is then calculated as R = V/I. In the following procedure, the
Model 6517 functions as a voltage source and an ammeter.
In the V/I function, the Model 6517 internally calculates the
resistance from the known voltage and current levels and dis-
plays the resistance value.
1. Remove all power from the switching matrix and
switching matrix cards.
2. Install the Model 7077 in the Model 708 or in slot 1 of
the Model 707.
3. Connect the Model 6517 to ROW A as shown in Figure
4-6 to measure differential isolation. Make sure the volt-
age source is in standby and there are no other connec-
tions to the card.
4. On the Model 6517, select the 20pA range, and enable
zero check and then zero correct. Leave zero correct en-
abled for the entire procedure.
5. On the Model 6517, set the voltage source for +42V, and
select the 200nA current range. Make sure the voltage
source is still in standby.
6. Place the Model 6517 in the V/I measurement function
by pressing SHIFT OHMS.
7. Turn on the Model 707/708, but do not program any
crosspoints to close. All crosspoints must be open.
8. On the Model 6517, disable zero check, and press
OPERATE to source 42V.
9. After allowing the reading on the Model 6517 to settle,
verify that it is >1G
Ω
. This measurement is the leakage
resistance (isolation) of ROW A.
10. Place the Model 6517 in standby and enable zero check.
11. Program the Model 707/708 to close crosspoint A1.
12. On the Model 6517, disable zero check and press
OPERATE to 42V.
13. After allowing the reading on the Model 6517 to settle,
verify that it is also >1G
Ω
. This measurement checks
the isolation of COLUMN 1.
14. Using Table 4-3 as a guide, repeat the basic procedure of
steps 11 through 14 for the rest of the columns and rows
(test numbers 3 through 32 of the table).
15. Place the Model 6517 in standby, and turn the Model
707/708 off.
16. Connect the Model 6517 to ROW A as shown in Figure
4-7, and repeat steps 8 through 16 to check common
mode isolation.