Service Information
6-4
Offset current tests
These tests check leakage current between HI (H) and LO
(L) (differential offset current) and from HI (H) and LO (L)
to chassis (common-mode offset current) of each pathway. In
general, these tests are performed by measuring the leakage
current with an electrometer. In the following procedure, the
Model 6517A is used to measure the leakage current.
Referring to Figure 6-2, perform the following procedure to
check offset current:
1. Turn off the Model 7001/7002 if it is on, and remove any
jumpers or wires connected to the card.
2. Connect the triax cable to the Model 6517A, but do not
connect it to the card at this time.
3. Turn on the Model 6517A and allow the unit to warm up
for two hours before testing. After warm up, select the
200pA range, enable zero check, and zero correct the in-
strument. Leave zero correct enabled for the entire pro-
cedure.
4. Connect the triax cable to bank A HI and LO, as shown
in Figure 6-2A.
5. Install the Model 7021 in slot 1 (CARD 1) of the main-
frame.
6. Turn on the Model 7001/7002.
7. Program the unit to close channel 1!1 (bank A, input 1).
8. On the Model 6517A, disable zero check and allow the
reading to settle. Verify that the reading is <100pA. This
measurement is the offset (leakage) current of the path-
way.
9. Enable zero check on the Model 6517A and open chan-
nel 1!1 from the front panel of the mainframe.
10. Repeat the basic procedure in steps 7 through 9 to check
the rest of the pathways of bank A (channels 1!2, 1!6,
1!7, 1!11, 1!12, 1!16, 1!17, 1!21, 1!22, 1!26, and 1!27).
11. Turn off the mainframe and change the electrometer
connections to bank B.
12. Repeat the basic procedure in steps 7 through 10 to
check the bank B inputs (channels 1!3-1!5, 1!8-1!10,
1!13-1!15, 1!18-1!20, 1!23-1!25, and 1!28-1!30).
13. Turn off the Model 7001/7002 and change the electro-
meter connections, as shown in Figure 6-2B. Note that
electrometer HI is connected to HI and LO of the bank
A output, which are jumpered together. Electrometer
LO is connected to chassis.
14. Turn on the Model 7001/7002.
15. Repeat steps 7 through 12 to check that the common-
mode offset current is <100pA.
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