Fluke 8842A Instruction Manual page 181

8842A
Instruction Manual
These tests put the 8842A in the respective range of the 2-wire ohms function. They
check that each range of the Ohms Current Source has enough compliance voltage to
overload the dc front end.
Puts the Ohms Current Source in the 500 nA range. The resulting current through Z302
(the 10 M input divider) causes a nominal divider output voltage of 50 mV. The T/H
Amplifier is in X10; thus the A/D Converter sees 50 mV, or 5000 counts. This test can be
susceptible to input capacitances above 1000 pF.
These three tests put the DC Scaling circuit in the 200V dc range and the Ohms Current
Source in the respective current range. The 10M ohm input divider (Z302) senses the
maximum open-circuit voltage of each range of the current source, and the T/H Amplifier
(X1) presents the compliance voltage divided by 100 to the A/D Converter. Nominal
readings should be 49 mV for Test 16, 53 mV for Test 17, and 59 mV for Test 18. All
three tests have a pass limit of 65 mV, insuring that no more than 6.5 volts appears at the
input terminals.
Test 18 (above) ties the 1 mA range of the Ohms Current Source into the 200V dc range,
with the three-pole analog filter on, such that the A/D reads 59 mV nominal. Test 19
decouples the DC Scaling circuit from the Ohms Current Source; the In-Guard C waits
28 ms and determines if the voltage at Z302-3 (the divide-by-100 point of the 10 M
input divider) has not discharged to zero volts, due to the long time constant of the filter.
The T/H Amplifier is configured in x100 mode. DC stimulus of 5 mV (similar to test 15)
is amplified to 500 mV, which is applied to the A/D converter. This test can be
susceptible to input capacitance above 1000 pF.
Test 20 ties the 1 mA range of the Ohms Current Source into the 200V dc range, with the
3-pole analog filter off, such that the A/D Converter reads 59 mV nominal. Test 21 then
decouples the DC Scaling circuit from the ohms current source; the In-Guard C waits 28
ms and determines if the voltage at Z302-3 is at zero volts.
A GALPAT test is performed on the internal RAM of the In-Guard C. If there are any
errors, ERROR 25 is displayed. This test is performed only upon powerup.
A pattern is written to the Display RAM and read back for comparison.If there are any
differences ERROR 26 is displayed.
A two-byte check sum is calculated over the entire 4K Internal Program Memory and
compared with the checksum bytes at the end of that memory. A special add and shift
algorithm minimizes the possibility of double errors cancelling. If something is wrong
with the Internal Program Memory, ERROR 27 is displayed.
6-44
TEST 15: 1000 VDC + X10 T/H + 20 M
TEST 16: 200 VDC + 200 k
TEST 17: 200 VDC + 20 k
TEST 18: 200 VDC + 2 k
TEST 19: 200 VDC, Filter On
TEST 20: 200 VDC + 2 k , Filter Off
TEST 21: 200 VDC, Filter Off
TEST 22: x100 T/H
TEST 25: In-Guard C Internal RAM (U202)
TEST 26: Display RAM (U212)
TEST 27: In-Guard C Program Memory (U202)