Fluke 8842a instruction manual 8842A

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.

∙TEST 15: 1000 VDC + X10 T/H + 20 MΩ

Puts the Ohms Current Source in the 500 nA range. The resulting current through Z302 (the 10 MΩ 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.

∙TEST 16: 200 VDC + 200 kΩ

∙TEST 17: 200 VDC + 20 kΩ

∙TEST 18: 200 VDC + 2 kΩ

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 19: 200 VDC, Filter On

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 μC waits

28ms and determines if the voltage at Z302-3 (the divide-by-100 point of the 10 MΩ

input divider) has not discharged to zero volts, due to the long time constant of the filter.

∙TEST 20: 200 VDC + 2 kΩ, Filter Off

∙TEST 21: 200 VDC, Filter Off

∙TEST 22: x100 T/H

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 μC waits 28 ms and determines if the voltage at Z302-3 is at zero volts.

∙TEST 25: In-Guard μC Internal RAM (U202)

A GALPAT test is performed on the internal RAM of the In-Guard μC. If there are any errors, ERROR 25 is displayed. This test is performed only upon powerup.

∙TEST 26: Display RAM (U212)

A pattern is written to the Display RAM and read back for comparison.If there are any differences ERROR 26 is displayed.

∙TEST 27: In-Guard μC Program Memory (U202)

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.

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