Fluke 8842a instruction manual Maintenance

Maintenance 6

TROUBLESHOOTING

Configures the 8842A in the mA DC function and the slow reading rate, and measures the reading across the 0.1Ω current shunt. This test should be fairly immune to outside noise because the total driving impedance is typically less than 1 kΩ. The reading is not a perfect zero because of the offsets generated by charge injection of U302 and the T/H Amplifier (X10 configuration).

∙TEST 5: 200 VDC, Zero

Configures the 8842A in the 200V dc range and slow reading rate. Input noise is divided by 100. Assuming no input of any kind, the T/H Amplifier is essentially shorted to ground by 100 kΩ and filtered by the 3-pole analog filter. Any non-zero reading under quiet input conditions is due to the offset of the T/H Amplifier (X1 configuration).

∙TEST 6: 1000 VDC, Zero

Configures the 8842A exactly as in the 1000V dc range and slow reading rate, with input noise being divided by 1000. The reading is very close to zero because of the inherent 2 kΩ driving impedance to the T/H Amplifier (X1 configuration).

∙TEST 7: 1000 VDC + 20 MΩ

Couples the 1000V dc range and 20 MΩ current source together. The result is nominally 500 nA through the 10 MΩ input divider. Since the 1000V dc range senses this voltage at the divide-by-1000 point of the scaling circuit, the reading should be 5 mV, or 500 counts at the A/D Converter. This test could indicate an error if input capacitance is greater than 1000 pF.

∙TEST 8: 20 VDC + 20 MΩ

Puts the DC Scaling circuit into the 20V dc range and the Ohms Current Source into the 20 MΩ range. The infinite input impedance of the 20V dc range causes the 20 MΩ current source to be clamped at its maximum open circuit voltage, typically 12V. The 20V dc range scales this voltage and presents the A/D Converter with 1.15V, or 115,000 counts.This is a good test to ensure that the maximum open-circuit voltage of the Ohms Current Source is less than 13V. This test is susceptible to capacitance greater than 0.01 μF at the input terminals.

∙TEST 9: 20 VDC + 2000 kΩ

Puts the DC Scaling circuit in the 20V dc range and the Ohms Current Source in the 2000 kΩ range. The infinite input impedance of the 20V dc range causes the 2000 kΩ current source to be clamped at typically 11.5V. The reading at the A/D Converter should be 1.15V. Again tests that the maximum open-circuit voltage of the Ohms Current Source is less than 13V. Capacitances greater than 0.1 μF at the input terminals can cause an error.

∙TEST 10: 2 VDC + 2000 kΩ

Puts the DC Scaling circuit in the 2V dc range and the Ohms Current Source in the 2000 kΩ range, except that its maximum open-circuit voltage is limited to less than 6.5V in this configuration. This test, as well as tests 11-13, checks clamps Q312 and Q313 and the analog filter. The reading at the A/D Converter should be an overload. Capacitances greater than 0.5 μF at the input terminals can cause an error.

∙TEST 11: 200Ω, Overrange

∙TEST 12: 2 kΩ, Overrange

∙TEST 13: 20 kΩ, Overrange

∙TEST 14: 200 kΩ, Overrange

6-43