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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
∙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
∙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
∙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
∙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
∙TEST 11: 200Ω, Overrange
∙TEST 12: 2 kΩ, Overrange
∙TEST 13: 20 kΩ, Overrange
∙TEST 14: 200 kΩ, Overrange