Fluke 8842a instruction manual 66. A/D Converter Troubleshooting, Output of A/D Amplifier TP101

Maintenance 6

TROUBLESHOOTING

6-66. A/D Converter Troubleshooting

If there is a failure of the A/D Converter, all power supply levels should be checked at the op amps (U102 and U103) and the A/D IC (U101). The A/D Converter has a total of seven supplies: +15V, -15V, +5V, +7.5V, -8.2V, +7.00000V, and -7.00000V. All supplies should be within 5% of their nominal values except for the +7.00000V and - 7.00000V reference supplies, which should be within +/-1000 ppm and +/-250 ppm respectively. The bootstrap supplies (lines BS1 and BS2) should be +7V and -7V (+/- 10%) referenced to the + input of the A/D amplifier (U103-3).

Troubleshooting the bootstrap supplies can often be made easier by putting the 8842A in EX TRIG (to stop the A/D Converter) and connecting the input of the A/D Converter (TP103) to INPUT LO (Reference Low on the schematic). The bootstrap supplies are then referenced to instrument common (Reference Low).

NOTE

For the following tests, set the 8842A to the VDC function and the 2V range, and trigger the oscilloscope from the falling edge of line not-TR (TP201).

If all supplies are correct, the next most useful troubleshooting tool is the A/D output waveform at TP101, which can be checked with an oscilloscope. The waveform should look like the one shown in Figure 6-14 when the input voltage is at 50% of the selected range. Various portions of the waveform correspond to different parts of the A/D cycle. By examining the waveform, problems in the A/D Converter can be isolated down to one or two components.

f6-14.wmf

Figure 6-14. Output of A/D Amplifier (TP101)

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