Fluke 5720A service manual 2808 A11 ADC Amp Gain Error, 2809 A11 DAC Monitoring Fault

Troubleshooting 5

Interpreting Diagnostic Fault Codes

2808 A11: ADC Amp Gain Error

The DAC is set to 0.1V on the 11V range and connected to the +input of the DACs adc amplifier circuit and the -input is connected to common. The adc amplifier has a nominal gain of 11 resulting in 1.1V at the adc amplifier output. This gain is checked to be 11 ±5%. A failure indicates the DAC assembly (A11) is probably at fault.

2809 A11: DAC Monitoring Fault

The DAC is set to 1.0V and routed to the adc circuit via DAC HI DIAG. This 1V from the DAC is divided by R70 and R84 on the DAC before going to the adc circuit where it is measured. The DAC is then set to 10V and another reading is taken by the adc. The exact gain of the divider resistors can now be determined by the formula:

Gain = (10-1V) / (adc10-adc1)

A failure occurs if the detected gain is not within 10% of the nominal 22.13. A failure indicates the DAC assembly (A11) is probably at fault.

2810 A11: +11V DC Range Fault

The instrument is configured in the +11V dc range. The DAC is sequentially set to 0V, 1V, and 10V. The DAC output is measure at each setting to be within 7% ±10 mV via DAC HI DIAG by the adc circuit. A failure indicates the DAC assembly (A11) is probably at fault.

2811 A11: -11V DC Range Fault

The instrument is configured in the -11V dc range. The DAC is sequentially set to 0V, -1V, and -10V. The DAC output is measure at each setting to be within 7% ±10 mV via DAC HI DIAG by the adc circuit. A failure indicates the DAC assembly (A11) is probably at fault.

2812 A11: +22V DC Range Fault

The instrument is configured in the +22V dc range. The DAC is sequentially set to 0V, 2V, and 21.9V. The DAC output is measure at each setting to be within 7.2% ±10 mV via DAC HI DIAG by the adc circuit. A failure indicates the DAC assembly (A11) is probably at fault.

2813 A11: -22V DC Range Fault

The instrument is configured in the -22V dc range. The DAC is sequentially set to 0V, -2V, and -21.9V. The DAC output is measure at each setting to be within 7.2% ±10 mV via DAC HI DIAG by the adc circuit. A failure indicates the DAC assembly (A11) is probably at fault.

2814 A11: 6.5V Buffered Reference Fault

The DAC's 6.5V buffered reference (BRF6 and BSRF6) is connected to the -input of the adc circuit and the DAC output (DAC SENSE CAL) is connected to the +input. The 6.5V buffered reference is measure by adjusting the DAC until a null is achieved. This fault occurs if the 6.5V buffered reference is over 10% of the nominal. A failure indicates the DAC assembly (A11) is probably at fault.

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