Fluke 8842a Automatic Settling Time Delay, External Trigger Input Option -05 Only

8842A

Instruction Manual

In the continuous trigger mode, the actual number of readings displayed per second for each reading rate is determined by the line-power frequency. At power-up, the 8842A senses the line-power frequency and adjusts the analog-to-digital converter timing characteristics for optimum normal-mode noise rejection. The resulting reading rates are shown in the specifications in Section 1.

2-21. Automatic Settling Time Delay

When the external trigger mode is selected, the 8842A automatically inserts a delay after receiving a trigger signal, but before starting the first input sample. The delay is just long enough so that the reading will be correct (within a specified number of counts of the final value) even if the trigger signal occurs as the input makes a step change between zero and full scale (10,000 counts and full scale in the ac functions).The length of the delay depends on the range, function, and reading rate, as shown in the specifications in Section 1. The delay is enabled only in the external trigger mode. It can be turned off with a remote command over the IEEE-488 interface bus to accommodate special timing considerations.

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Figure 2-8. Overrange Indication

2-22. External Trigger Input (Option -05 Only)

The rear panel EXT TRIG input is a TTL-level input which can be used to trigger measurements when the 8842A is in the external trigger mode. A measurement is triggered on the falling edge of the input. Since the EXT TRIG input is pulled high internally, it can also be controlled by a normally open switch. A measurement is triggered when the switch is closed. For special applications using the IEEE-488 Interface, the automatic setting time delay can be disabled using remote commands. (See Section 3.) Refer to Section 1 for timing details.

The polarity of the EXT TRIG input can be reversed by changing internal jumpers. Refer to the Maintenance section for instructions.

2-23. Sample Complete Output (Option -05 Only)

The SAMPLE COMPLETE output indicates when analog input sampling for a reading is completed. The output is a TTL-level signal which is pulsed low for approximately 2.5 μs when the input-sampling portion of the A/D conversion is completed. The signal is useful for interfacing with other equipment when the 8842A is used in external trigger mode in an instrumentation system. For example, the SAMPLE COMPLETE output could be used to advance a scanner to the next channel.

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