HYDRA

Service Manual

A3U8 switch S2, and the LO SENSE path of A3R35 and Analog Processor switch S19. Passive filtering is provided by A3C34, A3C27, and portions or all of the DC Filter block.

The voltage across the reference resistor for the 300Ω and RTD, 3-kΩ, and 30-kΩranges (the 1-kΩ, 10.01-kΩ, and 100.5-kΩresistances in A3Z4, respectively) is integrated for a variable period of time until the voltage across the integrate capacitor reaches zero. For the 300Ω and RTD range, the reference resistor voltage is switched in through Analog Processor switch S6 and applied to the A/D Converter by switch S8. For the 3-kΩrange, switches S9 and S11 perform these functions, respectively. For the 30- kΩ range, switches S13 and S14 are used. For all ranges, the voltage is routed through A3R34 to the RRS input.

The reference resistor for the 300-kΩ, 3-MΩ, and 10-MΩranges is the 1-MΩresistor in A3Z4, which is selected by S15. The voltage across this reference is integrated during its own minor cycle(s) and is switched to a passive filter and the A/D Converter by switches S1 and S18.

When 4-wire measurements are made on any of the six ranges, separate Source and Sense signal paths are maintained to the point of the unknown resistance. The 4-wire Source path measurement current is provided by the A3U8 ohms source through one of the A3U8 internal switches (S6, S9, S13, or S15) and the appropriate reference resistor in A3Z4. The current flows through relay A3K16, thermistor A3RT1, resistor A3R10, the HI Source instrument relay contacts (A3K1 - A3K3, A3K5 - A3K14), and the HI Source lead wire, to the unknown resistance to be measured. The current flows back through the LO Source lead wire, the LO Source path of the instrument relays (A3K1 - A3K3, A3K5 - A3K14), resistor A3R43, and analog ground, to the A3U8 ohms source.

The voltage that develops across the unknown resistance is sensed through the other 2 wires of the 4-wire set. HI is sensed through the HI Sense path made up of the users HI Sense lead wire, the HI Sense contacts in the instrument relays, resistor A3R11, relay A3K17, resistor A3R42, and Analog Processor A3U8 switch S2. LO is sensed through the users LO Sense lead wire, the LO Sense contacts in the instrument relays, protection resistor A3R35, and A3U8 switch S19.

Since virtually no current flows through the sense path, no error voltages are developed that would add to the voltage across the unknown resistance; this 4-wire measurement technique eliminates user lead-wire and instrument relay contact and circuit board trace resistance errors.

2A-55.AC Volts

AC-coupled ac voltage inputs are scaled by the ac buffer, converted to dc by a true rms ac-to-dc converter, filtered, and then sent to the a/d converter.

Refer to Figure 2A-6. Input HI is switched to the ac buffer by dc-blocking capacitor A3C31, protection resistor A3R11, and latching relay A3K15. Resistor A3R44 and A3K15 act to discharge A3C31 between channel measurements. LO is switched to the A3U8 A/D Converter through A3R34 and S18.

2A-28

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Fluke 2635A, 2625A service manual 2A-55.AC Volts
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2635A, 2625A specifications

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