Fluke 5720A System Interconnect Detailed Circuit Description, Ohms Functional Description

Theory of Operation 2

System Interconnect Detailed Circuit Description

2-15. Ohms Functional Description

Two assemblies function as one to supply the fixed values of resistance:

∙Ohms Main assembly (A10)

∙Ohms Cal assembly (A9)

All of the resistance values except the 1Ω, 1.9Ω, and short are physically located on the Ohms Main assembly. The 1Ω, 1.9Ω, and short are physically located on the Ohms Cal assembly. The desired resistance is selected by relays on these Ohms assemblies and is connected to the Calibrator binding posts by relays on the Analog Motherboard. The Ohms Cal assembly also contains the appropriate circuitry to enable the Calibrator to perform resistance calibration. Once calibrated, the Calibrator output display shows the true value of the resistance selected, not the nominal (e.g., 10.00031 kΩ, not 10 kΩ).

Four ohms measurement modes are available. For the two-wire configuration, measurement with or without lead-drop compensation sensed at the binding posts of the UUT (using the SENSE binding posts and another set of leads), or at the ends of its test leads is available for 19 kΩ and below. Four-wire configuration is available for all but the 100 MΩ value.

2-16. System Interconnect Detailed Circuit Description

The motherboard assembly contains the Digital Motherboard assembly (A4), and the Analog Motherboard assembly (A3). These two Motherboards are mechanically fastened together with screws. They are electrically connected by connectors P81 and P82 on the Digital Motherboard and connectors J81 and J82 on the Analog Motherboard. AC voltage taps from the Transformer assembly (A22) are connected to the Analog Motherboard through these connectors. Refer to Figure 2-4 for an overview of system interconnections. Figure 2-4 continues on the reverse side, showing system grounds.

2-17. Digital Motherboard Assembly (A4)

The Digital Motherboard contains the line-select switches, line fuse, power switch, a fiber-optic transmitter (J73), and a fiber-optic receiver (J74). It also contains connectors for the Transformer assembly (A22), Digital Power Supply assembly (A19), CPU assembly (A20), Front Panel assembly (A2), Rear Panel assembly (A21), and the two 24V dc fans mounted in the chassis.

The fiber-optic receiver and transmitter provide the serial communication link between the CPU on the unguarded Digital Motherboard and the Regulator/Guard Crossing on the guarded Analog Motherboard.

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