Fluke 5720A service manual Ohms Main Assembly A10, Selection of Resistance Values

Theory of Operation 2

Analog Section Detailed Circuit Description

These assemblies are also used to calibrate the Calibrator Current function. The Ohms Main assembly contains all the resistor values except the 1Ω, 1.9Ω, and short, which are located on the Ohms Cal assembly. It also contains the relays and their drivers to switch these values as requested under program control.

The Ohms Cal assembly contains all calibration circuits except for one op amp on the Ohms Main assembly. It also contains a circuit to provide accurate calibration of two- wire ohmmeters. In addition, there are relays, relay drivers, and logic to interface the Ohms assemblies to the digital bus. Refer to Figure 2-30 for a simplified schematic of both the Ohms Main and Ohms Cal assemblies.

2-169. Ohms Main Assembly (A10)

The Ohms Main assembly uses three Fluke hermetically-sealed thin film resistor networks (Z1, Z2 and Z3) to obtain values from 10Ω to 19 MΩ. The values are arranged in two strings, one for decade values and the other for the multiples of 1.9. The 100 MΩ value is achieved by inserting a 90 MΩ film resistor (R1) in series with the decade string.

2-170. Selection of Resistance Values

Refer to Figure 2-31 for the following discussion. Relays select the resistance values. All resistor values have four-wire connections except 100 MΩ, which is two-wire. The output high and sense high side of a resistance value is connected to INT OUT HI and INT SENSE HI by relays K1 and K2 in the reset position, except 100 MΩ, which is connected to INT OUT HI by K5. Relays on the motherboard connect INT OUT HI and INT SENSE HI to the OUTPUT HI and SENSE HI binding posts.

The low side of the 1x string is connected to OHMS SENSE LO and OHMS OUT LO by relay K27 (A and B) in the reset position. The low side of the 1.9x string is connected to the same lines by relay K39 (A and B). These lines are routed to the Ohms Cal assembly where they are connected via relays to the OUTPUT LO and SENSE LO binding posts.

The 10 MΩ value is selected by K7 (reset) and K8 (energized) and the 1 MΩ value by K9 and K10 (reset). Selection of decades below 1 MΩ is done by K15 and K16 (reset) plus a pair of relays from K17 through K26A. For example, to select 10 kΩ, relays K18A and K20A are reset.

The 19 MΩ value is selected by K11 and K12 (energized), and the 1.9 MΩ value by K13 and K14 (reset). Selection of 1.9 decades below 1.9 MΩ is done by K35 and K36 (reset) plus a pair of relays from K18B through K26B and K37.

Two lines, OHMS OUT HI and OHMS SENSE HI, are brought over to the Ohms Cal assembly to connect to 1Ω, 1.9Ω, and short. These lines also access resistance values during calibration. Relay K29 connects OHMS OUT HI to the 1x10n string when set, and to the 1.9x10n string when reset. Relay K30 connects OHMS SENSE HI to the 1.9x10n string during calibration.

Relays K3, K28, K31-K34, and op amp U1 and its associated components are only used during calibration. Operation of this circuitry is described in the "Calibration" part of the Ohms Cal theory.

Relays K4 and K6 are used during two-wire compensation. This is described in the "Two-Wire Compensation" part of the Ohms Cal theory.

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