Fluke 5720A Two-Wire Compensation Power Supplies, High Side Cancellation, Low Side Cancellation

Theory of Operation 2

Analog Section Detailed Circuit Description

2-177. Two-Wire Compensation Power Supplies

The floating supplies for U7 and U8 consist of a dc-to-dc converter made up of U6, T1 and associated components. Switching-regulator control IC U6 switches +17S through the primary of T1 at about 30 kHz as determined by R27 and C21. The switching creates high-frequency spikes on the +17S line which are filtered out by L2, R25, C18, C19, C20, and C52. When the two-wire compensation circuit is not in use, control line PB7 turns Q5 and Q6 off, which removes +17S, switching off U6. The secondary of T1 is rectified and filtered by CR1, CR2, CR9, CR10, C22, and C23. The voltage is then regulated by 8.2V zener diodes VR1 and VR2. Additional filtering is provided by L3, L4, R43, R44, C24, and C25. Two capacitors (C53 and C54) between A COM and S COM reduce switching noise that would otherwise appear across the calibration resistor.

2-178. High Side Cancellation

The high side voltage drop is eliminated by U7 and U8. Chopper-stabilized op amp U7 and buffer amplifier U8 supply up to 10 mA. The current from the high side of the ohmmeter being calibrated enters at the OUTPUT HI binding post, goes through K1 and K6 on the Ohms assembly, and is routed to the Ohms Cal assembly where it connects to A COM. The A COM connection is a common for the floating supply that powers U7 and U8. The current then flows out of U8 through 500Ω in Z2 and through K29 to the calibration resistor via the OHMS OUT HI line.

K29 connects the -input of U7 to the sense point of the calibration resistance via OHMS SENSE HI. Relays K2 and K4 on the Ohms Main assembly connect the +input of U7, AMP IN, to INT SENSE HI. Relays on the motherboard connect INT SENSE HI to the OUTPUT HI or SENSE HI binding post. Connected this way, U7 controls the voltage at the output of U8 so that the voltage at the input of U7 stays zero. This forces the voltage drop in the path to zero. Diodes CR3-CR6 and resistor R45 provide protection for U7.

2-179. Low Side Cancellation

The voltage drop in the low side path is canceled by current from high current op amp U9. U9 is driven by dual FET input op amp U10. The two non-inverting inputs of U10 are connected across the 500Ω resistor in Z2. This enables U10, in conjunction with the four 10 kΩ resistors in Z2, to sense the current in the high side path and to supply (through U9 and the 1 kΩ resistor in Z2) a current equal but opposite to the current through the low side path. This cancels the drop in that path. This canceling current goes through relay K28A to the low sense point of the calibration resistor via OHMS SENSE LO. From there it goes through the output low path to the OUTPUT LO binding post and then through the sense low path back to the Ohms Cal assembly and through K28B to S COM.

2-180. Ohms Calibration

The remaining circuitry is used only for calibration of the resistor values. All values except 1Ω and 1.9Ω are calibrated using a single external 10 kΩ resistance standard connected to the binding posts. The 1Ω and 1.9Ω values are calibrated using an external 1Ω standard. How to do the procedure is described in Section 7 of the Operator Manual.

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