Theory of Operation 4

Detailed Circuit Description

IC U156 is a dac whose digital input is under microprocessor control via data latch U155. The +15V supply is the dac’s reference voltage. Op amp U153A acts as a current- to-voltage converter, providing outputs between 0V and -6.375V. The desired output for the analog monitor function ranges from -2.56V to +2.56V. Level shifting and scaling is accomplished by U153B, by summing the output of U153A and the reference voltage for U156. Op amp U153B also forms an active two-pole filter to reduce wideband noise. The output of the dac is routed to both the multiplexer, U151, and the analog monitor comparator, U154A.

Resistors R171 and R170 provide a small amount of positive feedback around U154A to ensure noise-free operation for small input voltages. Q150 forms a simple inverter that converts the comparator output to 5V logic levels. This output, MONCOMP, is routed back to the 5725A Digital assembly (A5), where it is acted on by the microprocessor.

4-57. Serial Interface/Guard Crossing Section

The control link between the 5700A and the 5725A consists of a serial interface at the hardware level. Serial data signals arrive via B-RCV and leave via B-XMIT. These are translated to and from standard 5V logic levels by U160, an RS-232 interface chip that runs from a single 5V supply. Internal charge pumps use C158 through C161 to generate RS-232 compatible supply levels of ±10V.

A pair of dual optoisolators, U159 and U161, isolate the serial interface signals from the remainder of the 5725A circuitry. The serial interface uses half of each pair. The other half of U161 signals the status of the 5700A/5725A interface cable to the microprocessor.

A 5V supply is available from the 5700A via pins 19 and 20 of J101. This supply drives the optoisolator LED when the cable connections are intact. The 5V supply also provides coil drive for K153. This relay’s contacts are in series with the coils of the other relays on the High Voltage Sense assembly. Thus, if the cable becomes disconnected, all relays on the High Voltage Sense assembly automatically open. The relays are configured so that safety is maximized when they are open.

The remaining half of U159 signals to the 5700A when the 5725A is powered up. The 5725A +5V supply powers the LED of the optoisolator, whose output pulls B-CINT* (pin 21 of J101) toward the 5700A supply common, B+5VCOM. This system is a complement to the CABLEOFF signal provided to the 5725A processor.

4-58. Analog Input Switching Section

The analog input switching section is the primary analog interface with the 5700A output functions. Relay K108 serves exclusively to connect the output low lead of the 5725A to the load. Relay K106 does the same for the low sense line. Line MCOM, when switched to B-SENSE LO, serves as the low sense lead for the 5725A. Similarly, PACOM, which ties to the 5725A system ground VCOM at the power supply, is switched to B-PACOM, serving as the 5725A output low lead.

Signal BOOST IN is the input high lead from the 5700A signal sources, and is switched from the inputs to the High Voltage Amplifier or Current Amplifier and sense amplifier by K107. Relay K101 also selects the proper source for the B-FEEDBACK signal.

Signal B-FEEDBACK corresponds to the input high sense lead. Relay K111 provides the capability to provide local sensing directly by the 5700A on the 5725A High Voltage Sense assembly. Local sensing is used during ac voltage standby mode. This prevents the 5700A Oscillator from running without feedback when the 5725A High Voltage Amplifier assembly is in standby.

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Fluke 5725A instruction manual Serial Interface/Guard Crossing Section, Analog Input Switching Section

5725A specifications

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