Fluke 5725A instruction manual Sense Amplifier Section

5725A

Instruction Manual

∙Serial Interface/Guard Crossing Section

The serial interface/guard crossing section consists of an RS-232 interface integrated circuit that provides the hardware for 5700A/5725A communication within the 5725A. Two optoisolators provide the serial guard crossing to the Digital assembly (A5), and two more provide integrity checks for the interface. A relay remotely powered by the 5700A provides a software independent path to guarantee 5725A shutdown if the 5700A/5725A cable is disconnected.

∙Analog Input Switching Section

The analog input switching section consists of a bank of relays that route ac and dc voltages and dc currents from the 5700A to various functional blocks inside the 5725A. Assemblies that receive inputs from the Analog Input Switching Section are the High Voltage Amplifier assembly (A3); the Current Amplifier assembly (A2); and the sense amplifier section of the High Voltage Sense assembly.

∙Control Section

The control section is implemented with a bank of four latches connected to the microprocessor data bus. A pair of latches with their integral drivers control relays, a pair of latches control the 16-channel multiplexer and the bipolar output dac in the analog monitor section.

∙AC Line Voltage Switching Section

The ac line voltage switching section is independent of the other five sections of the High Voltage Sense assembly. The ac line voltage section contains three switches (labeled S2, S3, and S4 on the rear panel) that configure the input to the transformer to accept one of eight nominal line voltage levels. Also included in this section are a simple unregulated power supply and a time delayed relay driver circuit. These circuits control a shunt switch around a pair of surge current-limiting NTC thermistors.

Caution

To avoid meter damage, do not measure the ac line voltage section unregulated supply with a meter tied to earth ground.

4-55. Sense Amplifier Section

Op amp U102, transistors Q100 through Q103, and their associated parts comprise the forward gain elements of a precision ac amplifier. Since this monolithic device does not accept supply voltages large enough to accommodate output swings of ±15.6V, a quartet of discrete transistors is configured as an output stage with voltage gain. This provides the necessary output swing of 11V rms as well as limits the additional forward gain of the overall amplifier to simplify frequency compensation.

Inputs to the output voltage stage are derived from deviations of the supply current of U102 from its quiescent value. This topology depends on the increase in current from a particular power supply as the output of the monolithic op amp moves toward the value of that power supply. This increasing current develops a drop across R108 for positive output voltages or a drop across R116 for negative output voltages. These drops are amplified by Q100 and Q103. Transistors Q101 and Q102 buffer the output signal to present a low source impedance to the load.

Overall gain at low frequencies is controlled by Z100, a resistor network designed for good ac frequency response, low power coefficient, and low thermal settling time. This network provides a feedback ratio of 0.99, or a closed-loop gain of 0.01. As a result, the entire sense amplifier is running in a virtual unity gain configuration. Zeners VR103 and

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