Fluke 5725A instruction manual Output Stage Section, Monitor Section

5725A

Instruction Manual

(ICAL, TP208) with the same amplitude and polarity as the current amplifier input voltage. The calibration amplifier output is measured by circuitry within the 5700A during calibration of the 5725A current amplifier. The calibration amplifier output is fed back to the 5700A Oscillator during ac current operation.

The feedback signal from the precision shunt is applied to both the inner and outer loop error amplifiers (ISENSE HI, ISENSE LO , pages 1 and 2). The error amplifier section drives the output stage (TP204) so that the voltage across R272 is one-tenth of the Current Amplifier input voltage (VINI). Thus, the Current Amplifier is a transconductance amplifier: output current is proportional to input voltage. This transconductance is -5A/V.

4-36. Output Stage Section

Refer to page 1 of the Current Amplifier schematic. Components U203, R203, R204, and R209 convert the bipolar ground-referenced output signal from the error amplifier section into two unipolar supply-referenced waveforms at the inputs to U204A and U204B. IC U203 also isolates the output stage, which is referenced to a floating common (ICOM), from the rest of the circuitry, which is referenced to circuit common (VCOM). The operation of the positive and negative output halves are similar. Only the positive output section is described below.

In response to a positive polarity error amplifier output (TP204), the current through R209 increases. This same current flows into pin 2 of U203, causing the voltage drop across R203 to increase. The voltage across R203 is applied to the positive input of U204A. U204A drives Q203, Q205, and Q207 until the voltage at the emitter of Q207 is identical to the voltage at the non-inverting input of U204A. Test point TP204 is a test input for the output stage when in standby mode. The resulting output stage current can be monitored at TP208 with a scaling of 200 mV/A.

The quiescent operating point of the output devices is fixed by the bias current of U203 and current source CR205 at about 1A. There is no output stage bias adjustment.

Capacitor C235 and resistors R256 through R260 provide a tertiary feedback path that guarantees stability when the 5725A is driving the specified inductive loads.

4-37. Monitor Section

Refer to page 1 of the Current Amplifier schematic. Components U211C and D, U210C, RT203, CR207, and associated resistors and capacitors make up the temperature- monitoring circuit. Heat sink temperatures above approximately 85°C signal the ITEMPM fault condition.

Components U207, R280 through 284, and C234 generate a negative voltage at TP207 (DRIVE MONITOR) that remains less than 3.1V in magnitude when the output compliance voltage is within the linear operating range of the output stage. The output compliance voltage is not measured by the drive monitor circuit; an overcompliance condition is inferred from excessive error amplifier output.

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