Fluke 5725A, Instruction Manual, Page: 81 / 196

Theory of Operation
Detailed Circuit Description
4
4-23
Control and Switching Section 4-38.
Refer to page 2 of the Current Amplifier schematic. Relays K201 through 209, Q214
through 216, and U209 through 210 configure the Current Amplifier circuitry for the
following six modes:
•
Current Standby
Current Amplifier input is active through K201. Current Amplifier output is
disconnected from instrument output terminals but is routed through the sense path
through K202.
•
Operate1
Input is connected to BOOST IN and MCOM. Output is connected to front or rear
output terminals. FET switches Q211 and Q213 are off, enabling the error amplifier
circuitry. Operate1 is the active mode for delivering up to 11A (ac or dc) to a UUT.
•
Operate2
The Current Amplifier is configured as in standby mode, except K201 is off. 5700A
current source is routed to the 5725A current output terminals through K205.
Operate2 is the active mode for delivering 2.2A or less to the 5725A terminals
unless the 5725A 11A current range has been explicitly enabled from the 5700A
front panel or by remote command.
•
Calibrate1 (Shunt Calibration)
Input is shorted. Error amplifiers are disabled with Q211 and Q213. Output is routed
directly to current return path through K207, bypassing sense circuits. 5700A current
source is routed through K206 to the 5725A current sense path. Procedures
performed during Calibrate1 determine the value of R272. This is one of the factors
in the dc gain and the only factor in the ac gain of the Current Amplifier.
•
Calibrate2 (Offset Calibration)
The amplifier is configured as in Calibrate1, except with its output routed by K208
to the 5700A. Procedures performed during Calibrate2 determine the dc offset of the
Current Amplifier.
•
Calibrate3 (Gain Calibration)
Input is connected to BOOST IN, B-FEEDBACK, and MCOM as in Operate1, but
Output is routed through the current sense path through K202. Output is not
connected to output terminals. Procedures performed during Calibrate3 determine
the gain of the error amplifier, the other factor in the dc gain of the Current
Amplifier.
Power Supply Section 4-39.
Refer to page 3 of the Current Amplifier schematic. The unregulated, high-current
supply is made from K208 through 209, F201 through 202, CR206, and C218 through
C221.
The regulated
±
20V supplies are composed of RT201 through 202, U205, U206, CR212
through 215, C222, C223, and associated components. Thermistors RT201 and RT202
are PTC thermistors for current limiting and transformer protection.