Keysight 1143A, User'S And Service Manual

Page: 45 / 56

Positive Supply Refer to schematic 1. The reference for the positive
supply is the +1.5 V reference from the IC, fed to the noninverting input
of the error amplifier. The output voltage is divided through the
POSITIVE VOLTAGE ADJUST and fed to the inverting input of the error
amplifier. The output of the regulator IC is fed to pass transistor Q1. For
additional gain, Q1 is operated with the collector as the output.
The voltage across current sense resistor R42 is fed to the current sense
amplifier. The current is foldback-limited by R41 and VR1. The POSITIVE
CURRENT THRESHOLD adjustment sets the current limit threshold in the
amplifier.
Negative Supply Refer to schematic 2. The negative supply uses the
same IC as the positive supply. The control voltages however, are a sum
of the voltage from the positive supply, used as a reference, plus the
feedback from the output of the negative supply. The voltage input from
the positive supply makes the negative supply voltage track the positive
supply. Q3 is an additional gain stage in the output and Q4 assures a
clean start when power is applied.
The current limit circuit works the same as the one in the positive supply.
Offset Control
The probe 1 and probe 2 offset circuits are virtually identical. The probe 1
circuit will be described. Use schematic 3 (figure 10, page 50) for the
following discussion.
U3, a 1.23 V voltage reference, provides a reference for the internal offset
control. A fixed portion of the reference feeds the non-inverting input of
op-amp U1A. The fine and coarse offset controls also use the reference and
feed the variable offset into the inverting input of U1A. The output of U1A
swings
±
10 V, depending on the offset setting.
S1A selects the Variable offset or Zero (ground) and S1B selects the Local or
Remote offset voltage.
Op-amp U1B combines the selected offset signal with an offset adjustment.
PROBE 1 OFFSET ZERO is adjusted for zero offset current output when the
offset input selection is Local and Zero. To reduce introduction of errors
from ground loops, the ground from the remote input is returned to the input
circuits of U1B.
The output of U1B drives current source U2. The output of U2 swings
±
5 mA, depending on the offset control input. The offset control drives a
500-
Ω
load in the 54701A active probe.
Service
Troubleshooting and Repair
45