HP 3457A Service Manual page 173

to connect the amplifier for a zero measurement (see Autozero in this section). The switches are
controlled by circuits in UIO] which receive their control information from the A/D Controller, U501
(schematic A2/2).
The input terminals are switched to the measurement front ends (DC, OHMS, AMPS, AC) through K101
- K104. The coil drive for these latching relays is a 15ms pulse of 5V originating in the 16 bit serial
shift register U121 (schematic A2/2). This register is composed of 16 open drain VMOS devices which
pull to ground, The register is not strobed so a strobe operation is provided through Q121 which
provides a strobed +5V to the positive side of the relay coils and U121 provides the pull-down to
ground.
Reed relays K106, K107, K201, K301 and K302 are selected by MOSFETs located in input hybrid,
UIOI. These MOSFET coil select signals, UI01 pins 27-30 and 35, are only designed to be able to pull
down; therefore, drive current is provided by QI18. The combination of Q118, R118 and C118 provide
a power on delay of approximately
| second to allow the MPU (USO01) time to properly program the
desired state at initial power on. The combination of QI19, R122 and CI17 provide protection for
U402 and UIO01 in the event of an input hybrid latch-up condition. Should a latch-up condition occur,
all relays could be on at the same time and U402 could be damaged.
To prevent this, the combination
of QI19 and R122 form a sense circuit that will detect an imminent latch-up condition and disable the
+5V from all the reed relays controlled by UIOL.
The resistor R122 provides a sense of the +15V sup-
ply current for the input hybrid.
This current increases dramatically prior to the onset of latch-up and
causes a voltage drop across R122 that controls the gate of QI19. QUI9 is normally off except when
UIO0L is in a latch-up condition. Should UIOI enter a latch-up condition, QI19 will turn on and this
will rapidly turn off QII8.
Resistor R122 limits the +15V supply current to Ul01 during latch-up, thus
it protects UIOI. The following explains the various input paths.
Low Range Input Path
The low range input path consists of K106, R101 and R102 plus U10l MOSFET switch 4. The purpose
of the path is to connect the HI INPUT terminal to the input amplifier section.
This path is used in the
30mV, 300mV and 3V DC volts ranges, all ranges of 2-wire and 4-wire ohms measurements and self test
measurements,
High Range Input Path
The path consists of K107 and R103 plus UIOl MOSFET switch 5 and a 100:1 divider (9.9M ohm and
100K ohm resistors), The purpose of the path is to attenuate input voltages by a factor of 100 and to
connect the attenuated voltage to the input amplifier section. This path is used in the 30V and 300V
DC ranges, 300M ohm - 3G ohm 2-wire and 4-wire ohms measurements, while command FIXNEDZ is on
and during self test.
When this high range input path is selected, the input resistance is reduced from a
resistance that is greater than
10G ohms on the low range input path to 10M ohms on this high range
input path.
Ohms High Sense Path
This path consists of R201 and R202 plus UI01 MOSFET switch 6. This path connects the HI OHMS
SENSE
terminal to the input amplifier section in the 4-wire ohms function. UI01
MOSFET switch 6
(which is also part of the path) is used to connect the junction of U101 switch 6 to ground in all func-
tions except 4-wire ohms. This is used to shunt any possible voltage on switch 6 to ground.
Ohms Low Sense Path
This path consists of R203 and R204 plus UI0! MOSFET switch 7. This path connects the LO OHMS
SENSE terminal to ground in the 4-wire ohms function.
HP 3457A Multimeter
8-7