Overload Detection; Microprocessor; Battery Charger And Preregs - Stanford Research Systems SR570 User Manual

The final gain stage consists of U403 and output
buffer U406, configured for a gain of 5 and with
input attenuator U409 to select overall gains of
1, 2, or 5. The LM6321 (U406) provides the
output drive capability for the 50 ohm output.

OVERLOAD DETECTION

The overload detectors constantly monitor the I
to V amplifier output, front end output, filter 1
output, U402 (after the second filter) output, and
final stage output for excessive signal levels.
Comparators U408 and U410 compare both
positive and negative signal excursions against a
5 volt reference and light the front panel output
or input overload indicators if any levels are
excessive.

MICROPROCESSOR

The system processor U503 is a CMOS Z80
processor running at 4 MHz. The system clock
consists of Schmitt trigger U506A and an R-C
network. The oscillator is designed so that latch
U508A can shut down the clock oscillator
completely, thereby disabling all digital circuits
in the amplifier so that no digital noise will be
present. The processor and clock only run when
a front panel key is pressed and instrument
settings are to be changed, or while there is
activity on the RS-232 port.
The SR570 uses a 8 K x 8 CMOS
EPROM,(U504) containing system firmware
and calibration bytes, along with a 2 K x 8
CMOS RAM, (U505) which is battery backed-
up at all times to retain instrument settings.
U507 generates port strobes for system IO, and
U510 provides a buffered data bus. The
buffered data bus is active only during IO
instructions to keep digital noise in the amplifier
to a minimum while the processor is running.
U601 through U607 are control latches
providing the 56 DC control lines that configure
all of the instrument's hardware. U608 is an
input buffer that takes data from the front panel
and RS-232, and provides a processor input
indicating line operation.
BATTERY CHARGER
AND PRE-REGULATORS
The 17 volt AC line transformer provides
unregulated power for both amplifier operation and
battery charging. Diode bridge D706 and filter
capacitors C706 and C707 generate unregulated DC
voltages that are pre-regulated to ±12 VDC by U706
and U707 to take the place of the batteries when the
instrument is operating on AC line power. Relay
U705 switches the amplifier from battery to pre-
regulated AC whenever the AC line cord is plugged
in.
D712, D713 and C709, C710 provide unregulated
DC to charge the batteries. U701 and U702 operate
as "AC" regulators, limiting peak battery charging
voltage. As there are two positive batteries and one
negative battery, U701 is a LM350 regulator that
provides twice the current of the LM337 negative
battery regulator.
Charging is controlled by changing the set voltage of
the regulators based on battery charge status. Flip-
flop U703 determines whether the charge regulators
will be set to 15.5 volts for a quick charge or 13.8
volts for a trickle or "maintain" charge by grounding
the bottom of P701 and P702. C712 and R704
insure that the charger always powers up in the
"quick" charge mode. P701 and P702 are provided
to adjust the open circuit trickle charge voltage to
13.8 volts. D701 and D703 are blocking diodes for
the charging circuits while not charging, and D707
and D708 are clamps to guard against battery
polarity reversal.
U708 and U709 are LP365 micropower comparators
that monitor the battery voltage. A resistive divider
chain sets the four trip points for each comparator.
D709 provides a stable 2.5 volt reference against
which levels are compared. For each battery, three
level indications are provided, and are decoded by
multiplexer U704. The "trip" level is 14.5 volts.
The trip outputs control the state of U703 and switch
the battery charge voltage settings. The "low" level
is 11.3 volts and activates the front panel LOW
BATT indicator. R730 provides some level
hysteresis for the low battery indication to prevent
oscillation around the trip point. The "dead" level is
10.7 volts and is used to disconnect the load from
the batteries before they are damaged by an
excessively deep discharge. Q701 and Q703 are
power MOSFET switches used to disconnect battery
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