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PRINCIPLES OF OPERATION
The microcomputer
section consists of the 68008 MPU and
associated components. Software for the instrument is con-
tained in the ROM ICs. 'The MPU, with the help of the
software, supervises
all operating
aspects of the instru-
ment, including
A/D converter operation, front panel con-
trols and display, the IEEE-488 bus, the analog outputs,
as well as the operation of the second A/D channel, if that
module is installed.
The power supply
converts
the AC input
power line
voltage into various DC voltages used by the instrument.
Each channel has its own +5V and *15V analog supplies
to ensure proper isolation. Other supplies include the +5V
digital and additional
+15V supplies for switcher circuits
and analog outputs.
6.3 ANALOG
CIRCUITRY
For a detailed schematic of the analog circuitry,
refer to
drawing number 194.126, page 2 located at the end of Sec-
tion 8.
6.3.1 Block Diagram
A simplified block diagram of the analog circuitry is shown
in Figure 6.2. The analog input circuitry
can be divided
into six sections: input coupling,
attenuation,
protection,
x1.526 gain stage, filter, and switchable gain amplifier. The
coupling
circuitry
selects AC or DC coupling
at the input
stage, and the attenuator selects xl, x0.1, or 0.01 attenua-
tion factors for the 32V and ZOOV ranges. The protection
circuitry
protects the circuit from overvoltage inputs. The
xl.526 gain stage supplies a fixed gain. The filter circuitry
applies either 50kHz or 500kHz single-pole
filters. The
switchable gain amplifier can be set to xl or xl0 gain values,
depending
on the selected range.
6.3.2 Circuit
Description
Input
signals
are applied
through
JlOll
to the input
amplifier
via R328 or C384 and 1~21, depending
on the
positions of K303 and K304. From the input coupling
cir-
cuit, signals are applied the input attenuator
made up of
R312, R313, and R.318. Frequency
compensation
is pcr-
formed by C374, C375, C383, and C386. The actual attenua-
tion factor is selected by K302 and (1301. Attenuation
fat-
tars of x0.1 (32V range) or x0.01 (200V range) arc applied
by placing K302 in the correct position and switching Q301
on or off to take RX2 in or out of the circuit. Table 6-l sum-
marizes attenuator
values for the four ranges.
Input
protection
is ncccssary
in cases where ltn over
voltage input
might be applied.
Protection
circuit com-
ponents include CR302, CR302, VR301, VR302, Q302, and
0303.
The signal is next applied to a fixed gain stage made up
of U387 and associated components. U387 is an operational
amplifier
being used in the non-inverting
configuration
The gain of this stage is set to xl.526 by resistors R322 and
R323. This gain factor is applied
to convert a nominal
+32,767, +32,768 count input into a ?5V full scale value
to take full advantage of the A!D convertrr
range.
The analog filtering
is performed by lG2l, R327. and C387.
Actually, only one RC combination
can be in eifcct .It .I*!
given time. Q304 switches in the 5Okl lz filter, while Q307
controls the 500kHz filter. When the filter is disabled, both
FETs will bc off.
'The final stage in the input amplifier
is the ~1~x10 gain
amplifier,
which is made up of U386 and ass<)ciated con-
ponents.
The stage is also 'I non-inwrting
c>pcrational
amplifier. The gain of this stage is set to rither xl or ~10.
depending
on the selected range (WC Table h-l). Gain is
set by resistors R314 and 1~319, and FE% Q305 and Q306.
For example, when x~l0 gain is selected, Q305 will be on.
and Q306 will be off. Under these conditions.
the .Implifict
gain is:
A = ~1 -+ R314iR3~19
A = 1 + '18001200
A = 'IO
Flgure 6-2. Simplified
Block Diagram of Analog
Circuitry
6-3
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