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R
UNKNOWN
J125
J120
OHMS
SCALING
RESISTORS
R120, R121,
R122,R123
R165
S160
[ill
I
I
I
Theory of Operation-213 Service
0214, 0215
PRECISION
CURRENT
GENERATOR
0.5mA
1481-9
Fig. 3-1. Ohms Converter detailed block diagram.
output of U 175C is connected to J120 through switched re-
sistors R 120 through R 123. This 1 volt differential between
the Buffer Amplifier input and the output of U175C is con-
stant regardless of the scaling resistor selected; therefore, a
constant current is always applied to the unknown resistance.
This
results in a voltage input to the Buffer Amplifier that is
proportional to the unknown resistance. The Ohms Conver-
ter is protected against overvoltage by VR165 and series re-
sistors R 120 through R 123.
Input Buffer Amplifier
The Buffer Amplifier consists of a matched dual FET input,
0145; a common base stage, 0142 and 0144; a current
mirror, 0140 (A circuit whose characteristic is such that
whatever current flows in one path, an equal current also
flows,
or is mirrored, in
the
other path); a Darlington amplifi-
er, 0150 and 0154; and an emitter follower, 0155. These
circuits
combine to form a one times or ten times gain oper-
ational
amplifier type circuit. 0146 and 0156 are current
sources.
The input is through R 140 and R 142, which, in conjunction
with the low reverse current clamping diodes CR142 and
CR 143, provide input protection against overvoltage. C142
reduces the ac attenuation effects of this protection circuitry.
Amplifier feedback is frorn Lhe ernillet of 0155 Lhrough
R152 and R141-5 to the inverting input of the amplifier.
This feedback sets the current flow through O145B, 0144,
and O140B. As 0140 is a current mirror, whatever current
flows in O140B also flows in O140A, thus setting the signal
level
at the base of 0150. One times or ten times gain is
controlled by the grounded or ungrounded condition of
R 141-4. When grounded, only 10% of the output is fed
back to the inverting input giving the Buffer Amplifier a
REV APR 1982
closed loop gain of 10, which is adjusted by R152 during
calibration. When R141-4 is not grounded, all of the output
is fee:1 back and the amplifier has unity gain. The buffer am-
plifier operates at 1 0 times gain on the 5 and 1 0 millivolt
ranges, the 1 kilohm range, and on all current ranges.
The output of the buffer amplifier is connected to either
the DMM or oscilloscope function by S155, which also ap-
plies operating voltages to the selected function. The out-
put also goes to
the
overrange and clamping circuitry which
is discussed
later.
DIGITAL MULTIMETER
FUNCTIONS
0
<J>
Active Filter
U175B
and
its associated circuitry
is an operational
ampli-
fier
configured
as a
low-pass
two-pole active
filter. It
is
se-
lected by S185 in
the
de measuring
mode and removes
the
ac component from the signal before it enters the
Gm Con-
verter. When in the rms measuring mode,
C179 is
switched
across R 179 to improve the high frequency response into
the Gm Converter.
Gm Converter
The function of the Gm Converter is to generate an output
current that is proportional to the absolute value of the in-
put voltage. For ac signals the Gm Converter converts the
bipolar input voltage to a unipolar output current. The
main components are operational amplifiers U 180 and
U186;
transisto_
~s 0180, 0185, and 0188; diode CR187;
and resistor R 184.
3-3
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