Circuit Description; Input Switching; Voltage Coupling And Attenuation; Current Shunt Resistors - Tektronix 213 DMM Instruction Manual

Theory of Operation-213 Service
In the Oscilloscope function the input signal is further am-
plified by the Scope Preamplifier, which provides vertical
gain switching, variable gain control, and positioning. The
resultant signal drives the Vertical Output Amplifier which
causes vertical crt deflection to create a display.
The Sweep Generator produces a linear sawtooth sweep sig-
nal for horizontal deflection. It is triggered internally from
a portion of the vertical input signal, externally from a se-
parate trigger source, or free-runs in an internal ac automa-
tic mode of operation during the absence of an adequate
internal signal level. The output of the sweep generator is
amplified by the Horizontal Output Amplifier which causes
horizontal crt deflection to create a linear sweep. The Sweep
Generator also generates a signal to the Z-Axis Amplifier for
display blanking and unblanking.
All voltages required to operate the instrument are provided
by a battery operated power supply, which is recharged by
an integral battery charger when connected to the power
line.
CIRCUIT DESCRIPTION
Unless otherwise specified, the following detailed circuit
descriptions will make reference to the schematic diagrams
in the Diagrams section at the rear of this manual.
ATTENUATOR & INPUT AMPLIFIER
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The input section consists of the voltage and current-resis-
tance inputs and input switching; voltage input coupling
and attenuation; the current shunt resistor network; the
Ohms Converter; and the Input Buffer Amplifier.
Input Switching
Input switching is accomplished by a combination of physi-
cal input connections and self-cancelling pushbuttons. Vol-
tage inputs are made from the probe tip through probe input
P100, while current and resistance inputs are made through
J
120. Switch S 160 selects the type of input (voltage, cur-
rent, or resistance); connects the appropriate voltage atten-
uator, current shunt resistor, or the Ohms Converter with
its scaling resistor network; and connects the selected input
to the Buffer Amp I ifier.
Voltage Coupling and Attenuation
Voltage inputs from P100 are coupled to the attenuator
through INPUT COUPLING switch S100. In the DC posi-
tion of S100, all signals present at the probe enter the at-
tenuator. In the AC position, the de signals are blocked by
C101 and only the ac signals enter. In the GND position of
S100, the Buffer Amplifier side of S100 is grounded to provide
a ground reference without physically disconnecting the probe
3-2
to ground it. In this position, the input coupling capacitor,
C 101, is precharged through R 101 to ground.
The signal enters the attenuator section and passes through
without attenuation or is attenuated by 1000: 1, 100: 1, or
10: 1, depending upon the position of the S120 switch con-
tacts. When contact 18 is open and 16 is closed, 10: 1 atten-
uation is provided by R 108, R 109, and their associated
series-parallel components. When contact 21 is open and
19 is closed, 100: 1 attenuation is provided by R 103, R 106,
and their associated series-parallel components. When both
contacts 18 and 21 are open, and 16 and 19 are closed,
1000: 1 attenuation is provided. When 18 and 21 are closed,
and 16 and 19 are open, there is no attenuation. Resistor
pairs R 103-R 106 and R 108-R 109 are precision matched
pairs. Capacitors C104, C108, and C112 provide attenuator
frequency compensation. The output of the attenuator is
connected or disconnected from the Buffer Amplifier input
by the VOL TS section of S160.
Current Shunt Resistors
Current inputs are connected between the mA-Q jack, J 120,
and the COMMON jack, J 125. When the current mode is
selected by S160, an input voltage for the Buffer Amplifier
is generated by current flow through combinations of resis-
tor network R 115, R 117, and R 118 as selected by S 12p.
Clamping diodes CR 118 through CR 121 and fuse F 161 pro-
vide input protection in the current mode.
Ohms Converter
Figure 3-1 is a detailed block diagram of the Ohms Conver-
ter circuitry.
Resistance inputs share input connectors J120 and J125
with the current input. When S160 is in the OHMS mode,
a constant current source is connected to J 120, and resis-
tance is measured by passing a constant current through,
and measuring the voltage across the unknown resistance.
This measured voltage, which is directly proportional to the
unknown resistance, becomes the input voltage to the Buf-
fer Amplifier.
The Ohms Converter circuit consists of a precision current
generator 0214 and 0215, an operational amplifier U175C,
and the range sealing resistors R 120 through R 123.
U 17 5C is connected to the feedback summing junction of
the Buffer Amplifier; therefore, its inverting and non-inver-
ting inputs are at the same input voltage as the input of the
Buffer Amplifier. The precision current generator provides
a 0.5 milliampere current (adjusted by R215 during calibra-
tion) through R165which establishes and maintains a 1
volt drop across that resistor. This voltage drop keeps the
output of U 175C at 1 volt more negative than the potential
at J120, which is the input to the Buffer Ampl ifier. The
REV APR 1982