Rms Converter; A/D Converter - Tektronix 213 DMM Instruction Manual

Theory of Operation-213 Service
With a positive input to the converter, the 0185 end of
R 184 is maintained at the positive input voltage by U 180
and 0185, while the opposite end of R 184 is maintained
at zero volts by U186 and CR187. This voltage appearing
across R 184 sets the output current which appears at the
collector of 0185.
With a negative input to the converter, R184 has a negative
input voltage at its 0180 end and zero volts maintained at
the opposite end by U186 and 0188. This voltage appearing
across R 184 sets the output current which appears at the col-
lector of 0188.
R 186 and C186 provide high frequency compensation.
CR 183 through CR 185, C183, and R 183 shift the operating
point of U 180 to provide more dynamic range for the con-
verter. C181, C182, C187, and C188 are compensation ca-
pacitors for U 180 and U 186.
RMS Converter
The RMS Converter accepts the absolute value current input
from the Gm Converter and changes it to a de current which
is proportional to the true rms value. Operating voltage is
provided only when the DMM function is selected by S155,
and is connected into the circuitry only when the rms fea-
ture is selected by S185. Figure 3-2 is a detailed block dia-
gram of the RMS Converter showing how therms value is
derived. The RMS Converter is composed of an input current
splitter, a gain cell (A variable transconductance amplifier
that supplies an output current proportional to its input vol-
tage), an averaging circuit, a current mirror, and an output
current inverter.
Operation of the current splitter, 0198, depends upon its
matched pair devices for which a given emitter-base voltage
will result in the same collector current in each device. 0196
provides for beta compensation.
Transistor 0192 and 0194 make up a gain cell whose opera-
tion is dependent upon their matched pair devices, the loga-
rithmic characteristics of their emitter-base junction, and
feedback from O190A and O190B. In this configuration
the ratio of the collector current of O 194A to the collector
current of O194B is the same as the ratio of the collector
current of O192B to the collector current of O192A. Inte-
grated circuit U 188 is used to vary the offset voltage around
the loop of 0194 and 0192, and also serves as an rms gain
control.
The current from O194A is averaged by C190 and R190 in
the current mirror. U 175A and O 190, and the voltage de-
veloped sets the operation of U 175A. The output of U 175A
sets the voltage across R191 which controls the current
through O190A. The current through O190A is used to
balance the gain cell and, when balanced, this current is
equal to therms value of the input current. Since 0190 is
a current mirror, this same current appears in O190B and
appears at the input to the inverter.
The output current inverter, U 1750, divides its input current
by a factor to correct for the ratio of R 190 and R 191, and
for losses in the input current splitter. Inversion is accom-
plished by using the inverting input of the operational am-
plifier and amplification is set by the ratio of R 198 and R 199.
Resistors R194, R195, and R197 provide offset adjustment
and CR 197 prevents circuit latch-up if the offset is adjusted
such that the output would go negative. CR 199 keeps the
output from being fed to the A/0 Converter if it goes nega-
tive. 0195 detects crest factor overrange, and is part of the
overrange circuitry which is covered later.
A/D Converter
Figure 3-3 is a detailed block diagram of the A/0 Converter
which consists of a DMM reference, an integrator, a thres-
hold detector, a converter latch, a counter, and a clock gen-
erator.
The A/0 Converter operates on the modified dual-slope
integration principle which provides accuracy independent
of supply voltage and component changes. Modified means
25
r
2
42
=
z
5 . ~
z=2"\!I2
INPUT
I
3-4
CURRENT
SPLITTER
0196
0198
_!_
2
.l
2
X GAIN CELL
0192 XY
0194 Z
U188
z
12
4Z
AVERAGING
CIRCUIT
C190/R190
25!2
4Z
Fig. 3-2. RMS Converter detailed block diagram.
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