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Tektronix 213 DMM User Manual

Tektronix 213 DMM
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Theory
of
Operation-213
Service
+15
V
INTEGRATOR
REFERENCE
VOLTAGE
VR217,
CR217A
THRESHOLD
DETECTOR
CLOCK
0240
0245
0247
+15
V
REFERENCE
CURRENT
R234
R235
R236
CR217B
SWITCH
0254
C227
0230
COUNT
INPUT
TRANSFER
CONVERTER
LATCH
U250C
& D
INPUT
COUNTER
BCD
OUTPUT
U260
•
---TO
CHARACTER
GENERATOR
COUNT
EXTEND
______
__.OUTPUT
1481-11
Fig.
3-3.
A/D
Converter detailed block diagram.
that
the
unknown
potential
is
applied
at
all
times
and
not
switched
off
during
capacitor
discharge. A
current
directly
related
to
the
unknown
input
is
applied
to
an
integrator
which
causes
a
ramp.
When
the
counter
reaches
the
trailing
edge
of
the
9999
count,
a
reference
current
of
the
opposite
polarity
causes a
ramp
in
the
opposite
direction.
When
the
opposite
ramp
begins,
the
counter
is
at
0000.
When
the
in-
tegrated
waveform
reaches a negative
threshold,
the
number
in
the
counter
is
stored
as BCD
data,
and
the
reference
cur-
rent
is
turned
off.
Upon
receiving a scan
input
command,
this
stored
BCD
count
is
forwarded
to
the
Character
Gener-
ator
and
subsequently
displayed
as
the
value
of
the
measur-
ed
input.
The
DMM
reference
consists
of
VR217,
CR217A,
CR217B,
R235,
and
R236.
Temperature-stable
zener
diode
VR217
generates
a
reference
voltage (with
respect
to
the
+15
V
power
supply),
which
is
filtered
and
applied
to
the
positive
input
of
the
integrator
U220.
This
voltage also
appears
at
the
negative
input
of
U220
and across resistor
string
R235
and
R236,
which
causes
current
flow
through
them
and
CR217B
when
0254
is
not
conducting.
0254
acts
as a
switch
to
turn
the
reference
on
and
off.
When
0254
is
conducting,
CR217B
is
reverse biased and
the
cur-
rent
flow
is
through
0254
instead
of
CR217B,
which
essen-
tially
disconnects
the
reference
from
U220.
Rectifier
CR217
A
is
used
to
temperature
compensate
CR217B.
During
the
DC
Zero
calibration
adjustment,
R234
increases
the
A/D
Converter
sensitivity
by
a
ratio
of
about
10
to
1.
Refer
to
the
A/D
Converter
timing
diagram,
Figure 3-4
and
the
schematic
diagrams
for
the
remainder
of
the
A/D
Con-
verter
description.
Emitter
coupled
multivibrator
0240
and
0245,
generates
a
clock
pulse
of
approximately
70
kilohertz
which
is
applied
@
through
buffer
0247
to
pin 7
of
the
four
decade
counter
U260.
During
the
count,
C227
is
ramping
in
the
positive
direction
in
response
to
the
unknown
input.
After
the
coun
-
ter
reaches a
count
of
9999,
pin 8
of
U260
is
pulsed
HI
and
then
falls
LO
on
the
trailing edge
of
the
next
clock
pulse
at
which
time
the
counter
returns
to
count
0000.
The
trailing
edge
of
the
pulse
from
pin 8
is
differentiated
by
0258,
C258,
R258,
R252,
and
R253,
and
appears
as a LO pulse
at
pin 9
of
U250C.
A
LO
at
pin 9
of
U250C
causes a
HI
on
pin 8
which
turns
0254
off,
and
a
HI
on
pin
13
of
U250D
causes pin
11
to
go
LO. Pin 9
of
U250C
returns
HI
at
the
end
of
the
differentiated
pulse
from
pin
8
of
U260
which
causes
no
change.
With
0254
turned
off,
the
reference
cur-
rent
causes
U220
to
integrate
both
the
input
and reference
currents
as a negative
ramp.
When
the
negative
ramp
goes
low
enough
to
turn
on
CR230
and
0230,
pin 12
of
U250D
goes LO. This LO
at
pin
12
causes a
HI
at
pin
11,
a
HI
at
pin
10
of
U250C,
and
a LO
at
pin 8
of
U250C,
which
turns
0254
on.
Also,
this
LO
is
differentiated
by
C255
and
R255
and
applied
to
pin 5
of
U260
which
causes
the
BCD
count
of
the
counter
to
be
transferred
to
a
storage
register
(latch)
where
it
remains
for
display
until
the
next
transfer
pulse. Whenever pin 9
of
U260
receives a negative pulse
from
pin 1
of
U270,
the
BCD
data
is
transferred
serially
to
the
Character
Generator
one
character
at
a
time.
As each
character
is
transferred,
the
output
from
pins 1,
2,
3,
and
4
of
U260
provides
spacing
current
to
the
horizontal
am-
plifier
through
0260, 0262,
0264,
and
0266.
This
spacing
current
establishes
the
horizontal
position
on
the
crt
where
each
character
is
displayed.
Character Generator
The
Character
Generator
is
a
Tektronix
integrated
circuit
device
that
provides
X,
Y,
and Z signals
to
the
crt
deflection
system
to
form
the
numerical
readout
display.
The
Charac-
ter
Generator
provides
X
and
Y signals
to
form
the
seven
3-5

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Tektronix 213 DMM Specifications

General IconGeneral
BrandTektronix
Model213 DMM
CategoryTest Equipment
LanguageEnglish

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