Tektronix DM 502 User manual

Tektronix,
Inc .
P
.O .
Box500
Beaverton, Oregon 97005

Serial Number
070-1726-00
TEKTRONIX
D IVI 502
DIGITAL
IVI U
LT I IVI ET E R
INSTRUCTION
MANUAL
First
Printing Aug 1974

WARRANTY
All TEKTRONIX
instruments are warranted against
defective materials and workmanship for
one year .
Any questions
with respect
to the warranty
should
be taken up
with your TEKTRONIX Field Engineer or
representative .
All requests
for repairs and replacement parts
should be
directed to the TEKTRONIX
Field Office
or representative
in your area . This will assure you
the
fastest
possible service . Please include
the
instrument Type Number or Part
Number and Serial
Number
with all requests for parts or service .
Specifications and price change
privileges reserved .
Copyright '
1974 by Tektronix, Inc ., Beaverton,
Oregon
. Printed in the United States of America .
All
rights reserved . Contents of this
publication may not
be reproduced in any
form without permission of
Tektronix, Inc .
U .S .A
. and foreign TEKTRONIX products covered
by U .S . and foreign patents and/or
patents pending .
TEKTRONIX is a registered
trademark
of
Tektronix,
Inc .

CHANGE INFORMATION
REV . A, OCT . 1974
Accessories
Repackaging
Exploded View
DM 502
TABLE
OF
CONTENTS
Page
SECTION 1
OPERATING INSTRUCTIONS
Introduction
1-1
Instrument Description 1-1
Installation and Removal 1-1
Operating Considerations 1-2
Input Connections
1-2
Function's Available at Rear Connector
1-2
Sine Wave Response
1-3
Temperature Measurements
1-3
Making
Measurements
1-4
DC Voltage Measurements
1-4
DC Current
Measurements 1-4
AC
Voltage, Current and Resistance
Measurements
1-5
Decibel Measurements
1-5
Temperature
Measurements
1-5
Using a Transistor
as a Temperature-Sensing
Device 1-5
Accurate
Current Source
1-6
Specifications
1-6
SECTION 2
THEORY OF
OPERATION
Introduction
2-1
Attenuator
and Input Switching
2-1
AC Converter
2-1
DB
Converter
2-1
Ohms Converter
2-2
Temperature
Measurement Circuitry 2-2
Clock
2-3
Analog-To-Digital Converter
2-3
Display 2-5
Power Supplies
2-5
SECTION 3
ELECTRICAL
REPLACEABLEPARTS
LIST
SECTION 4
SERVICE INFORMATION
Internal
Adjustment Procedure
Rear Interface Connector
Assignments
Rear Interface
Connector Wire Locations
Front Panel Controls
and Connectors
Block
Diagram
Parts Location Grid
Attenuators and
Conversion Circuits Schematic
Integrator,
A/D Converter and Display
Schematic
Power Supply
and Switch Logic Schematic
SECTION 5
MECHANICAL REPLACEABLE
PARTS LIST

RA 502 nalm mn'

Instrument Description
The DM 502 Digital Multimeter measuresdo and ac
voltage and current, dBm, dBV, resistance and
temperature . The ac functions are average responding
and rms calibrated for sine waves . A single front panel
control selects
all ranges
.
A front-panel pushbutton
selects dB readout of ac functions in lieu of voltage or
current . Rear interface-connector input is also pushbut-
ton selected from
the front panel . Values of dB are
obtained by adding
the selected dB range value to
the
display
reading . Readout in dBm or dBV is chosen
by an
internal jumper . The unit is shipped
with the jumper in the
dBm position .
An internal jumper also permits selection of fet
input (>
1000 Mil) or 10 Mil input impedance on the
two lowest dc-
voltage ranges . The unit is shipped with the jumper in the
10 Mil position . See Fig . 1-2 .
Temperature measurements
are made using a
Tektronix P6430 probe (Tektronix Part Number 010-6430-
00) or other suitable sensing devices . Option 2 in-
struments delete the temperature measuring
capability .
The readout is a 3-1/2 digit display using seven-
segment LED's . The decimal point is automatically
positioned by the range-function switch . Polarity
indica-
tion is automatic . A usable 50% overrange exists in all
measurement modes except ac voltage and current, 1000
V
dc, and the 2 A do current range . In overrange, the
display blinks and the most significant digit
is suppressed .
Upon exceeding the available 50% overrange, a 1 appears
in the most significant digit position . Display readings are
invalid beyond this point .
REV .
A, OCT . 1974
INTRODUCTION
Section 1-113M 502
OPERATING
INSTRUCTIONS
Interlaced bcd output (1, 3, 2, 4) referenced
to
the low
input,
is available at the rear interface connector if user
wired .
NOTE
The
DM 502 is not directly compatible
with the
153
Calculator Interface .
The maximum input voltage is 1 kV, 350 V at the rear
connectors . The front-panel HI and LO connectors maybe
floated 1 kV maximum aboveground, the rear inputs 350 V .
Installation and Removal
The DM 502 is calibrated and ready for use when
received . It operates in any compartment of a TM 500
series power module . See the power module instruction
manual for line voltage requirements
and power module
operation . Fig . 1-1 shows the DM 502 installation and
removal procedure .
Turn the Power Module
off before inserting the DM
502
. Otherwise, arcing may occur at the rear inter-
face connectors, reducing their useful life and
damage may be done to
the
plug-in
circuitry .
Check that the DM 502 is fully inserted in the Power
Module . Pull the PWR switch on the Power Module . One or
more characters in the LED display should now be visible .
The Controls, Connectors and Adjustments foldout
page in Section 3gives a complete description of the front
panel .

Operating
Instructions-DIM 502
Input Connections
Three
binding posts provide measurement connec-
tions . The HI and LO posts are normally used for
all
measurements except temperature .
Normal measurement
conditions
will be with the LO terminal ungrounded . A
connection between the LO and GND posts may be made
to referencethe input against DM 502 chassis ground . Use
caution, as the LO terminal is then connected to
earth
ground through the three-wire powercord . Falsereadings
may be obtained due to ground loops .
If the INPUT button is pushed in, signal inputs are
made
via the rear interface connector
; if the button is out, signal
inputs are made
via the front panel .
Functions
Available at Rear Connector
HI, LO, GROUND,
temperature output at 10 mV per
degree and bcd connections are
available at the rear
Fig . 1-1 . DM 502 Installation
and removal .
OPERATING
CONSIDERATIONS
interface
connector . The Rear Interface Connector
Assignments page in the Service Section at the rear of this
manual gives data on the various connections . Only the
Temp Out, TempGnd and HI and LO inputsare factory-
wired . Jumper holes are provided
for making the desired
bcdoutput connections . See
the wire locations illustration
in the
foldout pages for the correct circuit board locations .
Simply
insert and solder an insulated wire strap in the
proper locations . Buffers are required before
the data
outputs are used to drive external
loads . See Fig . 1-3 for a
typical buffer
circuit .
The DM 502 has a slot between pins 17 and 18 . A barrier
in the corresponding position of the power module jack
allows only compatible plug-ins to be used
in that
compartment . This protects the plug-in,
should specializ-
ed connections be made to that
compartment . Consult the
Building A System section of the power module manual
for further information .

FET
input
jumper position
dBm jumper
position
dBV jumper
position
Connect jumperon
reverse side of board
as
shown
to
hard wire in
° F range .
Sine-wave Response
The DM 502
responds to the average value of a
sinusoidal
current or voltage . The readout indicates the
equivalent
rms value . The effective or rms value of a sine
wave is
0 .707 times the peak voltage or current . The
average value is 0 .636 of the peak value . The scale factor
of
the DM 502 is 0 .636/0 .707 or 0 .9 .
Multiply the DM 502
readout by 0 .9 to obtainthe
average value of a sinusoidal
input
voltage or
current .
Operating
Instructions-DM 502
Fig . 1-2 . Locations of dB reference jumper, input impedance
jumper, and hard wired ° C or ° F component or wire
changes .
Temperature Measurements
Connect
the P6430 (or other sensing device) to the
connector
marked TEMP PROBE . Use care to align the
connector
pins properly . Select calibration in °F or
°C
with the range-function switch . For best
accuracy,
calibrate the individual
instrument to the probe used . See
the Internal
Adjustment Procedure in the service section
of
this manual for the proper
procedure .

Operating Instructions-DM 502
Input
i-5 V
24 k
Floating ground
Output
2N3565 or other transistor
with minimum
beta of 150
Fig . 1-3 . Typical buffer
circuit for
rear connector data outputs . Up
to 20 mA may be used from the DM 502 +5 V supply .
DC Voltage Measurements
MAKING MEASUREMENTS
With the DM 502 properly installed in the power
module, allow twenty minutes warmup time for operation
to specified accuracy .
When the value
of
the quantity
being measured is unknown, select the highest range first .
Decrease the range setting until the display blinks in-
dicating over-range . Then increase the range switch to the
next higher position . This method obtains maximum
resolution . Resolution of the DM 502 is 0 .05% of full scale
setting, except temperature,
which
is
0 .1 ° .
Do
not
exceed
the maximum voltage ratings . With the range-function
switch in the 1 K DC VOLTS or 500 AC VOLTS position,
internal damage may result before overrange is indicated .
Select an appropriate full range do voltage position
with the range-function switch
.
Apply the voltage to be
measured to
the INPUT binding
posts .
Observe the
maximum voltage ratings as indicated on the front panel .
The readout displays a
+
if the HI input is positive with
respect to
the
LO
input . A -
is displayed
if the
LO
input
is
more positive . With the input shorted, the display reads
zero, ± one count .
The Temp
Out terminals atthe rear interface connector
are always connected to the
output of the temperature
circuitry . This feature provides continuoustemperature
monitoring capabilities
in °C even while using other
functions of the DM
502 . To avoid a momentary scale
change when a recorder or other device
is connected to
these terminals avoid switching through the °F
position .
The DM 502 may be hard wired in either the °C or °F
range independent of the range-function switch setting .
To hard wire in the ° F range, connect a jumper as shown in
Fig . 1-2 . To hard wire in ° C lift either end of R285 as shown
in Fig . 1-2 .
To prevent possible damage to switch contacts by
high currents, do not switch into, out of, or through
the current measurement modes from the other
measurement modes with a high current source, or
large inductive or capacitive load, connected to the
DM
502 input . This caution does not apply to
switching between individual current ranges up to
the maximum stated
input current, as
the switch
contacts are adequately protected
for this operation .
DC Current Measurements
Select an appropriate full scale DCmA position on the
range-function switch . Apply the do current to be
measured to the INPUT binding posts .
A
current (electron
flow) into the
LO
connector and out of the HI connector
indicates + on display . For opposite current flow, a - will
be displayed .
REV . A, OCT . 1974

AC
Voltage, Current and Resistance Measurements
Select an appropriate
full scale AC VOLTS, ACmA or
OHMS position on the range-function switch
. Connect the
unknown voltage, current or
resistance to the INPUT
binding posts .
Decibel
Measurements
To use this feature,
switch to an ac voltage or current
range and push the button marked dB on the
front panel .
Connect the voltage or current source
to the DM 502 . To
arrive at
the correct dB reading, algebraically add the
number on the outside
scale to the display reading . When
the dB switch is depressed, the decimal point is
fixed and
the maximum display reading is
19 .99 on all scales .
For example, if the range-function
switch is on the2 AC
VOLTS position and the
display reading is +13 .98, then
the correct dB reading
is :
+13 .98
-20 .00
-

6 .02 d B .
If the range-function switch is on the 200 ACmA
position and the display reads -4 .04, then :
- 4 .04
+20 .00
+15 .96 d B
Selection of dB readout
in dBV ordBm is accomplished
by a
jumper shown in Fig . 1-2 . The reference for dBV when
measuring
ac voltage is one volt and when measuring
current one milliampere . The reference
for dBm in the
voltage range is one milliwatt dissipated
in 6000 or0 .7746
volts ; in the current measuring
range the reference is
0 .7746 milliamperes . When dBm or
dBV
is
applied to
current measurements, they have no significance other
than to the reference current as stated above
.
Useful dB formulas are :
dB = 20 logio P 2 , 20 logio

2

and

10
logio
P
REV .
A, OCT . 1974
Operating Instructions-DM 502
For calculations substitute the reference voltage
or
current listed as the denominator
.
Temperature Measurements
With the P6430 probe
connected to the front panel
connector labeled TEMP PROBE, set the range-function
switch to the °F or °C in the TEMP position .
Apply the
probe sensor tip to the device
being measured . For
optimum temperature transfer, coat the
surface of the
device being measured with silicone grease
and apply the
probe tip squarely to the surface .
Allow sufficient time for
the probe
tip
to
stabilize before taking a reading . The time
required depends upon several factors . Generally,
when
the tip is first applied to
thedevice under test, the readings
change
rapidly . As the probe tip temperature approaches
the temperature of the device under test, the readings
change less rapidly, and finally
stabilize
.
The readings are
in °C or °F, depending on
the position of the range-
function switch
.
See
the P6430 manual for more informa-
tion on temperature
measurements and probe use .
Using a Transistor as a
Temperature-Sensing
Device
Certain npn transistors such as a
2N2484 can be used
as separate
sensors in place of the P6430probe with little
or no selection of the transistor . Connect thetemperature-
sensing transistor to the
DM 502 through the TEMP
PROBE
connector as shown in Fig . 1-4 . Accuracy without
recalibration of the
DM 502 is within ±8° C to 150'C . If the
Sensing
Transistor
(2N2484)
Connector
shell
Connector
TEKTRONIX Part Number
131-1653-00
Fig . 1-4 . Schematic
diagram of temperature-sensing transistor
connected to probe connector .

Operating
Instructions-DM 502
measurement to be made requires
greater accuracy,
recalibrate the temperature
circuitry or determine a
correction
factor . Place the sensing device in an
environ-
ment having a known ambient temperature
. Use any
difference between the known temperature
and the DM
502 readout as a correction factor
throughout the entire
measurement range .
Performance Conditions
The electrical
characteristics are valid only if the
DM
502 has
been calibrated at an ambient
temperature
between +20'C and +30° C and is
operating at an ambient
temperature
between +15°C and +40'C . Warm up time to
specified
accuracy is twenty minutes .
Display
The measurement rate
is 3 .33 per second . Resolution
on all
ranges
is
0 .05% of full scale setting ;
except for
temperature, which is 0 .1° .
RANGES :
DC
VOLTMETER
200 mV, 2 V, 20 V, 200
V, 1000 V .
ACCURACY :
±0 .1% of reading,
±1 count .
COMMON MODE REJECTION :
>100 dB at dc, 80 dB
at 50 or 60 Hz with a 1 kO
unbalance .
NORMAL MODE REJECTION :
>80 dB
at 50 or 60 Hz .
Accurate
Current Source
ELECTRICAL
SPECIFICATIONS
The OHMS positions
on the range-function switch
provide accurate
(typically 0 .2%) constant currents at the
INPUT terminals . See the Ohmmeter specifications for the
value of the currents and
maximum voltages across the
INPUT terminals
for full scale display readings . The
maximum voltage available across the INPUT terminals in
the
OHMS position is about 10 volts .
STEP RESPONSE TIME :
<-1/2 s .
INPUT RESISTANCE :
10
Mfg ±0
.25%
(jumper selectable for > 1000
Mfg on 0 .2
V and 2 V ranges)
.
MAXIMUM INPUT VOLTAGE ON ANY RANGE
:
Front Panel : 1 kV
.
Rear
Connector : 350 V .
RANGES :
AC
VOLTMETER
Voltage :

200 mV, 2 V, 20 V, 200 V, 500 V .
dB :

-40, -20, 0, 20, 40 . Reference is dBV
(1 V) or
dBm
(1 mW dissipated in 600 0, 0 .7746
V) .
ACCURACY
:
Voltage :

±0 .5% of reading, ±1
count, for a40 Hz to 10
kHz sine wave .
±1 .0% of reading, ±1 count, for a 20
Hz to 20 kHz sine
wave .
Usable to 100 kHz . Typically <10% down
between 40
mV and 500 V at 100 kHz .
REV . A, OCT . 1974

DB
:
COMMON MODE REJECTION :
X60 dB at 50 or 60 Hz with a 1 kQ unbalance
.
RESPONSE TIME :
<5s .
INPUT IMPEDANCE :
10 MO ±1% paralleled by < 60 pF
.
MAXIMUM INPUT VOLTAGE ON ANY RANGE :
Front Panel : 500
V
ac
rms or +56 dBm or 600 V dc, not
to exceed
1 kV do plus peak ac .
Rear Connector : 350 V do plus peak ac or
+50 dBm .
REV . A, OCT . 1974
OHMMETER
RANGES :
200
S2,
2 kQ, 20 kQ, 200 kQ, 2 MQ, 20 Mil .
ACCURACY :

RANGES :
Operating Instructions-DM 502
RESPONSE TIME :
200 S2 through 2 Mil ranges,
<_1/2s . 20
Mil
range, <-5s .
MAXIMUM INPUT VOLTAGE ON ANY
RANGE :
130 V
do and ac rms indefinitely .
250 V do and ac
rms for one-half hour .
RANGES :
200 /jA, 2 mA,
20
fnA,
200 mA, 2 A .
ACCURACY :
±0 .2% of reading, ±1 count .
RESPONSE TIME :
<1/2 s
.
INPUT IMPEDANCE :
DC AMMETER
0 .2 V

+

0
.1 S2
Range Setting
except <2 .0 kQ on
the 200 pA range .
MAXIMUM INPUT CURRENT ON ANY RANGE :
Fused at 2 .5 A .
AC AMMETER
200
IZ
range, 0 .5% ±1
count, +0 .1 S2 .

Current : 200 pA, 2 mA, 20 mA, 200 mA, 2 A .
2 kQ through 2 MQ ranges, ±0 .5%, ±1
count .

dB : -40, -20, 0, 20, 40 . Reference is 1 mA (dbV)
or
20 MQ range, 1 .0%, ±1 count .

0.7746 mA (dBm) .
MEASUREMENT CURRENT AND VOLTAGE :
Range Current
Full
Scale
Voltage
200
IZ 1 mA 0 .2 V
2kQ 1 mA 2V
20 kit 100,uA 2 V
200 kO
10
pA 2
V
2MQ
1/jA 2V
20 MO 0 .1 luA 2 V
Display
Frequency Maximum
Reading Range
0 to -+-20 20 Hz to 20 kHz 0 .5 dB
0to-10 20 Hz to2kHz 0 .5 dB
2 kHz to 20 kHz 1 .0 dB
-10 to - 20 20 Hz to 2 kHz 0 .5 dB
2 kHz
to
7
.5
kHz 1
.0 dB
7 .5 kHz to 20 kHz 2 .0 dB

Operating
Instructions-DM 502
ACCURACY :
Current : f0 .6% of reading, ±1
count for a 40 Hz to 10
kHz sine wave . Usable
to 100 kHz .

RANGES :
DB :
RESPONSE TIME :
<_5 s .
INPUT IMPEDANCE :
Centrigrade : -55 ° to +150 ° .
Fahrenheit : -67 ° to +200 ° .
ACCURACY :
MAXIMUM INPUT
VOLTAGE :
0 .2
V

+

0
.1
n
Range Setting

TEMPERATURE :
Operating : +15 ° C to +40 ° C .
except < 2 .0 KQ on the 200
NA range .

Non-operating : -40 ° C to +75 ° C .
MAXIMUM INPUT CURRENT ON ANY RANGE
:

POWER CONSUMPTION
:
Fused at 2 .5 A .

6 .5 watts .
THERMOMETER
With any P6430 probe, ±8 1 C or ±14 .4'F .
With instrument calibrated to
probe, ±2°C or ±3 .6 0 F .
Up to
1 kV may be safely applied to the front panel
INPUT terminals (350 V to the rear input
connectors) when
the range-function switch is in
the TEMP positions .
ENVIRONMENTAL
SPECIFICATIONS
REV . A, OCT . 1974
Display
Frequency Maximum
Reading Range
0 to +20
20 Hz to 20 kHz 0 .5 dB
0 to
-10 20 Hz to 2 kHz
0
.5 dB
2 kHz to 20 kHz
1 .0 dB
-10 to -20 20 Hz to 2 kHz
0 .5 dB
2 kHz to
7
.5
kHz 1 .0 dB
7 .5 kHz to 20 kHz 2 .0 dB

Introduction
The DM 502
is
basically an analog-to-digital converter
with the logic, display devices, and power supplies
necessary to display, in digital form, the value of a do
voltage . The analog-to-digital converter is an integrating
amplifier . Current is measuredby passing the unknown
current through a known resistance and measuring the
resultant voltage drop,
using a four terminal configura-
tion . Resistance is measured by passing a constant
current through an unknown resistance and measuring
the voltage drop across the unknown resistance .
Temperature is measured by obtaining a do voltage
proportional to the temperature and measuring that
voltage . Ac voltages and currents are
rectified and the
resulting do is measured, then read out in appropriate ac
units . The dB circuitry converts the output of the ac
rectifier to a logarithmic output that is applied to the
analog-to-digital converter . Refer to the Block
Diagram
in
the foldout pages for an overall view of the DM 502
operation
.
Use the following circuit description with the
schematics to gain an understanding of the operation of
the DM 502 .
Attenuator and Input Switching
The HI and LO binding posts are connected across
R147, R146 and R145 serving
as a
voltage divider . The
various capacitors switched across these resistors arethe
compensating capacitors used in the ac modes of opera-
tion . C100 is switched in series with the divider when the
unit is operating in the ac
voltage modes .
R105, R106, R107, R110 and R142 serve as current
shunts in the current modes
.
A four terminal technique
is
used . The current passes through the shunts from the HI
terminal to the LO terminal . The voltage is applied to the
analog-to-digital converter through S110, 17, 16, 9,15 and
18 to ground . F100, in
series with shunts R105, R106, R107
and R110 along with CR110 and CR112, protect the shunts
from over current . The diodes conduct, opening the fuse
before the voltage
across
the higher value shunts
damage
them . Lamp RT100 protects shunt R142, in the lowest
current range, and the Ohms Converter .
REV
.
A, OCT .
1974
THEORY OF OPERATION
Ac
Converter
Section 2-DM 502
THEORY
OFOPERATION
In the ac measurement modes, the ac signal is applied
to fet Q165 . CR165 and CR166 protect 0165 from over
voltage . Q170, connected
from the
source to the
drain
of
Q165, provides bootstrapping
.
The output of 0165 is fed to the negative inputof U195,
an operational amplifier . A small amount of negative do
feedback is provided by R192, R190 and the
filtering
action
of C190 . The gain of U195 is extremely high until CR187
and CR195 conduct . This high gain allows rectification of
extremely
small signals
. More negative do feedback is
provided through the action of CR187 and CR195 . The do
feedback
stabilizes the operational amplifier gain
and
compensates for amplifier offset voltages or capacitor
leakage .
The positive half cycle is applied through CR195 to the
filter network . The do voltage from the filter output is
switched to
the
analog-to-digital converter input in the ac
measurement mode, and the dB converter in thedB
mode .
The gain of U195 is set by R180 and R181 to produce a do
output equivalent to the rms value of a sine-wave input .
DB Converter
This circuitry converts the linear do current output from
the ac converter to a logarithmic output voltage . A
collector current change with respect to a base to emitter
voltage change in a bipolar transistor is logarithmic .
VR310 serves as the voltage reference diode at 6 .2 V .
This voltage is applied to the negative input of operational
amplifier U210 through R203 and
R205 . R205 sets
the
current through Q120A . Should
the
current through
Q120A increase, the voltage at pin 2 of U210 goes
negative . This causes the output of U210 to go positive
reducing the current through the transistor to thevalue set
by R205 . Q120A and 0120B are
housed in the same case
so that their base
to
emitter voltages remain
constant
with
temperature (for equal collector currents) .

Theory of Operation-DM
502
The
rectified and filtered do current from
the ac
converter
is applied to the plus input
of operational
amplifier U220, through R212 .
R212 causes a reference
current shift in the dBm
mode and is shorted out in the
dBV
mode . As pin 3 of
U220 goes positive, the output
goes
positive . The base
of Q120B goes positive and
the
collector negative providing negative
feedback
. The
collector
current of Q120A always
equals the current
through R212 at 0 dB
. The necessary voltage at theoutput
of
U220 is logarithmic due to
the logarithmic
characteristics of Q120B
in the feedback loop . As the
temperature increases
the gain of Q120B decreases
. This
is
compensated for by RT220 decreasing
in value with
increasing temperature .
Ohms Converter
U135 is an operational
amplifier . The current setting
resistances are connected
from the output to the plus
input terminal of
U135 . The unknown resistances
are
connected
from the plus terminal to floating
ground . See
Fig . 2-1 .
Negative feedback occurs from
the output of
U135 to
the negative input terminal . R130 is
the calibration
adjustment . R135 and R136 are switched in
and out of the
negative feedback loop to provide bias
current compensa-
tion .
The value of the current
setting resistance is always
equal to one half of the full scale
measurement value,
except on the 200
f2 range, as shown on the range-
2-2
Fig . 2-1
. Simplified diagram
of Ohms
Converter .
function switch . The 200
Q scale uses 1 kQ and the gain of
the
analog-to-digital converter is increased by a
factor of
ten . R130 adjusts the gain of
the amplifier to produce one
volt across the
current setting resistance (0 .1 V in the 200
Q
range) .
As long as the unknown resistance is
within the
measurement range, the circuitry adjusts
the current so
that the drop across the
current setting resistance is one
volt . This
current flows through the unknown
resistance,
varying the voltage at the plus terminal of U135 .
This
voltage is then fed to the analog-to-digital
converter input .
Temperature Measurement
Circuitry
This circuitry
operates on the principle that a change
in
base to
emitter voltage of a bipolar transistor is
propor-
tional to the temperature of the junction
as the collector
current is changed between
two levels .
A square
wave (about 2 kHz) is generated by Q230
and
Q235 operating as an astable
multivibrator . The square
wave is applied to the gates
of Q255 and Q260 .
The
collector to base voltage of the temperature
sensing
transistor in the probe is held constant
. When fet
0255 is off, the current to the probe
through R255 and
R257 is approximately one-tenth the
on current .
HI
LO
-12 V
REV . A, OCT . 1974

When Q230
is off and 0235 is on, Q255 is off and the
voltage at pin 2 of U250A
goes negative . The output of
U250A goes positive, reducing the
temperature-variable
base-emitter current of the sensing transistor until
pin 2 of
U250A
is at ground . A positive-going square wave results
at the output
of U250A, with an amplitude proportional
to
the temperature of the
base-emitter junction . Q260 is on
and
0245 is off, due to the action of Q230 and
Q235 . A do
voltage, whose amplitude
is proportional to temperature,
occurs across C263 . The
opposite occurs whenQ230 and
Q235 change state, with the exception that
Q245 turns on,
shorting the negative
voltage to ground ; and Q260 is off,
preventing the discharge
of C263 . The do voltage across
C263 is applied to operational amplifier U250B .
R280
adjusts the
gain . Offset voltage in U250B is compensated
for by R270 . R268 and
R285, through S110, 44, change
offset and increase gain for ° F readout
vs ° C . The output
of U250B at
10 mV/ O is applied to the analog-to-digital
converter input
and the rear interface connectors .
Clock
U325 is a monolithic timing circuit
used to generate the
clock frequency for the DM 502 . The frequency
is
20 .48 kHz . C330, the timing capacitor, charges through
R328, R329 and R330, and discharges through R330 . R328
sets the frequency so that it is a multiple of the power line
frequencies . This insures minimization of line frequency
interference in the analog-to-digital converter .
REV . A,
OCT . 1974
Analog-To-Digital Converter
Theory of Operation-DM
502
U290 comprises the
analog-to-digital converter and
U335 contains the necessary control
logic . U290 contains
the integrating
amplifier, comparator, auto-zero amplifier
and input buffer
amplifier
.
See Fig . 2-2 . C300 is the
integrating capacitor . The clock
frequency is divided by
the time base
counter into groups of 6144
pulses .
Measurement takes place
for 4096 of these pulses and
automatic zeroing takes place for 2048
pulses . The auto-
zeroing
feature provides current to
compensate for offset
currents in the
buffer amplifier .
Basic sensitivity
of the analog-to-digital converter is 2 V
for a full scale display reading
. When S110, 36 is closed,
the
sensitivity becomes 0 .2 V full
scale .
During auto zero, the input
of the buffer amplifier is
grounded . The buffer offset current
also offsets the output
of the integrator . The
offset at the integrator output passes
through a low-pass filter comprised
of C300, R300 and
R302 to the
plus input of the auto-zero amplifier . The
output of the unity gain auto-zero amplifier
compensates
for this offset current
at the summing node (pin 9) at the
integrator input . The reference
current through R304 and
R305 from the reference current source,
is applied forfour
clock cycles and disconnected for the
next four cycles by
the
U/D control logic . The referencecurrent
is connected
to R304 when
the up-down logic is low . Equilibrium
forthe
VIN
5
M/Z M/Z
2-u CLOCK
GENERATOR
U/D U/D
1511 "1131

11 21 31 4
15
V1
HI-OUAL .
GND
ELF
OUT
V2
1
AZ FILTER
R5
REFOUT VREF
10
VSS SIGN
IS
BCD OUMT
87 02 B3
84
DIGIT
STROBES
D1 D2 D3 DM
GND

CLOCKIN
Fig . 2-2 . Analog to digital converter
and control logic block diagram .
Reprinted by permission of Siliconix Inc
.
2-3

Theory of Operation-DM 502
entire integrating and auto-zero
system is obtained when
the sum of the
average currents at the integrator summing
junction
equals zero . C316 charges to the
equilibrium
voltage and maintains this voltage at
the auto-zero
amplifier input during the measurement
interval when the
switch from the integrator output is open
.
See
Fig . 2-3 for
functional timing during the auto-zero
interval . Notice that
the fifty percent duty cycle
of the up-down counter is
overridden
at the start of the auto-zero interval .
This
override period permits the output of
the integrator to
come to V,,, g and C316 to assume
this voltage .
Upon
completion of the 2048 clock pulse auto-zero
interval, the measurement interval commences .
The
measure-zero logic switches the analog
input voltage to
the buffer input and
disconnects the integrator output
from the auto-zero amplifier input . The additional
current
resulting at the summing junction of
the integrator causes
the integrator
output voltage to move away from the
equilibrium voltage
obtained during auto-zero interval
and maintained
during the measurement interval by C316 .
The
comparator senses and transmits this deviation to
the
control logic . The control logic changes the duty cycle
of
the reference voltage in an attempt to
re-establish
equilibrium at the integrator summing
junction . The up-
down logic is up
(integrator output voltage is also up) for
one
clock cycle and down for seven cycles when
the
comparator output was high during the
preceding set of
eight clock cycles . This is
shown as duty cycle A in Fig . 2-
4 . When the comparator output is
low in clockcycle seven,
the up-down logic is up
for seven cycles and down for one
cycle during the
following eight clock cycles . This is
shown as duty cycle B . Fig . 2-4 shows the
result of these
actions on the integrator output
.
2-4
A counter, synchronous with the up-down
logic, in-
crements by each
clock pulse when the up-down logic is
up and decrements by each clock pulse when the up-down
logic is down . The net count increases
by six counts for
each B duty cycle and decreases by
six counts for each A
duty cycle, to a maximum
count of about 3100 . This
counting procedure
is reversed for negative input
voltages .
Polarity of the input voltage is determined by the state
of the up-down logic when the bcd counter state is zero .
This information is stored in a flip-flop and loaded
into the
static latch once each measure-zero
cycle .
The bcd
counter accumulates a number of counts
proportional to the input voltage during the measure
interval while the control logic works to
maintain
equilibrium
. It can be seen that equilibrium is achieved in
steps and usually a residual voltage remains at
the end
of
the
measurement cycle . This residual voltage is compen-
sated for by a short override interval at the beginning of the
auto-zero period . The counter
continues until the in-
tegrator output equals
the auto-zero equilibrium voltage
and the up-down logic is down . The bcd counter is now
put on hold and its contents loaded into the
latches
.
The
counter is then cleared .
Fig . 2-3 . Auto-zero timing intervals .
Reprinted by permission of Siliconix Inc .

CLOCK
INPUT
CYCLE 7 0 1
COMPARATOR
INTEGRATOR
OUTPUT
2 3 4 5 6 7 0 1 2345 6 7 0 1 23456 7 0 1 2345 6 7 0
B B
Display
Fig . 2-4 . Measurement interval timing . Reprinted by permission of Siliconix, Inc .
The bcd data from the data buffers is applied to U340, a
bcd to seven segment
decoder . All segments of digits one,
twoand three are connected in parallel to U340 . The high
outputs from the digit buffers are inverted by U345A, B, C
and D, which drive Q350 for digit one, Q355 for digit two,
Q360 for digit three and Q365 forthe most
significant digit .
When the collectors of these transistors are high, the
particular digit is displayed . This is accomplished in a one,
three, two, four sequence . The common terminal for the
polarity signs is also activated by Q365 at the most
significant digit time . S110, 37, 38 and S200 disconnectthe
polarity signs when not appropriate for the
measurement
mode in use . S200 is on the dB switch . When pin 5 of U335
is high the positive sign is lighted . When pin 5 of U335 is
low, the negative sign is illuminated by the action of Q375
and U345F .
The
decimal
positions are changed by contacts in the
range-function and dB switches through 0370 . As pin 16
of U335 is the bcdone line to the seven segment decoder,
when the most significant
digit
is present,
pin 16 is high .
This is converted to a low and connected to the cathodes
of the onessegments in
the
most
significant digit module .
The contents of the static latches aremultiplexed to the
data output buffers . The data output format is an interlac-
ed scan of digits one, three, two and four . Four is
the
most
significant digit . With a count of 2000 or greater in the
counter (100% of full scale) the3-1/2 digit display blinks
off during the zero cycle .
Power Supplies
Theory of Operation-DIM 502
The entire DM 502
circuitry, with the exception of the
temperature
circuitry can be elevated up to 1kV above
chassis
ground . Isolation is accomplished with
transformer T410 . 50 V ac from the power module (two 25
V ac windings in series) is applied to
the transformer
primary . Ac for the +5
V and ±12 V supplies is bridge
rectified .
U420, for the ±12 V supplies, is a dual tracking
voltage regulator . R418 is a current shunt for the
+12 V
supply and R427 is the same
for the -12 V supply . R422
sets the levels . U420 is
internally protected against short
circuit damage .
Power for the display modules is not
electronically
smoothed, regulated or filtered .
Q440 serves as the
series pass transistor for the +5 V
supply
. The output voltage is set through the
divider
consisting of R452 and R453
connected to the base of
Q450 . Should the
+5 V output drop, Q450 increases
conduction . This causes Q440 to also increase
conduc-
tion restoring the output voltage to the preset level
.
The temperature
measuring circuitry obtains ±15 V
from ±33 .5 V through zener diodes VR400 and
VR405 .
2-5

REPLACEABLE
ELECTRICAL PARTS
PARTS ORDERING INFORMATION
Replacement parts are available from or through your
local Tektronix, Inc . Field Office
or representative .
Changes to Tektronix
instruments are sometimes made to accommodate improved
components as they become available,
and to give you the benefit of the latest circuit
improvements developed in our
engineering department . It is therefore important, when
ordering parts, to include the following information
in your order : Part number, instrument
type or number, serial number, and
modification number if applicable .
If a part you have ordered
has been replaced with a new or improved part, your local
Tektronix, Inc . Field
Office or representative will contact you concerning any change in part
number .
Change information, if any, is located at the rear of this manual .
SPECIAL NOTES AND SYMBOLS
X000

Part first added at this serial number
00X

Part removed
after this serial number
ITEM NAME
In the Parts
List, an Item Name is separated from the description by a colon
( :) .
Because of space
limitations, an Item Name may sometimes appear as incomplete . For
further Item
Name identification, the U .S . Federal Cataloging Handbook H6-1
can be
utilized where possible .
ABBREVIATIONS
Section 3-DM
502
ACTR ACTUATOR
PLSTC PLASTIC
ASSY ASSEMBLY
QTZ QUARTZ
CAP CAPACITOR RECP RECEPTACLE
CER CERAMIC
RES RESISTOR
CKT CIRCUIT
RF RADIO FREQUENCY
COMP COMPOgITION SEL SELECTED
CONN
CONNECTOR SEMICOND SEMICONDUCTOR
ELCTLT ELECTROLYTIC
SENS SENSITIVE
ELEC ELECTRICAL
VAR VARIABLE
INCAND INCANDESCENT
WW WIREWOUND
LED LIGHTEMITTJNG
DIODE XFMR TRANSFORMER
NONWIR NON WIREWOUND
XTAL CRYSTAL

Electrical
Parts
List-DM
502
CROSS INDEX
MFR . CODE NUMBER TO
MANUFACTURER
3- 2
REV . B JAN . 1975
MFR .CODE MANUFACTURER
ADDRESS
CITY,STATE,ZIP
00853 SANGAMO
ELECTRIC CO ., S . CAROLINA DIV .
P . 0 . BOX 128
PICKENS, SC 29671
01121
ALLEN-BRADLEYCO .
1201 2ND ST . SOUTH
MILWAUKEE, WI 53204
01295 TEXAS
INSTRUMENTS, INC .,
SEMICONDUCTOR GROUP
P . O . BOX 5012
DALLAS, TX 75222
03508
GENERAL ELECTRIC CO ., SEMI-CONDUCTOR
PRODUCTSDEPT .
ELECTRONICSPARK
SYRACUSE, NY 13201
03888
KDI PYROFILMCORP .
60 S .
JEFFERSON RD . WHIPPANY,
NJ 07981
04713 MOTOROLA, INC .,
SEMICONDUCTOR
PRODUCTS DIV .
5005 E .
MCDOWELL RD . PHOENIX, AZ
85036
07263 FAIRCHILD
SEMICONDUCTOR,
A DIV . OF
FAIRCHILD CAMERA AND INSTRUMENT
CORP . 464 ELLIS ST .
MOUNTAIN VIEW, CA
94042
07910
TELEDYNE SEMICONDUCTOR
12515 CHADRON AVE .
HAWTHORNE, CA 90250
14752 ELECTRO
CUBE INC .
1710 S . DELMAR
AVE . SAN GABRIEL, CA
91776
15281 GENERAL INSTRUMENT
CORP ., DELETED,
BUSINESS DISCONTINUED
15454 RODAN
INDUSTRIES, INC . 2905BLUE
STAR ST . ANAHEIM,
CA 92806
15818 TELEDYNE
SEMICONDUCTOR
1300 TERRABELLA AVE .
MOUNTAIN VIEW, CA 94040
17856 SILICONIX,
INC .
2201 LAURELWOOD RD :
SANTA CLARA, CA 95054
18324
SIGNETICS CORP .
811 E . ARQUES
SUNNYVALE, CA 94086
19396
ILLINOIS TOOL WORKS, INC ., PAKTRON
DIV . 1321 LESLIE AVE .
ALEXANDRIA, VA 22301
27014 NATIONAL
SEMICONDUCTOR CORP .
2900 SAN YSIDRO WAY
SANTA CLARA, CA 95051
28480
HEWLETT-PACKARD CO ., CORPORATE HQ .
1501PAGE MILL RD .
PALO ALTO, CA 94304
34553
AMPEREX ELECTRONIC CORP .,
COMPONENT DIV . 35 HOFFMAN AVE .
HAPPAUGE, NY
11787
49956
RAYTHEONCO .
141 SPRING ST .
LEXINGTON, MA 02173
50522
MONSANTOCO ., ELECTRONIC
SPECIAL
PRODUCTS
10131 BUBBRD .
CUPERTINO, CA 95014
56289 SPRAGUE
ELECTRIC CO .'
NORTH ADAMS, MA
01247
58474 SUPERIORELECTRIC CO ., THE
383 MIDDLE ST .
BRISTOL, CT 06010
71400 BUSSMAN MFG ., DIVISIONOF
MCGRAW-
EDISON CO .
2536 W . UNIVERSITYST
. ST . LOUIS, MO 63107
71590 CENTRALAB
ELECTRONICS, DIV .
OF
GLOBE-UNION, INC .
5757 N . GREEN BAY
AVE . MILWAUKEE, WI
53201
71744 CHICAGO MINIATURE LAMP
WORKS 4433RAVENSWOOD
AVE . CHICAGO, IL
60640
72982 ERIE TECHNOLOGICAL PRODUCTS,
INC . 644 W . 12TH
ST . ERIE,
PA 16512
73138 BECKMAN INSTRUMENTS,
INC ., HELIPOT DIV . 2500
HARBOR BLVD .
FULLERTON, CA 92634
75042 TRW ELECTRONIC COMPONENTS,
IRC FIXED
RESISTORS,
PHILADELPHIA DIVISION
401 N . BROAD ST .
PHILADELPHIA, PA 19108
80009 TEKTRONIX, INC .
P .
0 .
BOX500
BEAVERTON, OR 97077
80031 ELECTRA-MIDLAND
CORP ., MEPCO DIV .,
A NORTH
AMERICAN PHILLIPS CO . 22
COLUMBIA RD .
MORRISTOWN,NJ 07960
80294 BOURNS, INC .,
INSTRUMENT DIV .
6135 MAGNOLIA AVE .
RIVERSIDE, CA 92506
81483 INTERNATIONAL
RECTIFIER CORP . 9220
SUNSET BLVD .
LOS ANGELES, CA 90069
86684 RCA CORP
., ELECTRONICCOMPONENTS
415 S . 5TH ST .
HARRISON, NJ 07029
90201 MALLORY
CAPACITOR CO ., DIV . OF
P . R .
MALLORY CO ., INC .
3029 E . WASHINGTONST .
INDIANAPOLIS, IN 46206
91637 DALE
ELECTRONICS, INC .
P . 0 . BOX609
COLUMBUS, NB 68601