Tektronix AM 502 User manual

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BEFORE READING
070-1582-01
PLEASE
CHECK FOR CHANGE INFORMA TION
AT THE REAR OF THIS MANUAL
.
THIS MANUAL REPRINTED MARCH 1978
TEKTRONIX
A I~ 5®~
DIFFERENTIAL
AlUIPL1FIER
INSTRUCTION
MANUAL
Tektronix, Inc .
P.O . Box 500
Beaverton, Oregon 97077

Serial Number
First Printing MAY 1973

WARRANTY
This Tektronix instrument is
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 .
Copyright ~~
1973, 1978 by Tektronix,
Inc . All rights reserved :
Contents
of this publication may not be
reproduced in any
form
without
the permission of Tektronix,
lnc :
Products of Tektronix, Inc .
and its subsidiaries
are covered by U .S .
and foreign patents and/or
pending patents
.
TEKTRONIX,
TEK, SCOPE-M08fLE,
TELEQUIPMENT,
and
are registered
trademarks of
Tektronix, Inc . Printed
in U .S :A :
Specification and price
change privileges
are reserved .

REV . B MAR 1978
Page
SECTION
1

OPERATING
INSTRUCTIONS
SECTION
2

SPECIFICATION
AND PERFORMANCE
CHECK

2-1
WARNING
TABLE
OF
CONTENTS
The remaining
portion of this Table of
Contents lists servicing instructions
that
expose personnel
to hazardous voltages .
These instructions are for qualified
service personnel
only .
SECTION 3

ADJUSTMENT

3
- ~
SECTION 4

MAINTENANCE
AND INTERFACING
INFORMATION

4-1
SECTION 5

CIRCUIT DESCRIPTION

5-1
SECTION 6

OPTIONS

6'1
SECTION 7

REPLACEABLE ELECTRICAL
PARTS

7-1
SECTION
8

DIAGRAM
AND CIRCUIT BOARD
ILLUSTRATION

8-1
SECTION 9

REPLACEABLE MECHANICAL
PARTS AND
EXPLODED VIEW

9-~
CHANGE
INFORMATION
AM
502

AM
502 Differential Amplifier plug-in module
.
i!

REV .
B MAR 1978

Description
INTRODUCTION

Installation and Removal
The AM 502
is a dc-coupled differential
amplifier with
excellent common-mode rejection capabilities
and high
gain for
low voltage measurements . The do
offset capabili-
ty
permits nulling up to 1 volt dc, so
that low level, low
frequency signals impressed on
a do level can be
amplified without the
degradation often introduced by ac
input coupling .
High andlow frequency -3 dB points
can
be selected at
the front panel to suit the application
. Signal
inputs and
outputs are available at the rear
connector as
well as at
the front panel . A front panel
lamp indicates
most
overrange conditions of excessive
input signal,
excessive gain, or excessive
offset .
The input circuit
can be represented by approximately
1 megohm to
ground paralleled by
approximately 47
picofarads .
The input impedancecan be raised to
about
200
megohms with the removal of a jumper .
Overload protection is provided
by fuses in series with
the input which open when
large amounts of currentflow
due to
overloading conditions .
A STEP GAIN DC BALANCE
control is provided to
adjust for zero shift as thegain switch
is changed from one
position to another .
The DC
OFFSET COARSE and FINE controls give
offset up
to fi volt do potential difference
at the input
connectors . The amplifier internal
bias is changed to
accomplish the offset . The LF -3dB switch
mustbe in the
DC OFFSET position to actuate these
controls .
The HF -3 dB switch is used
to reduce the upper
bandwidth limit as
necessary to improve the signal-to-
noise
ratio when using the AM 502 in
low-frequency
applications . The LF -3 dB control
increases the lower
bandwidth frequency .
Use of the pre-charging
feature prevents surge
currents, due to
charging the ac coupling capacitor,
from
damaging the circuit under test .
REV . CMAR
1978
Overheating
Section 1-AM 502
OPERATING
INSTRUCTIONS
CAUTION
Turn the power module off
before inserting the plug-
in ; otherwise, damage may occur
to the plug-in
circuitry . Because of the high current
drawn by the
AM 502, it is also
recommended that the power
module be turned
off before removing the AM 502
.
Refer to Fig .
1-1 . Check to see that
the plastic
barriers
on the interconnecting jack of theselected
power
modulecompartment match
the cut-outs in
the AM 502 circuit board edge
connector .
Align the AM 502
chassis with the upper and
lower
guides of the selected
compartment . Push
the module in
and press firmly to
seat the circuit
board in the inter-
connecting
jack .
To remove the AM 502,
pull on the release latch
located
in the lower left `corner,
until the interconnecting
jack
disengages and the
AM 502 will slide out .
Controls and Connectors
Refer to
Fig . 1-2 . Even though the
AM 502 is fully
calibrated
and ready to use, the
functions and actions of
the
controls and connectors should be reviewed before
attempting to use it . Pull the
Power switch on the power
module to apply power to the
AM 502 . The POWER
indicator light indicates when
power is applied to the AM
502 .
OPERATING
CONSIDERATIONS
The AM 502 is
designed to operate at
an ambient
temperature
from 0° to+50°C . However,
when operating
several power supplies in a multi-plug-in
power module,
especially
at low output voltages,
orwhen operating close
to
other heat-producing equipment,
internal temperature
may exceed safe limits and
actuate a thermalcutout in
the
power module . Refer to
the power module instruction
manual for more
complete information .

Operating Instructions-AM 502
Fig . 1-1 . Plug-in installation and removal .
Input Connections

Probes
Consider the change in the source operating
characteristics due to loading by the signal input cables .
The circuit
at
the input
connectors can be represented by
approximately
1 megohm to ground paralleled by ap-
proximately 47 picofarads . Two feet of 50 ohm coaxial
cable increases the parallel capacitance by
about 60
picofarads, which could be excessive in many situations .
To minimize these effects, use a higher impedance cable
or an attenLator probe .
Bottom
G roove
Input Overloading
Slot
AM 502
(1412)1582-2
Unshielded test leads canbeused to connect the AM

Attenuator probes decrease the
resistive-capacitive
502 to a signal source when a high-level, low-frequency

loading
of
a signal source .
They also
extend the measure-
signal
is
monitored
at a
low impedance point . However,

ment range of the AM 502 to substantially highervoltages .
when any of these factors is missing, it becomes in-

Some measurements require a higher resistance input to
creasingly important to use shielded signal cables . In all

the AM 502, with very little source loading or signal
cases, the signal-transporting leads should be kept as

attenuation . In such situations use a FET probe or the
short as practical .

high-impedance input provision of the AM 502 . Contact
your Tektronix Representative for further
information
on
When making single-ended
input measurements
probes .
(conventional amplifier operation), be sure
to
establish a
common ground connection between the device under
test and the AM 502 . The shield of a coaxial cable is

High Impedance Input
normally used for this purpose . See Fig .1-3 for reference .

To
raise the internal input impedance of the AM 502 to
Push the GND button for the
input not connected to the

about 200 megohms, remove the P40 plug (Fig .3-1) . Make
device under test .

certain the attenuator is in the NORM mode . Signal source
impedance now becomes an important factor
. For exam-
In some cases differential measurements require no

pie, a 100 picoampere gate
current through 10 megohms
common chassis ground connection, and therefore are

produces
a one-millivolt offset . This offset may result in
less susceptible to interference by ground-loop currents .

significant error when small voltages are measured .
When measuring unknown do voltages, push the=100
pushbutton in, and start with the 100 position on the GAIN
switch . Increase the GAIN switch setting and finally
release the =100 pushbutton until a suitable output signal
is
obtained . If the input circuit of the AM 502 is overdriven,
large amounts of current will flow, opening the protective
fuses .
REV . B MAR
1978

STEPGAIN DC BAL Control
Variable Gain
Control con-
tinuously
variable .
When lit, indicates most over-
range conditions .
AC Pushbutton
Selects the input signal (ac or
dc) into the negative side
of the
preamplifier
.
+ Input Connector
BNC connector for the
positive side of
the amplifier .
GND Pushbutton
Grounds
+ amplifier input .
G~ Fund Binding Post
- Input
Connector
BNC connector for the
negative side of the amplifier .
REV . B MAR 1978
Release Latch
Pull
to
remove plug-in .
100 Pushbutton
Divides GAIN switch setting
by 100 .
Ground
- amplifier input .
Fig . 1-2 . AM 502
controls and connectors .
Operating
Instructions-AM 502
DC
OFFSET COURSE and FINE
Controls
Provides internal
offset of
approximately
t1 V to input
signal .
LF -3dB switch must
be in the DC OFFSET
position
for controls to
operate .
When lit, indicates power
is
applied .
HF --3dB Switch
Selects
upper -3dB point of
the amplifier .
LF -3 dB Switch
~ ~ j ~''~

Selects lower -3dB point of
the amplifier and activates DC
OFFSET controls .
OUTPUT Connector
Selects the
input signal (ac or
dc) into
the positive side of the
preamplifier
.
7 582-9

Operating Instructions-AM 502
7-4
Waveforms applied to the (+) and
(-)
input
connectors .

The resultant display .
Two signals of
different amplitude .
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Signal (square wave) with an
inter-
fering
line-frequency component is
applied to the (+)input . The interfering
signal (sine wave) is also applied to the
(-) input connector .
Fig .1-3 . Waveforms
showing differential rejection ofa
common-mode signal . Resultant Waveforms show
the difference between
the two signals .
Step Gain DC Balance
1582-5
Output Connections

If this control is misadjusted,
the do output level will
Make output connectionsusing
a bnc to dual binding

shift as the GAIN switch
position is changed . Push both
post connector, or
a coaxial cable with at least onebnc

GND buttons and place the
GAIN switch in the 100
connector
.
To
prevent current limiting in theoutput
stage,

position . Rotate the GAIN
switch from stop to stop while
do not load
the output with less than 250 ohms .
Output

adjusting the STEP
GAIN DC BAL control for nodo
shiftat
current is limited to 20 milliamperes .

the OUTPUT
terminal .
REV . B MAR 1978

DC Offset Coarse and Fine
Use
these controls to offset up to f1 volt do potential
difference at the input connectors . The amplifier internal
bias is changed to accomplish the offset . The
differential
rejection capabilities of the AM 502 are
not affected . The
LF -3dB switch must be in the DCOFFSET
position to
activate these controls .
HF and l-F Bandwidth Reduction
Use the HF-3dB switch to reduce the upper bandwidth
limit, as necessary, to improve the signal-to-noise ratio
when using the AM 502 in low-frequency applications .
The LF -3dB control increases the lower bandwidth
frequency . Use this control to reduce do drift, when
raising the lower bandwidth does
not undesirably reduce
the bandwidth for the signal being measured .
Pre-Charging
Use of this feature prevents
surge currents, due to
charging the
ac coupling
capacitor in the AM 502, from
damaging the circuit under test
. Before connecting the
AM
502 to
a signal containing a do component, push the
AC and GND pushbuttons . Connectthe input to the circuit
under test . Wait about one second for the coupling
capacitor to charge . Release the GND pushbutton, and
the coupling capacitor is charged to the value of the do
voltage to be measured .
Differential Operation
A differential measurement is made by
connecting
each of the two inputs to
selected points in the test circuit .
The input to the amplifier will then be the difference in
voltage of the two selected points . Consideration should
be given to the proper connection
method used between
the AM 502 and the circuit under test
; otherwise improper
measurement results may occur
. See Fig . 1-4 for
reference .
Differential voltage measurements are made by apply-
ing the signals to the +input and -input connectors . Set
the input coupling switches to the same position, AC or
DC, depending on
the measurement being
made .
In
differential
measurements,
only the
voltage
difference
between the two signals is
amplified
. Common mode
signals (common in amplitude, phase, and frequency) are
rejected . See
Fig .
1-3
A, B,
and C
for reference
.
REV . B MAR
1978
(A) Right
circuit
under
test
(B) Wrong
I

I'
___J
circuit
under test
(C) Wrong
I
I

probe shields
I

tied
together
i probe shields
should not be
floating
Operating Instructions-AM 502
U
common ground
I
I

\
I
___J
circuit

probe shields should
under test

not be grounded to
equipment under test
Differential
Amplifier
Differential
Amplifier
I
Differential
`,

Amplifier
(0931)1582-13
Single-ended measurements often yield unsatisfactory

Fig . 1-4 . Connecting a differential amplifier to a signal source .
results because of the interference resulting from
ground-
loop currents between the
AM
502 and
the device under

capacitor,
which
could
limit the low-frequency response .
test . In other cases, it may be
desirable to eliminate a do

The limitations of single-ended measurements are effec-
voltage by
means other than the use of a blocking

tively eliminated by using differential measurements .