Tektronix AM 502 User manual

Tektronii. Inc
P.O Box SOO
Beaverton. Oregon
l^ktronix-
COMMTTTED TO E)CELLENCE
AM 502
DIFFERENTIAL
AMPLIFIER
INSTRUCTION MANUAL
97077 Serial Number
070-1582.01
PtodjCi Group 75 First Printing MAY i983
Revised OCT i966

Copyright *1973. 1976 TeKtronix. Ir>c All rights m*rveO
Cont«r>it of this publication may rwt be reproduced in any
form without the written permission o< TMIronix, Inc
Products Of Tektronix. Inc and iissubsidiaries are covered
by USand foreign patents and/or pending patents
TEKTRONIX TEK SCOPE-MOeiLE. and
isiered trademarxs ot Tektronix, Inc are reg-
Printed in uSASpecification and price change privileges
are reserved
INSTRUMENT SERIAL NUMBERS
Each instrument has aserial number on apanel insert, tag.
or stamped on the chassis. The first number or letter
designates the country of manufacture. The last five digits
of the serial number are assigned sequentially and are
unique to each instrument. Those manufactured in the
United States have six unique digits. The country of
manufacture is identified as follows:
BOOOOOO Tektronix. Inc.. Beaverton, Oregon. USA
100000 Tektronix Guernsey. Ltd., Channel Islands
200000 Tektronix United Kingdom. Ltd., London
300000 Sony/Tektronix. Japan
700000 Tektronix Holland. NV. Heerenveen.
The Netherlands

AM 502
TABLE OF CONTENTS
Pag«
Saction 1OperaUng Inatructlona 1-1
ENGLISH VERSION
FRENCH VERSION
GERMAN VERSION
JAPANESE VERSION
Saction 2SPEaFICATIONANDPERFORMANCE
CHECK 2-1
WARNING
THE FOLLOWING SERVICING INSTRUCTIONS
ARE FOR USE BY QUALIFIED PERSONNEL ONLY.
TO AVOID PERSONAL INJURY. DO NOT
PERFORM ANY SERVICING OTHER THAN THAT
CONTAINED IN OPERATING INSTRUCTIONS
UNLESS YOU ARE QUAUFIED TO DO SO.
Section 3ADJUSTMENT 3-1
Page
Section 4MAINTENANCE AND INTERFACING
INFORMATION 4-1
Section 5CIRCUIT DESCRIPTION 5-1
Section $OPTIONS 6-1
Section 7REPLACEABLE ELECTRICAL PARTS 7-1
Section 8DIAGRAM AND CIRCUIT BOARD
ILLUSTRATION 8-1
Section 9REPLACEABLE MECHANICAL PARTS
AND EXPLODED VIEW 9-1
CHANGE INFORMATION
REV CSEP 1980 I

AM 502
isa2i
AM 502 Oiftervntlal Amplifier plug>in module.
ii REV CSEP 1960

Section 1—AM 502
OPERATING
INSTRUCTIONS
INTRODUCTION
Description
The AM 502 i$ adc-couple<j differential amplifier with
excellent common-mode rejection capabilities and high
gain for low voltage measurements. The dc offset capabili-
ty permits nulling up to 1volt dc, so that low level, low
frequency signals impressed on adc level can be
amplified without the degradation often introduced byac
Input coupling. High and low frequency -3d6 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. Afront panel lamp indicates
most overrange conditions of excessive input signal,
excessive gain, or excessive offset.
The input circuit can be represented by approximately
1megohm to ground paralleled by approximately 47
picofarads. The Input impedance can be raised to about
200 megohms with the removal of ajumper.
Overload protection is provided by fuses In series with
the input which open when largeamountsof current flow
due to overloading conditions.
ASTEP GAIN DC BALANCE control iS provided to
adjust for zero shift as the gain switch is changed from one
position to another.
The DC OFFSET COARSE and FINE controls give
offset up to ±1 volt dc potential difference at the input
connectors. The amplifier internal bias is changed to
accomplish the offset. The LF -3 dB switch must be 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.
Installation and Removal
Turn lh» powermodule offbefore 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 forned off before removing the AM 502.
Refer to Fig. 1-1. Check to see that the plastic
barriers on the interconnecting jack of the selected
power module compartment 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 comer, 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 theAM
502
OPERATING CONSIDERATIONS
Overheating
The AM 502 is designed to operate at an ambient
temperature from 0” to -^-SO^C. However, when operating
several power supplies In amulti-plug-in power module,
especially at low output voltages, or when operating close
to other heat-producing equipment, internal temperature
may exceed safe limitsandactuatea thermal cutout in the
power module. Refer to the power module instruction
manual for more complete information.
REV DSEP 1980 M

Operating Instructions
—
AM S02
Fig. t*l. Pkjg-in Installation and removal.
Input Connections
Unshielded test leads can be used to connect the AM
502 to asignal source when ahigh-level, low-frequency
signal is monitored at alow impedance point. However,
when any of these factors is missing, it becomes in-
creasingly important to use shielded signal cables. In all
cases, the signal-transporting leads should be kept as
short as practical.
When making single-ended input measurements
(conventional amplifier operation), be sure to establish a
common ground connection between the device under
test and the AM 502. The shield of acoaxial cable is
normally used for this pun>ose. See Fig. 1-3 for reference.
Push the GND button for the input not connected to the
device under test.
In some cases differential measurements require no
common chassis ground connection, and therefore are
less susceptible to interference by ground-loop currents.
Probes
Attenuator probes decrease the resistive-capacitive
loading of asignal source. They also extend the measure-
ment range of the AM 502 to substantially higher voltages.
Some measurements require ahigher resistance input to
the AM 502. with very little source loading or signal
attenuation. In such situations use aFET probe or the
high-impedance input provision ot the AM 502. Contact
your Tektronix Representative for further Information on
probes.
High Impedance Input
To raise the internal input impedance of the AM 502 to
about 200 megohms, remove the P40plug (Fig. 3-1 ]. Make
certain the attenuator is in the NORM mode. Signal source
impedance now becomes an important factor. For exam-
ple, atoo picoampere gate current through 10 megohms
produces aone-mllllvolt offset. This offset may result in
significant error when small voltages are measured.
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 1megohm 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 ahigher impedance cable
or an attenuator probe.
input Overloading
When measuring unknown dc 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 -rt 00 pushbutton until asuitable output signal
IS obtained. If the input circuit of the AM 502 is overdriven,
large amounts of current will flow, opening the protective
fuses.
1-2 REV CSEP 1980

operating Instructions—AM 502
STEP GAIN DC BAL Control
Adiusts dc balance In the
preampliller stage.
Variable Gain Control con-
tinuously variable.
Calibrated in thecw position.
GAIN Switch
Selects amplifier gain.
OVER RANGE Indicator
When lit, indicates most over-
range conditions.
+100 Pushbutton
Divides GAIN switch setting
by 100.
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OC OFFSET COURSE and FINE
Controls
fProvides Intental olfset ol
approximately ±1 Vto input
signal. LF —3 dB switch must
be In the OC OFFSET position
for controb to operate.
POWER Indicator
When lit. Indicates power Is
applied.
AC Pushbutton
Selecb the input signal (ac or
dc) into the negaUve side ol the
preamplifier.
4- Inpul Conrtector
6NC connector tor the
positive side of the amplifier.
GND Pushbutton "
Grounds Tamplifler input
Ground Binding Post
Provides achassb return for
signab.
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HF -3dB Switch
Selecb upper -3dB point of
the amplifier.
LF -3 dB Switch
'Selecb lower —3 dB point of
the amplifier and activates DC
OFFSET controls.
OUTPUT Connector
BNC output terminal con-
nector.
-Input Connector /
BNC connector for the
negativa side ol the amplifier.
onus AM 502f DIFFERENTIAL AWPL AC Pushbutton
Selecb the inpul signal (ac or
dc) into the positive side ol the
preampllfbr.
Release Latch
Pull to remove plug-ln.
GND Pushbutton
Ground -amptitier inpuL
REV CSEP 1980 1-3

Operating Inatructions—AM 502
Fig. 1-3. Waveformeahowlrtg dltferenttsttejeclion ofacofnmen-fliodeiJgnal.ReMltanlweMtormchowttiedillerence between
ttie two tignalt.
Step Gain DC Balance
Output Connections Kcontrol is misadjusled, the dc output level will
Make output connections using abnc to dual binding shitt as the GAIN switch position is changed. Push both
post connector, or acoaxial cable with at least one bnc GNO buttons and place the GAIN switch in the 100
connector. To prevent current limiting in the outputstage. 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 node shitt at
current is limited to 20 milliamperes. the OUTPUT terminal.
1-4 REV CSEP 1980

operating Instructions—AM 502
DC Offset Coarse and Fine
Use these corjtrols to offset up to =1 volt dc 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 DC OFFSET position to
activate these controls.
HF and LF 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 -3dS control increases the lower bandwidth
frequency. Use this control to reduce dc 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 conrtecting the
AM 502 to asignal containing adc component, push the
AC and GND pushbuttons. Conrwct the 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 dc
voltage to be measured.
Differential Operation
Adifferential 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 Cfor reference.
(A) Rlehl
(B) Wrong
(C) Wrong
undor lest net bo grounded to
oquipmont under teet log3i)isS2
Single-ended measurements often yield unsatisfactory Fig. i-4. Connecting adltterentlel emplliler to •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-lrequency response
test. In other cases, It may be desirable to eliminate adc The limitations of single-ended measurements are effec
voltage by means other than the use of ablocking tively eliminated by using differential measurements.
REV CSEP 1980 1-5

Operating Instructions—AM 502
DQ Offset Operation
By using the FINE and COARSE DC OFFSET controls,
it Is possible to use the AM 502 differentially in aslide-back
mode, to observe asmall signal whose dc potential
difference may be considerable. The offset voltage is
continuously adjustable from plus 1volt to minus 1volt,
and Is internally available for all of the GAIN switch
positions. The LF -3dB switch must be in the DC OFFSET
position to activate the offset circuit.
Common Mode Rejection Ratio
The ability of the AM 502 to reject common-mode
signals Is Indicated by the common-mode rejection ratio
(cmrr). For example, assume that asignal consists of
unwanted 60 hertz at 10 volts peak-to-peak (common
mode connected to both inputs), plus adesired signal at 1
millivolt peak-to-peak (differentially connected to one
input). The AM 502 gain is setat200.TheoutputoftheAM
502 shows the desired signal at an amplitude of 0.2 volt (1
millivolt X200), and the 60 hertz signal is viewed at an
amplitude of 0.02 volts. The cmrr in this application is
100,000 to 1. This figure is calculated by multiplying the
value of the common-mode signal (10 volt) by the gain of
the amplifier (200) for aproduct of 2000 volts. This product
is divided by the observed 60 hertz voltage at the output
(0.02 volt] and result is the cmrr. 100.000 to 1. It would, of
course be impossible to view the 1millivolt signal
superimposed on the 60 hertz signal by using single input
methods.
Degradation of Common Mode Rejection
There are a number of factors that degrade common-
mode rejection ratio (cmrr). The principal requirement for
maximum rejection is for the common-mode signal to
arrive at the input FET gates at the same phase and
amplitude. Adifference of only 0.01% in the attenuation
ratios of the input attenuators will reduce the rejection
ratio to 10,000 to 1. Also, any difference in source
impedance will degrade the rejection ratio. Figures 1-5
and 1-6 show common-mode rejection degradation due to
differences in source impedance. The frequency of the
common-mode signal also affects the common-mode
rejection ratio. Generally, as the frequency of the input
signal increases, the common-mode rejection ratio is
more difficult to maintain.
Fig. 1-5. Relallonthip ol twt point wure* Impodanee to the
amplifior Input Impodanca end thaapparenl CMRR caused by (A)
large dlttaranca batwcon lasl-point Impedancas and (B) low
knpadanca Mat points.
1-6 REV 0SEP 1980