Tektronix AM 502 Instruction Manual page 7

O p I n 5 0
F ig. 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 can be used 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