ORTEC 420 - Output Signal Termination

3-2
3.4 Linear Output Signal Connections and Terminating impedance Considerations
The source impedance of the 0-10 volt standard linear outputs of most 400
Series modules is approximately 1 ohm. Interconnection of linear signals
is, thus, non-critical since the input impedance of circuits to be driven is
not important in determining the actual signal span, e.g., 0-10 volts, de
livered to the fol lowing circuit. Paralleling several loads on a single out
put is therefore permissible while preserving the 0-10 volt signal span.
Short lengths of interconnecting coaxial cable (up to approximately 4 feet)
need not be terminated. However, if a cable longer than approximately
4 feet is necessary on a linear output, it should be terminated in a resistive
load equal to the cable impedance. Since the output impedance is not
purely resistive, and is slightly different for each individual module, when
a certain given length of coaxial cable is connected and is not terminated
in the characteristic impedance of the cable, oscil lations wil l generally
be observed. These oscillations can be suppressed for any length of cable
by properly terminating the cable either in series at the sending end or in
shunt at the receiving end of the line. To properly terminate the cable
at the receiving end, it maybe necessary to consider the input impedance
of the driven circuit, choosing on additional paral lel resistor to make the
combination produce the desired termination resistance. Series terminating
the cable at the sending end may be preferable in some coses where re
ceiving and terminating is notdesirable or possible. When series terminat
ing at the sending end, ful l signal span, i.e., amplitude, isobtainedat
the receiving end only when it is essential ly unloaded or loaded with an
impedance many times that of the cable. This may be accomplished by
inserting a series resistor equal to the characteristic impedance of the ca
ble internally in the module between the actual ampiifier output on the
etched board and the output connector. It must be remembered that this
impedance is in series with the input impedance of the load being driven,
and in the case where the driven load is 900 ohms, a decrease in the signal
span of approximately 10% will occur for a 93-ohm transmission line. A
more serious loss occurs when the driven load is 93ohms and the transmis
sion system is 93 ohms. In this case, a50% loss will occur. BNC connec
tors with internal terminators are available from a number of connector
manufacturers in nominal values of 50, 100, and lOOOohms. ORTEC stocks
in limited quantity both the 50 and 100 ohm BNC terminators. The BNC
terminators are quite convenient to use in conjunction with a BNC tee.